Unlocking the Future: Carbon Nanotubes as Pioneers in Sensing Technologies

Abstract

Carbon nanotubes (CNTs) have emerged as pivotal nanomaterials in sensing technologies owing to their unique structural, electrical, and mechanical properties. Their high aspect ratio, exceptional surface area, excellent electrical conductivity, and chemical tunability enable superior sensitivity and rapid response in various sensor platforms. This review presents a comprehensive overview of recent advancements in CNT-based sensors, encompassing both single-walled (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). We discuss their functional roles in diverse sensing applications, including gas sensing, chemical detection, biosensing, and pressure/strain monitoring. Particular emphasis is placed on the mechanisms of sensing, such as changes in electrical conductivity, surface adsorption phenomena, molecular recognition, and piezoresistive effects. Furthermore, we explore strategies for enhancing sensitivity and selectivity through surface functionalization, hybrid material integration, and nanostructuring. The manuscript also covers the challenges of reproducibility, selectivity, and scalability that hinder commercial deployment. In addition, emerging directions such as flexible and wearable CNT-based sensors, and their role in real-time environmental, biomedical, and structural health monitoring systems, are critically analyzed. By outlining both current progress and existing limitations, this review underscores the transformative potential of CNTs in the design of next-generation sensing technologies across interdisciplinary domains.

1. Introduction

Carbon nanotubes (CNTs) have emerged as a pivotal class of nanomaterials since their discovery by Iijima in 1991 [1,2,3,4]. Structurally, CNTs are cylindrical allotropes of carbon, consisting of rolled graphene sheets with sp²-hybridized carbon atoms arranged in a hexagonal lattice [5]. Based on the number of concentric graphene layers, CNTs are classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) [6]. Their unique quasi one-dimensional (1D) structure offers extraordinary physical, chemical, and mechanical properties that surpass many conventional materials. CNTs exhibit remarkable tensile strength (~100 times stronger than steel at one-sixth the weight), superior elasticity, excellent thermal conductivity (~3000–3500 W/mK for SWCNTs), and exceptional electrical conductivity, depending on their chirality and diameter [7]. Their high aspect ratio and large specific surface area make them highly sensitive to surface phenomena, which is a fundamental advantage in sensor applications [8]. Moreover, CNTs possess tunable electronic properties, acting as either metallic or semiconducting materials based on their structural configuration (chirality and diameter) [9]. This versatility has fueled extensive research into their applications across multiple scientific domains, ranging from electronics and energy storage to drug delivery and sensing platforms [10]. The unique physicochemical characteristics of CNTs have positioned them at the forefront of nanotechnology-enabled sensing platforms. Sensing technologies rely fundamentally on the interaction of analytes with the active sensing material, leading to measurable changes in electrical, optical, or mechanical properties [11]. CNTs, owing to their large surface area-to-volume ratio and high electron mobility, offer unparalleled sensitivity to even trace levels of target analytes [12]. Furthermore, the presence of defect sites, functional groups, or heteroatoms on the CNT surface can significantly enhance their interaction with specific chemical or biological species [13]. Functionalization strategies both covalent and non-covalent allow tuning of CNT surface chemistry, enabling selective detection in complex environments [14]. The integration of CNTs into sensor architectures has demonstrated significant advantages in detecting gases (e.g., NO₂, NH₃, and CO), volatile organic compounds (VOCs), biomolecules (e.g., DNA, proteins, and glucose), heavy metals, and environmental pollutants [15]. Their rapid response time, low detection limits, stability, and potential for miniaturization align well with the growing demand for portable and real-time monitoring devices in healthcare, environmental monitoring, food safety, and industrial applications [4]. Additionally, the combination of CNTs with other nanomaterials, such as metal nanoparticles, polymers, or 2D materials, has resulted in synergistic effects, further enhancing sensor performance metrics such as selectivity, sensitivity, and stability [16]. As illustrated in Table 1, carbon nanotubes (CNTs) exhibit remarkable advantages over conventional sensing materials, making them highly attractive candidates for advanced sensor development. The exceptional electrical conductivity of CNTs, ranging from 10² to 10⁵ S/m, significantly surpasses that of most metal oxides and conducting polymers, enabling rapid electron transfer and enhanced signal transduction in sensor systems [17]. Additionally, the specific surface area of CNTs exceeds 1000 m²/g, far greater than that of traditional materials like metal oxides or noble metals, thereby providing an abundance of active sites for analyte adsorption and improving sensitivity to trace-level targets [18]. Mechanically, CNTs possess an ultra-high Young’s modulus of approximately 1 TPa, granting them outstanding robustness and flexibility, which are essential for the development of flexible and wearable sensing devices [19]. Moreover, CNTs offer excellent functionalization capability through both covalent and non-covalent approaches, allowing for selective detection of specific analytes in complex environments, a feature often limited in metal-based or polymeric materials [20]. In contrast, although materials like graphene share similar mechanical strength and electrical conductivity, CNTs provide distinct advantages in 1D morphology and higher aspect ratios, which further amplify their sensing performance [21]. These comparative attributes collectively justify the growing research focus on CNT-based sensors [22,23,24,25,26] and underpin their potential to revolutionize sensing technologies across environmental, biomedical, and industrial sectors. Despite the numerous advantages of carbon nanotubes (CNTs) in sensing applications, several critical limitations continue to challenge their widespread adoption in practical devices. One of the most significant drawbacks is poor intrinsic selectivity, which hampers the ability of CNTs to distinguish between chemically similar analytes, particularly in complex or mixed environments. This issue arises due to the inherently non-specific nature of pristine CNT surfaces, which readily interact with a broad range of chemical and biological species through π–π interactions, van der Waals forces, or electrostatic attractions. Additionally, batch-to-batch variability in CNT synthesis, including inconsistencies in diameter, chirality, and purity, often leads to non-uniform sensor responses and impairs reproducibility. Another drawback is the difficulty in precisely controlling CNT alignment and dispersion on sensor substrates, which affects device performance and scalability. Moreover, CNTs tend to agglomerate due to strong van der Waals forces, limiting effective surface area and active site accessibility. Environmental factors such as humidity, temperature, and interference from background gases can also influence the sensor signal, leading to signal drift or reduced stability. From a fabrication standpoint, integrating CNTs into device architectures requires complex processing steps and sometimes harsh chemical treatments, which may degrade their structural and electronic integrity. Furthermore, while functionalization strategies have been developed to enhance selectivity and sensitivity, these modifications can inadvertently alter the intrinsic properties of CNTs, introducing trade-offs between performance metrics. Finally, concerns regarding toxicity and environmental persistence of CNTs remain, particularly for biomedical or wearable applications, necessitating comprehensive toxicological assessments and safe disposal practices. Addressing these challenges through controlled synthesis, advanced functionalization, and hybrid material strategies is essential for translating CNT-based sensors into reliable, scalable, and selective sensing technologies.

Table 1. Comparative summary of Carbon Nanotubes (CNTs) with Conventional Sensing Materials for Sensor Applications.

Material Type	Electrical Conductivity	Surface Area	Mechanical Strength	Sensitivity to Analytes	Functionalization Possibility	References
Carbon Nanotubes (CNTs)	High (102–105 S/m)	Very High (>1000 m2/g)	Exceptional (Young’s modulus~1 TPa)	Very high (down to ppb/ppt levels)	Excellent (Covalent and Non-Covalent)	[17]
Graphene	High (~104 S/m)	High (~2630 m2/g)	High (Young’s modulus~1 TPa)	High	Excellent	[21,27]
Metal Oxides (e.g., SnO2, ZnO)	Moderate to Low (~10−2–100 S/m)	Moderate (~10–50 m2/g)	Brittle	Moderate	Limited (Surface Modification)	[28]
Conducting Polymers (e.g., Polyanilin, Polypyrrole)	Low to Moderate (~10−3–102 S/m)	Moderate (~50–100 m2/g)	Low to moderate	High (selective in certain environments)	Moderate	[29]
Noble Metals (e.g., Au, Pt, Ag)	High (~107 S/m)	Low (<10 m2/g)	High (ductile)	High (surface plasmon effect)	Limited (Surface Adsorption)	[30]

Table 1. Comparative summary of Carbon Nanotubes (CNTs) with Conventional Sensing Materials for Sensor Applications.

Material Type	Electrical Conductivity	Surface Area	Mechanical Strength	Sensitivity to Analytes	Functionalization Possibility	References
Carbon Nanotubes (CNTs)	High (102–105 S/m)	Very High (>1000 m2/g)	Exceptional (Young’s modulus~1 TPa)	Very high (down to ppb/ppt levels)	Excellent (Covalent and Non-Covalent)	[17]
Graphene	High (~104 S/m)	High (~2630 m2/g)	High (Young’s modulus~1 TPa)	High	Excellent	[21,27]
Metal Oxides (e.g., SnO2, ZnO)	Moderate to Low (~10−2–100 S/m)	Moderate (~10–50 m2/g)	Brittle	Moderate	Limited (Surface Modification)	[28]
Conducting Polymers (e.g., Polyanilin, Polypyrrole)	Low to Moderate (~10−3–102 S/m)	Moderate (~50–100 m2/g)	Low to moderate	High (selective in certain environments)	Moderate	[29]
Noble Metals (e.g., Au, Pt, Ag)	High (~107 S/m)	Low (<10 m2/g)	High (ductile)	High (surface plasmon effect)	Limited (Surface Adsorption)	[30]

Figure 1 illustrates the multifaceted role of carbon nanotubes (CNTs) in modern sensing technologies by highlighting their integration across various sensor types. At the center, the schematic emphasizes the two primary forms of CNTs single-walled (SWCNTs) and multi-walled (MWCNTs) both known for their unique structural, electrical, and surface properties. From this central structure, application branches extend to show how CNTs are employed in gas sensors for detecting hazardous gases like NO₂ and NH₃, chemical sensors for identifying pollutants and monitoring reactions, biosensors for detecting biomolecules such as glucose and DNA, and pressure sensors for flexible and wearable electronics. Additionally, the figure showcases hybrid sensor platforms where CNTs are combined with other nanomaterials to enhance sensitivity and specificity. This visual encapsulates the versatility and transformative impact of CNTs in advancing real-time, miniaturized, and highly sensitive sensor systems.

Objectives of the Review

Given the rapidly expanding body of literature on CNT-based sensors, this review aims to provide a comprehensive and critical overview of their role in advancing sensing technologies. The objectives of this review are:

• To provide a fundamental understanding of CNT structures, properties, and their relevance to sensing mechanisms.
• To systematically discuss the integration of CNTs in various sensor types, including gas sensors, chemical sensors, biosensors, and pressure sensors.
• To highlight recent advancements in CNT-based sensor design, fabrication techniques, and performance optimization.
• To explore current challenges and limitations hindering the commercialization and large-scale deployment of CNT-based sensors.
• To present future perspectives and emerging trends in the development of CNT-enabled sensing platforms for real-time and smart sensing applications.

Through this review, we aim to bridge the gap between fundamental CNT science and practical sensor engineering, offering insights into future research directions and potential solutions for overcoming current barriers.

2. Properties of Carbon Nanotubes (CNTs)

2.1. Structural Characteristics of CNTs

Carbon nanotubes (CNTs) are cylindrical nanostructures composed entirely of carbon atoms arranged in a hexagonal lattice, resembling a rolled-up sheet of graphene. Based on the number of graphene layers, CNTs are primarily categorized into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). SWCNTs consist of a single graphene cylinder with a diameter typically ranging from 0.4 to 2 nm, while MWCNTs are composed of multiple concentric graphene cylinders, leading to diameters between 2 and 100 nm [31]. The length of CNTs can range from a few micrometers to several millimeters, providing a high aspect ratio that is critical for sensing applications. A distinctive feature of CNTs lies in their unique chirality, which describes the angle at which the graphene sheet is rolled. Chirality significantly influences the electronic properties of CNTs, dividing them into metallic or semiconducting types [32]. This property is governed by the chiral vector (n,m), which defines the wrapping angle of the graphene sheet. When (n – m) is a multiple of 3, the CNT exhibits metallic behavior; otherwise, it demonstrates semiconducting characteristics [33]. The ability to control the chirality of CNTs is vital in designing sensors with specific electrical responses, although this remains a challenge in large-scale synthesis. Structurally, SWCNTs provide a single active surface for interaction with analytes, facilitating highly sensitive detection at the molecular level. Conversely, MWCNTs possess interlayer spaces and multiple walls that offer structural robustness and increased surface functionality [34]. The inner and outer walls of MWCNTs can interact differently with analytes, allowing for multiplexed sensing capabilities. From a morphological perspective, CNTs can exhibit defects, such as vacancies, Stone-Wales defects, or functional groups attached during synthesis or post-treatment processes. While pristine CNTs exhibit ideal electrical properties, the presence of defects often enhances their chemical reactivity and adsorption capability, which is beneficial for sensing applications [35]. Moreover, the structural flexibility and mechanical integrity of CNTs allow them to form networks or composite materials that can be integrated into various sensing platforms. Techniques like chemical vapor deposition (CVD), arc discharge, and laser ablation are commonly employed for synthesizing CNTs with controlled structural features [36]. The optimization of synthesis conditions directly affects their purity, aspect ratio, and defect density, ultimately influencing their sensing performance. CNTs can also be assembled into different configurations, including vertically aligned CNT arrays, random networks, and horizontally aligned structures, depending on the targeted application. Vertically aligned CNTs offer uniform and high-density active sites, making them suitable for biosensors and chemical sensors where direct analyte contact is required [37]. So, the structural characteristics of CNTs, including their chirality, aspect ratio, defect density, and configuration, play a vital role in determining their suitability for various sensing applications. Advanced synthesis techniques and post-functionalization approaches continue to evolve to tailor these structural attributes for specific sensor designs.

Figure 2 provides a comparative visualization of the structural configurations and microscopic features of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), which are fundamental to understanding their functional behavior in sensing applications. SWCNTs consist of a single graphene sheet seamlessly rolled into a cylindrical tube, typically 0.4–2 nm in diameter, leading to a high surface area and quantum confinement effects that significantly enhance their electronic sensitivity [38]. In contrast, MWCNTs are composed of multiple concentric graphene cylinders, resulting in diameters that can exceed 100 nm. This multi-layered configuration imparts superior mechanical durability and higher chemical stability, making MWCNTs more suitable for robust and long-term sensing under harsh conditions. The accompanying transmission electron microscopy (TEM) images in parts C and D of the figure validate the structural distinctions, revealing the uniform, single-layered morphology of SWCNTs versus the nested architecture of MWCNTs. Furthermore, the figure illustrates the sp²-hybridized bonding in MWCNTs, where σ bonds provide structural integrity and π bonds facilitate delocalized electron mobility, critical for electrical conduction. These structural and electronic differences directly influence the nanotubes’ interaction with analytes, signal transduction efficiency, and suitability for specific sensor types. Thus, the figure underscores the intrinsic structure–property relationships that guide the rational selection of SWCNTs or MWCNTs in designing high-performance CNT-based sensors.

2.2. Electrical and Mechanical Properties of CNTs

Carbon nanotubes (CNTs) possess extraordinary electrical and mechanical properties, which have positioned them as one of the most promising materials in the field of sensing technologies. The unique one-dimensional (1D) structure of CNTs and their sp²-hybridized carbon–carbon bonding configuration endow them with exceptional performance attributes that surpass many conventional nanomaterials [39].

2.2.1. Electrical Properties of CNTs

The electrical behavior of CNTs is inherently dependent on their chirality (n,m), diameter, and electronic band structure. Specifically, CNTs can exhibit either metallic or semiconducting properties based on the manner in which the graphene sheet is rolled to form the nanotube [40]. Metallic CNTs possess zero bandgap, allowing them to conduct electricity with minimal resistance, while semiconducting CNTs display a bandgap typically ranging from 0.5 eV to 1 eV, depending on their diameter and chirality [41]. This unique tunability of electrical properties makes CNTs highly adaptable for different sensor types. Metallic CNTs are preferred for applications requiring fast electron transport and conductivity, such as electrochemical sensors, while semiconducting CNTs are crucial for field-effect transistor (FET)-based sensors, where changes in local charge density due to analyte binding modulate their conductivity [42]. The ballistic transport property of electrons within CNTs further enhances their electrical performance. In metallic SWCNTs, electrons can traverse along the length of the tube with negligible scattering, enabling extremely high current densities exceeding 10⁹ A/cm², much higher than that of copper or other metals [43]. This property is particularly advantageous for constructing highly sensitive, low-power sensors with rapid response times. Moreover, the electrical conductivity of CNT networks can be influenced by various factors such as tube-to-tube junction resistance, contact resistance with electrodes, and the presence of defects or functional groups [44]. Defect sites or functionalization, while often reducing intrinsic conductivity, can provide reactive sites for specific analyte binding, facilitating signal generation in chemical and biosensing applications. In gas sensing, for instance, the adsorption of gas molecules onto CNT surfaces can induce charge transfer or modulation of local electronic states, leading to detectable changes in conductivity or resistance [45]. This mechanism enables the development of highly sensitive gas sensors capable of detecting trace concentrations of toxic gases like NO₂, NH₃, and CO.

2.2.2. Mechanical Properties of CNTs

In addition to their outstanding electrical characteristics, CNTs exhibit exceptional mechanical strength, making them highly durable and suitable for flexible and wearable sensing platforms. Young’s modulus of SWCNTs is reported to be approximately 1 TPa, comparable to or exceeding that of diamond, while their tensile strength can reach values up to 100 GPa [46]. These remarkable mechanical properties originate from the strong sp² covalent bonding between carbon atoms within the hexagonal graphene lattice structure of CNTs [47]. The hollow cylindrical geometry of CNTs enables them to sustain large strains without structural failure, allowing for their integration into stretchable, bendable, and foldable sensors that maintain performance even under mechanical deformation. MWCNTs, due to their multiple concentric layers, exhibit enhanced resistance to mechanical damage and fracture, making them preferable for applications where mechanical stability is crucial, such as pressure sensors or strain gauges [48]. Their multi-layered structure also provides a mechanism for energy dissipation, increasing their fatigue resistance and operational lifetime. Additionally, the high aspect ratio of CNTs (length-to-diameter ratio) facilitates the formation of percolating networks in composite materials, allowing them to maintain mechanical integrity while providing electrical conductivity [49]. Such composite systems have been utilized in developing flexible piezoresistive sensors, where the electrical resistance changes in response to mechanical deformation, enabling detection of pressure, strain, or touch stimuli [22,50,51,52].

2.2.3. Synergistic Role of Electrical and Mechanical Properties in Sensing

The integration of superior electrical and mechanical properties within CNTs provides a synergistic advantage for sensor development. For example, CNT-based flexible sensors can simultaneously maintain high sensitivity, fast response, and mechanical resilience, enabling their deployment in wearable electronics, smart textiles, and human health monitoring devices [53,54,55,56]. In pressure sensors, CNT films or composites exhibit a piezoresistive effect, where external mechanical stress alters the contact resistance between CNTs, generating a measurable signal proportional to the applied pressure [57]. Similarly, in strain sensors, the stretching or bending of a CNT-based device modifies its conductive pathways, allowing precise monitoring of mechanical deformation. The robustness of CNTs also allows for their application in harsh environmental conditions, including high temperature, humidity, or corrosive environments, where conventional sensing materials might fail [58]. Their chemical stability and resistance to degradation further extend their applicability in long-term monitoring applications, such as structural health monitoring of bridges, aerospace components, or biomedical implants. Furthermore, the combination of electrical conductivity and mechanical flexibility facilitates the development of multi-functional sensors capable of detecting multiple parameters simultaneously, such as pressure, temperature, and chemical analytes, within a single CNT-based platform [59].

Figure 3a illustrates the synergistic integration of electrical conductivity and mechanical flexibility in carbon nanotube (CNT)-based sensors, which enables their application across various advanced sensing platforms. The schematic demonstrates how CNTs respond to mechanical stimuli such as pressure and strain through piezoresistive and conductive pathway modulation mechanisms, converting mechanical deformations into electrical signals. It also highlights their application in flexible electronics and wearable devices, where durability under bending, stretching, or compression is critical. Moreover, the figure showcases their utility in multi-functional sensing systems capable of detecting multiple physical and chemical parameters simultaneously. The ability of CNTs to maintain performance under harsh conditions, including high temperatures and corrosive environments, further underscores their robustness and long-term reliability in structural health monitoring and biomedical diagnostics.

Figure 3b,c provide a comprehensive depiction of the resistance behavior and sensing mechanisms in CNT-based composites as the concentration of carbon nanotubes varies [60]. In Figure 3b, the resistance model illustrates two primary components influencing the electrical conductivity: contact resistance, which arises at the physical junctions between adjacent CNTs, and tunneling resistance, which is due to electrons hopping across small insulating gaps between nearby but non-touching nanotubes. The interplay of these resistive elements determines the overall sensor response under mechanical deformation. Figure 3c elaborates on how these mechanisms evolve with different CNT loadings. At low CNT concentrations, the network is sparse, and the electrical conductivity is primarily governed by tunneling effects due to limited contact points. As CNT content increases, more percolative pathways form, lowering resistance and enhancing signal stability. Eventually, at high CNT loadings, the network reaches saturation, where additional CNTs contribute minimally to conductivity, and the sensor exhibits a more stable and linear response. This progression highlights the critical role of nanotube distribution and density in optimizing the electrical performance and sensitivity of CNT-based strain sensors.

2.2.4. Influence of Defects and Functionalization on Properties

While pristine CNTs exhibit optimal electrical and mechanical properties, real-world applications often necessitate functionalization to enhance their sensitivity and selectivity towards specific analytes. Covalent functionalization involves the attachment of chemical groups to the CNT surface, which can disrupt the sp² bonding network and slightly reduce conductivity but provides enhanced chemical reactivity [61]. Non-covalent functionalization, such as π–π stacking interactions, preserves the intrinsic electrical properties while imparting specificity through the adsorption of recognition elements like polymers, biomolecules, or metal nanoparticles [62]. These strategies enable the tailoring of CNT properties to suit particular sensing requirements without compromising their mechanical stability. It is evident that the exceptional electrical and mechanical properties of CNTs make them highly attractive for the development of next-generation sensors. Their tunable conductivity, high current-carrying capacity, mechanical robustness, and flexibility enable the design of highly sensitive, durable, and multi-functional sensing devices. Future research efforts are focused on optimizing synthesis techniques, controlling defect density, and developing advanced functionalization methods to further harness the full potential of CNTs in diverse sensing applications.

2.3. Surface Area and Chemical Reactivity of Carbon Nanotubes (CNTs)

The high surface area and chemical reactivity of carbon nanotubes (CNTs) represent pivotal characteristics that significantly contribute to their superior performance in sensing technologies. These properties enable CNTs to interact effectively with a wide range of chemical and biological species, making them ideal candidates for sensor development, especially in applications requiring high sensitivity, selectivity, and rapid response times [63]. CNTs, due to their nanoscale dimensions and large surface-to-volume ratio, provide abundant active sites for molecular adsorption and chemical interaction. This property is crucial for the transduction mechanism in chemical sensors, gas sensors, and biosensors, where the detection signal is generated from surface interactions between the analyte and the sensing material [64]. Furthermore, the intrinsic chemical inertness of pristine CNTs, along with the possibility of surface modification, allows for tunable chemical reactivity, enabling the detection of specific target molecules in complex environments [65]. The surface area of CNTs is among the highest recorded for any nanomaterial, reaching values as high as 1000 m²/g for single-walled carbon nanotubes (SWCNTs) and 400–800 m²/g for multi-walled carbon nanotubes (MWCNTs), depending on their synthesis method and purification process [66]. This ultra-high surface area is primarily attributed to their hollow cylindrical morphology, high aspect ratio, and ability to form porous networks in bulk materials. Such a vast surface area plays a critical role in sensing applications, as it provides a large number of adsorption sites for gas molecules, chemical species, or biomolecules. This is particularly advantageous in gas sensors, where the interaction between the CNT surface and gas molecules induces changes in electrical conductivity or resistance that can be readily measured [67]. Moreover, the high surface area facilitates rapid diffusion of analyte molecules to the active sites, enabling fast sensor response times. In biosensors, the large surface of CNTs provides an ideal platform for the immobilization of enzymes, antibodies, or DNA strands, enhancing the sensitivity and stability of the biosensing device [68]. Pristine CNTs are composed of sp²-hybridized carbon atoms arranged in a hexagonal lattice, similar to graphene. This configuration renders them chemically inert under ambient conditions. However, the chemical reactivity of CNTs can be significantly enhanced through the introduction of structural defects, functional groups, or through covalent and non-covalent surface modification techniques [69]. Defects in CNTs, such as vacancies, Stone-Wales defects, or open tube ends, provide chemically active sites where reactions can readily occur [70]. Moreover, CNTs produced by chemical vapor deposition (CVD) or arc-discharge methods often contain a variety of defect sites, which can enhance their chemical reactivity and facilitate their application in sensing. Chemical functionalization methods are commonly employed to modify the CNT surface to improve solubility, dispersion, or to impart specific chemical affinity towards target analytes [71]. Functionalization strategies can be broadly categorized into covalent and non-covalent approaches. Covalent functionalization involves the direct attachment of chemical groups to the carbon atoms of the CNT lattice. This process usually requires oxidative treatments using strong acids (e.g., HNO₃ and H₂SO₄) to introduce carboxyl (-COOH), hydroxyl (-OH), or carbonyl (-C=O) groups onto the CNT surface [72]. Such functional groups enhance the chemical reactivity of CNTs, enabling the binding of biomolecules, metal nanoparticles, or polymers, thus tailoring the CNT surface for specific sensing applications. However, covalent modification may disrupt the π-conjugated system of CNTs, leading to a decrease in their electrical conductivity, which needs to be carefully controlled depending on the sensing application [73]. Non-covalent functionalization methods preserve the intrinsic electrical properties of CNTs while modifying their surface chemistry. These methods rely on π–π interactions, van der Waals forces, or electrostatic interactions between CNTs and functional molecules, such as polymers, surfactants, or biomolecules [62]. Non-covalent approaches are highly advantageous for biosensors, as they allow the immobilization of sensitive biological molecules without altering their functional activity or the electrical properties of the CNT substrate [74]. The synergistic combination of high surface area and tunable chemical reactivity enables CNTs to detect a wide range of analytes, from toxic gases and heavy metals to biological macromolecules. In gas sensing, the adsorption of gas molecules on the CNT surface induces charge transfer or dipole interactions, leading to measurable changes in electrical resistance or capacitance [75]. For instance, CNT-based sensors have demonstrated remarkable sensitivity towards gases like NO₂, NH₃, CO₂, and volatile organic compounds (VOCs), often detecting concentrations as low as parts-per-billion (ppb) [76]. The high surface area ensures abundant adsorption sites, while functionalization enhances the selectivity towards specific gases. In biosensing applications, functionalized CNTs provide a versatile platform for the detection of glucose, cholesterol, DNA sequences, and cancer biomarkers. The large surface facilitates the immobilization of bioreceptors, while their chemical reactivity allows for specific binding interactions essential for selective sensing [77]. Moreover, the high surface area enables the development of CNT-based electrochemical sensors with low detection limits and fast response times, essential for real-time monitoring in medical diagnostics, environmental monitoring, and food safety [37]. Despite their excellent properties, the utilization of CNTs in commercial sensors faces several challenges. Achieving controlled and reproducible functionalization remains a critical issue, as over-functionalization can degrade the electrical properties of CNTs [78]. Additionally, the aggregation of CNTs due to van der Waals interactions can reduce their effective surface area and hinder their dispersion in sensing matrices. Therefore, strategies to prevent CNT bundling, such as polymer wrapping or surfactant-assisted dispersion, are actively being explored [79]. Emerging techniques such as plasma functionalization, click chemistry, and molecular imprinting offer promising approaches to fine-tune the surface reactivity of CNTs without compromising their electrical performance [80]. Furthermore, the development of hybrid nanomaterials combining CNTs with metal nanoparticles, graphene, or metal–organic frameworks (MOFs) can further enhance their surface area and chemical reactivity for advanced sensing applications [81]. The high surface area and versatile chemical reactivity of CNTs form the cornerstone of their application in advanced sensing technologies. These properties enable the design of highly sensitive, selective, and rapid-response sensors for diverse applications in environmental monitoring, medical diagnostics, and industrial safety. Future research should focus on optimizing functionalization strategies, improving dispersion methods, and exploring novel CNT-based hybrid materials to unlock their full potential in next-generation sensor platforms.

2.4. Functionalization of Carbon Nanotubes (CNTs) for Sensing Applications

Functionalization of carbon nanotubes (CNTs) is a critical strategy for enhancing their performance in sensing technologies. While pristine CNTs possess outstanding physical and chemical properties, including high electrical conductivity, mechanical strength, and large surface area, their inherent chemical inertness limits their direct interaction with target analytes in many sensing scenarios. Functionalization allows tailoring the surface of CNTs to improve their solubility, dispersion, biocompatibility, and chemical affinity, making them highly versatile materials for various sensor platforms [82]. This section provides a comprehensive discussion of functionalization techniques of CNTs, their classifications, mechanisms, and their roles in advancing sensing technologies. The functionalization of CNTs is broadly categorized into two main approaches: covalent and non-covalent functionalization. Each method offers distinct advantages and challenges based on the desired application in sensing systems [83]. Covalent functionalization involves the chemical modification of the CNT sidewalls or ends through the formation of stable chemical bonds between the CNT and functional groups. Typically, this process requires the generation of reactive sites on the CNT surface, often introduced via oxidation treatments using strong acids such as nitric acid (HNO₃) or sulfuric acid (H₂SO₄) [84]. The most common functional groups introduced include carboxyl (-COOH), hydroxyl (-OH), and amine (-NH₂) groups. These reactive sites enable the attachment of various molecules, including polymers, nanoparticles, and biomolecules, facilitating selective detection in sensor applications [85]. Covalent functionalization provides strong, stable attachment of functional groups, enhancing sensor stability and reproducibility. However, it may disrupt the π-conjugated system of CNTs, leading to reduced electrical conductivity, which can affect sensor performance if not carefully controlled [86]. Non-covalent functionalization preserves the intrinsic electronic properties of CNTs while modifying their surface chemistry. This method involves physical adsorption or wrapping of functional molecules around CNTs through van der Waals forces, π–π stacking, hydrophobic interactions, or electrostatic interactions [87]. Non-covalent methods are particularly useful in biosensors, where the functionalization process should not interfere with the biological activity of immobilized biomolecules. Molecules such as surfactants, polymers, aromatic compounds, or DNA strands can non-covalently interact with CNTs to enhance solubility and improve interaction with target analytes [88].

In addition to conventional covalent and non-covalent methods, advanced functionalization strategies have been developed to further enhance the sensing capabilities of CNTs. Polymers, such as polyaniline (PANI), polypyrrole (PPy), or polyethylene glycol (PEG), are frequently used to functionalize CNTs due to their excellent film-forming abilities, chemical stability, and ability to introduce selective binding sites for specific analytes [89]. For example, PANI-functionalized CNTs have shown improved sensitivity and selectivity towards ammonia (NH₃) detection due to enhanced charge transfer interactions between the analyte and the sensing layer [90]. Decorating CNTs with metal nanoparticles, such as gold (Au), platinum (Pt), palladium (Pd), or silver (Ag), enhances their sensing performance, particularly for gas sensors and biosensors. Metal nanoparticles act as catalytic centers, improving analyte adsorption, electron transfer kinetics, and sensitivity [91]. Au-decorated CNTs have been successfully employed in electrochemical biosensors for glucose and DNA detection, offering high sensitivity and fast response times [92]. Supramolecular chemistry offers a non-covalent approach to functionalize CNTs using host–guest interactions. Cyclodextrins, calixarenes, or crown ethers can form inclusion complexes with CNTs, enabling the selective detection of small molecules or ions [93]. This technique allows for the development of highly selective sensors capable of discriminating between structurally similar molecules, a key requirement in complex sensing environments [94]. Gas sensors based on functionalized CNTs demonstrate excellent performance in detecting toxic gases, volatile organic compounds (VOCs), and environmental pollutants. Covalent or non-covalent functionalization introduces specific binding sites that enhance the interaction between gas molecules and the CNT surface [95]. For instance, carboxylated CNTs exhibit enhanced sensitivity towards nitrogen dioxide (NO₂) due to increased charge transfer during gas adsorption. Similarly, functionalization with polymers such as polyethyleneimine (PEI) improves the selectivity of CNT-based sensors towards carbon dioxide (CO₂) [96]. In biosensing, functionalization of CNTs is essential for immobilizing biomolecules such as enzymes, antibodies, aptamers, or DNA probes. Covalent attachment ensures strong and stable binding, while non-covalent approaches preserve biomolecular activity [97]. Functionalized CNTs have been widely used in glucose sensors, cancer biomarker detection, and DNA hybridization assays. For example, glucose oxidase (GOx) immobilized on carboxylated CNTs provides a sensitive platform for glucose detection in medical diagnostics [98]. In DNA sensors, the functionalization of CNTs with complementary DNA strands enables highly specific hybridization events, resulting in measurable changes in electrical or electrochemical signals [99]. Functionalized CNTs are employed in chemical sensors for detecting heavy metals, pesticides, explosives, and toxic chemicals. The introduction of selective chelating groups or molecular recognition elements on the CNT surface enhances their selectivity towards target chemicals [100]. For example, thiol-functionalized CNTs exhibit strong binding affinity towards mercury (Hg²⁺) ions, enabling sensitive detection in environmental monitoring applications [101]. Despite significant advancements, challenges remain in the functionalization of CNTs for sensing applications. Achieving controlled and uniform functionalization without compromising the structural integrity or electrical properties of CNTs is a major concern [102]. Moreover, the scalability of functionalization processes for commercial sensor fabrication needs further optimization. Developing environmentally friendly and cost-effective functionalization methods is critical for the large-scale deployment of CNT-based sensors [103]. Future research should focus on exploring novel functional molecules, hybrid materials, and green chemistry approaches for CNT functionalization. Integration of CNT-based sensors with flexible electronics, wearable devices, and Internet of Things (IoT) platforms offers exciting opportunities for real-time monitoring and smart sensing systems [81]. Functionalization of CNTs is a transformative approach for tailoring their properties to meet the diverse requirements of modern sensing technologies. By enabling enhanced sensitivity, selectivity, and stability, functionalized CNTs serve as a powerful material platform for developing next-generation sensors in environmental, biomedical, and industrial applications.

Table 2. Comparative summary of Functionalization Strategies of Carbon Nanotubes (CNTs) for Sensing Applications.

Functionalization Method	Mechanism	Advantages	Limitations	Typical Applications	References
Covalent Functionalization	Formation of chemical bonds (e.g., carboxyl, amine groups) on CNT surface	Strong and stable attachment, improved dispersion, enhanced sensitivity	Disruption of CNT π-conjugation, reduced conductivity	Gas sensors, biosensors, chemical sensors	[104,105,106]
Non-Covalent Functionalization	Physical adsorption via π–π stacking, van der Waals, hydrophobic interactions	Preserves electrical properties, maintains structural integrity, easy processing	Relatively weaker attachment, potential desorption under harsh conditions	Biosensors, flexible and wearable sensors	[107]
Polymer Functionalization	Wrapping or grafting of conductive or selective polymers on CNTs	Enhanced selectivity, improved analyte interaction, tunable properties	Possible decrease in conductivity, complex synthesis	Chemical sensors, environmental sensors, gas sensors	[108,109]
Metal Nanoparticle Decoration	Decoration of CNT surface with metal nanoparticles (Au, Pt, Pd, Ag)	Improved catalytic activity, increased sensitivity, enhanced electron transfer	Aggregation of nanoparticles, cost of noble metals	Electrochemical biosensors, gas sensors, glucose sensors	[110]
Supramolecular Functionalization	Host–guest chemistry using cyclodextrins, calixarenes, crown ethers	High selectivity, reversible interactions, minimal damage to CNT structure	Selectivity limited to specific analytes, complex synthesis	Chemical sensors, ion detection, small molecule sensing	[111]

Table 2. Comparative summary of Functionalization Strategies of Carbon Nanotubes (CNTs) for Sensing Applications.

Functionalization Method	Mechanism	Advantages	Limitations	Typical Applications	References
Covalent Functionalization	Formation of chemical bonds (e.g., carboxyl, amine groups) on CNT surface	Strong and stable attachment, improved dispersion, enhanced sensitivity	Disruption of CNT π-conjugation, reduced conductivity	Gas sensors, biosensors, chemical sensors	[104,105,106]
Non-Covalent Functionalization	Physical adsorption via π–π stacking, van der Waals, hydrophobic interactions	Preserves electrical properties, maintains structural integrity, easy processing	Relatively weaker attachment, potential desorption under harsh conditions	Biosensors, flexible and wearable sensors	[107]
Polymer Functionalization	Wrapping or grafting of conductive or selective polymers on CNTs	Enhanced selectivity, improved analyte interaction, tunable properties	Possible decrease in conductivity, complex synthesis	Chemical sensors, environmental sensors, gas sensors	[108,109]
Metal Nanoparticle Decoration	Decoration of CNT surface with metal nanoparticles (Au, Pt, Pd, Ag)	Improved catalytic activity, increased sensitivity, enhanced electron transfer	Aggregation of nanoparticles, cost of noble metals	Electrochemical biosensors, gas sensors, glucose sensors	[110]
Supramolecular Functionalization	Host–guest chemistry using cyclodextrins, calixarenes, crown ethers	High selectivity, reversible interactions, minimal damage to CNT structure	Selectivity limited to specific analytes, complex synthesis	Chemical sensors, ion detection, small molecule sensing	[111]

Table 2 presents a comparative analysis of the most prominent functionalization strategies employed to enhance the sensing capabilities of carbon nanotubes (CNTs). Covalent functionalization offers robust chemical stability due to the formation of strong chemical bonds with the CNT structure; however, this method may compromise the electrical properties of CNTs due to partial disruption of their π-conjugated network [112]. In contrast, non-covalent functionalization maintains the intrinsic conductivity of CNTs while facilitating facile surface modification, which is highly desirable for biosensing and flexible sensor applications [113]. Polymer functionalization introduces selective chemical environments and amplifies analyte interaction, proving useful in environmental and chemical sensing, though it may introduce additional resistance to the CNT network [114]. Decoration with metal nanoparticles significantly enhances the catalytic activity and sensing sensitivity of CNTs, especially for gas and electrochemical sensors, but challenges like nanoparticle aggregation and high costs must be addressed [115]. Supramolecular functionalization, leveraging host–guest chemistry, provides reversible and highly selective binding sites for small molecules or ions, though its applicability is often limited to specific targets [116]. Overall, the choice of functionalization strategy is closely dictated by the targeted sensing application, desired sensitivity, selectivity, and operational stability.

3. Types of Carbon Nanotubes and Their Applications

3.1. Single-Walled Carbon Nanotubes (SWCNTs)

Single-walled carbon nanotubes (SWCNTs) are a unique class of carbon nanomaterials consisting of a single graphene sheet rolled into a seamless cylindrical structure with diameters typically ranging from 0.4 nm to 3 nm and lengths extending up to several micrometers or even millimeters [117]. The atomic structure of SWCNTs is governed by their chirality, which determines their electronic properties whether metallic or semiconducting making them particularly attractive for a wide range of sensing applications [118]. The exceptional characteristics of SWCNTs, such as high aspect ratio, excellent electrical conductivity, chemical stability, and mechanical robustness, position them at the forefront of nanotechnology research for sensor development [119]. SWCNTs possess remarkable electronic characteristics due to the quantum confinement of electrons along their tubular structure. Depending on the chiral vector (n,m) of the rolled graphene sheet, SWCNTs can exhibit metallic or semiconducting behavior, influencing their suitability for different types of sensors, including electrochemical, chemical, and biological sensors [120]. The high surface-to-volume ratio of SWCNTs enhances their sensitivity, as the majority of their atoms are exposed on the surface, making them highly responsive to changes in their environment [121]. Moreover, SWCNTs exhibit ballistic electron transport, particularly in metallic tubes, resulting in minimal scattering and ultra-high electron mobility [122]. This property is vital for developing fast-response sensors with low detection limits. Their one-dimensional structure facilitates efficient charge transfer between the target analyte and the nanotube surface, an essential mechanism in electrochemical and field-effect transistor (FET)-based sensing platforms [123]. The synthesis technique plays a critical role in determining the purity, chirality, and defect density of SWCNTs, all of which directly impact their sensing performance. Common synthesis methods include chemical vapor deposition (CVD), laser ablation, and arc discharge techniques [124]. CVD is particularly favored for sensor applications due to its scalability, relatively low cost, and the ability to grow vertically aligned SWCNT arrays that provide uniform sensor architectures [125]. Post-synthesis purification techniques are essential to remove metallic catalysts and amorphous carbon that can interfere with sensing performance [126]. Pristine SWCNTs exhibit hydrophobicity and chemical inertness, which can limit their interaction with target analytes. To overcome this limitation, functionalization strategies—both covalent and non-covalent—are employed to modify the SWCNT surface, improving selectivity and sensitivity towards specific molecules [127]. Covalent functionalization involves attaching functional groups directly to the carbon framework, which can introduce defects but enhances chemical reactivity. Non-covalent functionalization preserves the electronic properties of SWCNTs and involves π–π stacking or van der Waals interactions with polymers, surfactants, or biomolecules [128]. For instance, SWCNTs functionalized with carboxyl, amine, or hydroxyl groups have been utilized for detecting heavy metals, gases, and biological targets due to their ability to form specific interactions with these analytes [129]. Additionally, the immobilization of enzymes, antibodies, or DNA aptamers onto SWCNTs has enabled the development of highly selective biosensors for medical diagnostics [130].

Applications of SWCNTs in Sensing Technologies: The unique properties of SWCNTs have been harnessed across various sensing platforms, including:

Gas Sensors: SWCNT-based gas sensors exhibit rapid response times, low detection limits, and the ability to operate at room temperature. They have been used for detecting gases such as NH₃, NO₂, CO, H₂, and volatile organic compounds (VOCs) [131]. Functionalization with metal nanoparticles (e.g., Pt, Pd, and Au) enhances their sensitivity and selectivity towards specific gases by providing catalytic sites for adsorption [132].

Chemical Sensors: SWCNTs have demonstrated excellent capabilities in detecting chemical species such as glucose, hydrogen peroxide, and neurotransmitters. Their integration into electrochemical sensors allows for sensitive detection based on redox reactions, with significant applications in environmental monitoring and food safety [133].

Biosensors: SWCNTs provide a versatile platform for biosensors, enabling the detection of biomolecules such as DNA, proteins, glucose, and pathogenic microorganisms [134]. The high electrical conductivity of SWCNTs facilitates the transduction of biological interactions into measurable electrical signals, critical for point-of-care diagnostic devices [135].

Pressure and Strain Sensors: Due to their mechanical flexibility and high tensile strength, SWCNTs are employed in flexible and wearable sensors capable of monitoring strain, pressure, and tactile stimuli. These sensors are valuable in healthcare monitoring systems and wearable electronics [136]. Despite their promising properties, several challenges hinder the widespread commercialization of SWCNT-based sensors. These include issues related to the scalability of production, control over chirality and purity, reproducibility of sensor fabrication, and integration with existing electronic systems [137]. Future research is focused on developing cost-effective synthesis methods, advanced functionalization techniques, and hybrid nanocomposites to enhance the performance and reliability of SWCNT-based sensors [138]. Ultimately, SWCNTs represent a significant advancement in the field of sensing technologies. Their exceptional structural, electrical, and chemical properties provide a versatile platform for developing highly sensitive, selective, and miniaturized sensors for a broad range of applications. Continued research and technological innovations are expected to overcome the existing limitations, paving the way for the large-scale commercialization of SWCNT-based sensing devices [139].

3.2. Multi-Walled Carbon Nanotubes (MWCNTs)

Multi-walled carbon nanotubes (MWCNTs) have emerged as one of the most versatile and widely explored nanomaterials in the field of sensing technologies due to their unique structural configurations, superior physicochemical properties, and ease of functionalization. MWCNTs consist of multiple concentric layers of rolled graphene sheets, with an interlayer distance of approximately 0.34 nm, resembling a nested cylindrical structure similar to Russian dolls [140]. This multi-layered configuration distinguishes MWCNTs from single-walled carbon nanotubes (SWCNTs), endowing them with remarkable mechanical strength, chemical stability, and tunable electronic characteristics suitable for diverse sensing platforms. The typical diameter of MWCNTs ranges from 2 nm to over 100 nm, while their lengths can extend to several micrometers depending on the synthesis method employed [141]. The multiple graphene walls not only increase the mechanical robustness of MWCNTs but also provide a larger surface area, enabling enhanced interaction with target analytes during sensing processes. Their graphitic walls possess both metallic and semiconducting behaviors, which can be exploited in fabricating various types of sensors, including electrochemical, gas, biosensors, and strain sensors [142,143,144,145]. The synthesis techniques for MWCNTs primarily include chemical vapor deposition (CVD), arc discharge, and laser ablation, among which CVD is the most popular due to its cost-effectiveness, scalability, and control over nanotube growth parameters [146]. The structural integrity, purity, and defect density of MWCNTs significantly influence their electrical and sensing performance, thus necessitating post-synthesis purification and functionalization treatments. MWCNTs possess superior electrical conductivity attributed to their delocalized π-electron cloud along the graphene walls, which facilitates rapid charge transfer processes critical for sensing applications [147]. Additionally, the presence of multiple conducting channels in MWCNTs improves their resistance to structural defects compared to SWCNTs, making them more reliable for practical sensor devices [148]. Their mechanical properties are equally impressive, with tensile strengths reaching up to 150 GPa and Young’s modulus values ranging between 1 TPa and 1.8 TPa, depending on the degree of alignment and structural perfection [149]. These properties make MWCNTs ideal for strain, pressure, and flexible wearable sensors. To improve their dispersibility in solvents, enhance biocompatibility, and increase selectivity towards specific analytes, MWCNTs often undergo various functionalization strategies. Covalent functionalization involves the introduction of chemical groups such as carboxyl (-COOH), hydroxyl (-OH), or amine (-NH2) groups onto the surface of MWCNTs through oxidation or chemical reactions [150]. While covalent modification can alter the intrinsic conductivity of MWCNTs, it enables strong binding with biomolecules or polymers, facilitating the fabrication of biosensors. Non-covalent functionalization, on the other hand, preserves the electrical conductivity of MWCNTs by employing π–π stacking interactions, van der Waals forces, or electrostatic interactions to attach sensing molecules, polymers, or nanoparticles onto their surface [151]. This approach is widely used in constructing gas sensors, where MWCNTs are coated with metal oxides or conducting polymers to improve selectivity and sensitivity. MWCNTs have been extensively explored in a wide array of sensing applications due to their outstanding electronic and surface characteristics. In gas sensing, MWCNT-based sensors have demonstrated high sensitivity and rapid response times towards gases such as NH₃, NO₂, H₂S, and CO₂ [152]. Their ability to interact with gas molecules through charge transfer or adsorption-induced conductivity changes is the fundamental mechanism driving their gas sensing behavior. Electrochemical sensors utilizing MWCNT-modified electrodes exhibit enhanced electron transfer rates, making them highly suitable for detecting biomolecules like glucose, dopamine, cholesterol, and heavy metal ions [153]. Moreover, MWCNT-based biosensors have been developed for the detection of DNA, proteins, pathogens, and cancer biomarkers due to their high surface area, biocompatibility, and capability for immobilizing biomolecules [154]. Pressure and strain sensors incorporating MWCNTs benefit from their exceptional mechanical flexibility and piezoresistive properties, which allow for real-time monitoring of physiological signals such as heartbeat, respiration, and body movement in wearable health monitoring systems [155]. Despite their tremendous potential, the practical deployment of MWCNT-based sensors faces several challenges, including aggregation in solution, batch-to-batch variation during synthesis, and toxicity concerns for biological applications [156]. Addressing these challenges through optimized functionalization methods, controlled synthesis protocols, and comprehensive toxicity assessments will pave the way for the commercialization of MWCNT-based sensing devices. Furthermore, the integration of MWCNTs with advanced technologies such as flexible electronics, Internet of Things (IoT), and artificial intelligence (AI)-based data analysis offers exciting opportunities for developing smart sensing systems capable of real-time monitoring and remote diagnostics. The continued advancement in the scalable production of high-purity, defect-free MWCNTs will further facilitate their transition from laboratory research to industrial-scale sensor applications.

Applications of MWCNTs in Sensing Technologies

Multi-walled carbon nanotubes (MWCNTs), characterized by their concentric cylindrical structure of multiple graphene layers, have emerged as vital components in the development of high-performance sensors. The robust mechanical integrity, enhanced thermal and chemical stability, and high electrical conductivity of MWCNTs render them highly adaptable for diverse sensing applications. In comparison to single-walled carbon nanotubes (SWCNTs), MWCNTs exhibit a larger diameter and a more complex structure, which allow for increased surface functionalization and higher defect density features advantageous for the anchoring of functional groups or recognition elements in sensor platforms [157].

In gas sensing, MWCNTs have demonstrated remarkable sensitivity and selectivity towards various gaseous analytes such as ammonia (NH₃), nitrogen dioxide (NO₂), carbon monoxide (CO), and volatile organic compounds (VOCs). Their tubular architecture offers an expansive surface area and multiple active sites, which facilitate adsorption-based interactions. For instance, MWCNT-based sensors functionalized with metal oxide nanoparticles such as SnO₂ or ZnO have shown enhanced gas adsorption and response characteristics due to synergistic effects at the heterointerface [158,159]. Moreover, their multi-layered structure contributes to better charge transport and recovery behavior, essential for real-time and repeatable gas detection.

In chemical sensing, MWCNTs are increasingly utilized for the detection of hazardous and environmentally relevant chemicals. Their compatibility with a wide array of chemical functionalization strategies such as acid treatments, amine or carboxyl group modification, and polymer coatings allows for tailored interaction with specific analytes [160]. This enhances the chemical affinity and sensor response towards pollutants like formaldehyde, heavy metals, and phenolic compounds. MWCNTs modified with conductive polymers such as polyaniline or polypyrrole have also been employed to create hybrid chemical sensors with improved sensitivity, reproducibility, and linearity over a broad concentration range.

The role of MWCNTs in biosensing is equally significant. Their large surface area, high electron mobility, and biocompatibility make them suitable for the immobilization of biomolecules such as enzymes, antibodies, and nucleic acids. Biosensors based on MWCNTs have shown excellent performance in detecting biomolecules including glucose, dopamine, DNA sequences, and various disease biomarkers. For example, glucose sensors utilizing MWCNTs functionalized with glucose oxidase demonstrate enhanced electron transfer and lower detection limits due to the proximity of the enzyme active site to the conductive surface. Furthermore, antibody-functionalized MWCNTs have enabled sensitive and selective immunosensors for early disease diagnostics. MWCNTs have also been extensively explored for pressure and strain sensing applications. Their piezoresistive behavior i.e., changes in resistance under mechanical deformation is harnessed in flexible, wearable, and structural health monitoring devices. The interlayer interactions and multiple conduction paths within MWCNTs allow for enhanced signal stability under repetitive mechanical stress. Composites of MWCNTs with elastomers or polymers such as PDMS (polydimethylsiloxane) have yielded flexible sensor films capable of detecting subtle strain variations, making them ideal for applications in prosthetics, robotics, and aerospace. Additionally, the robustness of MWCNTs makes them suitable for integration into harsh environments, where long-term durability and resistance to fatigue are critical.

Another advantage of MWCNTs in sensor development lies in their relatively simpler and scalable synthesis compared to SWCNTs. This makes them more commercially viable for industrial sensor fabrication. Furthermore, their stability against oxidation and chemical degradation ensures long-term reliability, especially in environmental and industrial monitoring settings. So the MWCNTs offer multifaceted benefits for sensing technologies across a wide range of analytes and physical stimuli. Their ability to be functionalized extensively, combined with their mechanical resilience and stable electrical properties, makes them indispensable in the design of next-generation sensors.

3.3. Comparison of Single-Walled Carbon Nanotubes (SWCNTs) and Multi-Walled Carbon Nanotubes (MWCNTs) in Sensing Applications

Carbon nanotubes (CNTs), particularly single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), exhibit distinctive physicochemical characteristics that significantly influence their performance in sensing technologies. The comparative analysis of SWCNTs and MWCNTs in sensing applications is crucial to guide the optimal selection of CNT type for specific sensor platforms, considering sensitivity, selectivity, stability, fabrication cost, and functionalization capability. SWCNTs consist of a single layer of graphene rolled into a seamless cylindrical structure with a diameter typically ranging between 0.4 and 2 nm. In contrast, MWCNTs comprise multiple concentric graphene layers rolled into tubes, with an outer diameter ranging from 2 nm to 100 nm [109]. These structural differences directly affect the surface area, defect density, and electronic properties of the nanotubes, thereby influencing their sensing capabilities. SWCNTs provide a higher surface area-to-volume ratio compared to MWCNTs, which translates into increased exposure of active sites for analyte interaction, resulting in enhanced sensitivity [161]. However, the presence of multiple layers in MWCNTs provides them with greater mechanical robustness and higher chemical stability under harsh operating conditions [162]. SWCNTs exhibit remarkable semiconducting or metallic behavior depending on their chirality and diameter, which enables them to exhibit high sensitivity in field-effect transistor (FET)-based sensors and resistive-type sensors. Their unique one-dimensional electronic structure allows significant changes in conductivity upon binding with target analytes, making SWCNTs highly suitable for detecting gases, biomolecules, and chemical vapors [163]. Conversely, MWCNTs generally display metallic behavior due to interlayer interactions and reduced bandgap effects. While their sensitivity is often lower than SWCNTs, MWCNTs exhibit better electrical stability and lower noise levels in sensor operation, making them preferable for certain electrochemical sensor applications [164]. The ability to functionalize CNTs is critical for improving sensor selectivity and specificity towards target analytes. SWCNTs, owing to their pristine surface and fewer defects, are ideal for covalent and non-covalent functionalization with chemical groups, biomolecules, and polymers [165]. This enhances their potential in biosensing and chemical detection. However, MWCNTs offer a larger surface for multi-point functionalization due to their multiple layers and inherent defect sites, allowing for the immobilization of various recognition elements, such as enzymes, antibodies, and aptamers [166]. This makes MWCNTs highly suitable for biosensors and immunosensors operating in complex biological environments. SWCNT-based sensors often exhibit ultra-high sensitivity and low detection limits, sometimes down to the single-molecule level, particularly in gas sensing and FET-based biosensing platforms [64]. For instance, SWCNT sensors have demonstrated detection limits for ammonia (NH₃) and nitrogen dioxide (NO₂) gases in the parts-per-billion (ppb) range, outperforming many MWCNT-based sensors [167]. However, MWCNTs, due to their structural robustness and stable conductivity, offer higher reproducibility and stability over long-term operation, which is advantageous in industrial monitoring applications [168]. Mechanical strength and durability are critical factors in the practical deployment of sensors. MWCNTs exhibit superior mechanical properties due to their multi-layered structure, allowing them to withstand repeated stress, bending, and chemical exposure without significant performance degradation [169]. This makes MWCNTs more suitable for flexible and wearable sensor applications where mechanical deformation is inevitable. In contrast, SWCNTs, though mechanically strong at the individual tube level, are more prone to damage during sensor fabrication and integration processes, affecting long-term reliability [170]. The large-scale production and purification of SWCNTs remain more expensive and technically challenging than MWCNTs. The high cost associated with SWCNTs is attributed to their stringent synthesis conditions, need for chirality control, and complex purification steps to remove metallic impurities [171]. On the other hand, MWCNTs are relatively easier to synthesize in bulk using chemical vapor deposition (CVD) methods and are commercially available at a lower cost, making them attractive for cost-sensitive sensing applications [172]. SWCNTs are more susceptible to environmental factors such as humidity, temperature, and oxidative degradation, which can affect their sensing performance and reproducibility [119]. MWCNTs, with their robust structure and higher chemical stability, are better suited for outdoor sensing applications and harsh environments. Moreover, the presence of multiple layers in MWCNTs offers a protective effect against environmental degradation, enhancing sensor lifespan [173]. So it is clear that the choice between SWCNTs and MWCNTs in sensing applications depends on the specific requirements of the sensor platform. Future research efforts should focus on developing hybrid CNT-based materials that combine the advantages of both SWCNTs and MWCNTs to create next-generation sensing platforms with optimized performance, stability, and cost-effectiveness.

Figure 4a illustrates the fundamental differences and complementary characteristics of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) that influence their suitability in various sensing platforms. SWCNTs, characterized by a single graphene layer structure, offer high surface area and unique electronic properties either semiconducting or metallic making them ideal for applications requiring ultra-high sensitivity, such as field-effect transistor (FET)-based gas and biosensors. However, their high cost and environmental susceptibility limit large-scale use. Conversely, MWCNTs, composed of multiple concentric graphene layers, exhibit superior mechanical strength, thermal and chemical stability, and multi-point functionalization capability. These features make MWCNTs better suited for robust biosensing and industrial applications requiring long-term durability. The infographic underscores the importance of selecting the appropriate CNT type based on application-specific requirements and suggests that combining both types in hybrid materials may yield optimized performance across diverse sensing scenarios. The Figure 4b is the orbital structure of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) is fundamentally derived from the sp² hybridization of carbon atoms, forming a hexagonal lattice akin to graphene. In both SWCNTs and MWCNTs, each carbon atom contributes three sp² hybrid orbitals that establish strong in-plane σ-bonds with neighboring atoms, ensuring structural integrity and high mechanical strength. The remaining unhybridized p-orbital lies perpendicular to the tube surface and overlaps with adjacent p-orbitals to form a delocalized π-electron system, which is crucial for the electronic properties of CNTs. In SWCNTs, the simplicity of a single cylindrical shell allows for well-defined π-band structures that depend on chirality and diameter, enabling either metallic or semiconducting behavior. Conversely, MWCNTs consist of multiple concentric SWCNTs, where the interlayer π–π interactions and the presence of various chiralities within the walls result in a more complex band structure, typically with metallic conductivity. The orbital overlap between layers in MWCNTs can lead to charge transfer and screening effects, affecting electron transport pathways. These distinct orbital configurations significantly influence the electronic, optical, and chemical reactivity profiles of SWCNTs and MWCNTs, making orbital considerations vital in their application in nanoscale sensing devices.

3.4. Hybrid CNT-Based Sensing Materials

The integration of carbon nanotubes (CNTs) with other functional materials has emerged as a promising strategy to overcome the limitations of pristine CNTs and enhance their performance in sensing applications. Hybrid CNT-based sensing materials combine the extraordinary electrical, mechanical, and surface properties of CNTs with the unique functionalities of other nanomaterials, including metals, metal oxides, polymers, and 2D materials like graphene. This synergistic approach facilitates improved sensitivity, selectivity, and stability, making hybrid CNT materials highly attractive for advanced sensor platforms. Hybrid CNTs leverage the intrinsic advantages of CNTs such as high aspect ratio, excellent electrical conductivity, and chemical stability, while incorporating other components to address specific sensing challenges like selectivity towards particular analytes, resistance to environmental interferences, and long-term durability. For instance, metal nanoparticles (NPs) such as gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) have been extensively used to decorate CNT surfaces, providing catalytic activity and enhancing electron transfer processes in electrochemical sensors [174]. These metal/CNT hybrids are particularly beneficial in gas sensing and biosensing applications due to their ability to promote catalytic reactions and improve sensitivity to target molecules. Similarly, metal oxide–CNT hybrids represent another class of highly functional sensing materials. Metal oxides such as ZnO, SnO₂, TiO₂, and Fe₂O₃ are known for their excellent sensing properties, including high sensitivity to gases and chemicals [175]. However, pure metal oxides often suffer from poor conductivity and response stability. Integrating these oxides with CNTs mitigates such drawbacks, as CNTs provide a conductive network that facilitates rapid charge transport, while the metal oxides offer active sites for chemical interaction with analytes [176]. Moreover, polymer–CNT hybrids have garnered significant interest due to the flexibility, processability, and specific functionalization capabilities of polymers. Conducting polymers like polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) have been widely used in combination with CNTs for sensing applications [177]. These hybrids exhibit enhanced sensitivity and selectivity, especially in chemical and biological sensors, owing to the synergistic interaction between the polymer matrix and CNTs, which improves the charge transfer mechanism and provides functional binding sites for target molecules. In recent years, 2D materials such as graphene and transition metal dichalcogenides (TMDs) have also been hybridized with CNTs to develop advanced sensing platforms. Graphene–CNT hybrids combine the high surface area and flexibility of graphene with the superior conductivity and mechanical strength of CNTs, resulting in high-performance sensors capable of detecting ultra-low concentrations of analytes [178]. The presence of graphene also improves the dispersion of CNTs, addressing the common challenge of CNT agglomeration, thereby enhancing the reproducibility and uniformity of the sensing material. In addition to material combinations, hybridization strategies have been extended to the functionalization of CNTs with biological molecules, including enzymes, antibodies, and DNA sequences. Such bio-functionalized CNT hybrids are crucial for the development of biosensors aimed at medical diagnostics, environmental monitoring, and food safety applications [179]. The high surface area of CNTs provides a suitable platform for the immobilization of biomolecules, while hybridization with nanoparticles or polymers enhances the stability and activity of these biological components. The fabrication methods for hybrid CNT-based sensing materials vary depending on the type of hybridization and the desired application. Common synthesis techniques include chemical vapor deposition (CVD), in situ polymerization, electrochemical deposition, and solution-based methods such as drop-casting, spin-coating, and layer-by-layer assembly [180]. The choice of fabrication method significantly influences the morphology, surface properties, and sensing performance of the resulting hybrid materials. The application of hybrid CNT-based sensing materials spans a wide range of fields, including gas sensing, biosensing, chemical detection, pressure sensing, and environmental monitoring. For instance, AuNP-decorated CNT hybrids have demonstrated exceptional sensitivity in glucose sensors due to their excellent catalytic activity and electron transfer capabilities [181]. Similarly, ZnO–CNT hybrids have been utilized in gas sensors for detecting volatile organic compounds (VOCs) with high sensitivity and selectivity [182]. In the biomedical domain, polymer–CNT hybrids have been employed in electrochemical biosensors for the detection of cancer biomarkers and pathogens [183]. Despite the significant advancements in hybrid CNT-based sensing materials, several challenges remain. One of the primary issues is achieving uniform dispersion and stable integration of CNTs with other materials, which is critical for ensuring reproducibility and consistent sensor performance. Moreover, scalability and cost-effectiveness of fabrication methods need to be addressed for commercial deployment. The potential toxicity and environmental impact of certain hybrid materials also require thorough investigation to ensure safe and sustainable applications [184]. Future research in hybrid CNT-based sensing materials is expected to focus on the development of multifunctional sensors capable of simultaneous detection of multiple analytes, integration with flexible and wearable electronics, and enhancement of wireless and real-time sensing capabilities. The advancement of nanofabrication techniques and the exploration of novel hybridization strategies will play a crucial role in unlocking the full potential of CNT-based hybrid materials in next-generation sensing technologies.

Table 3. Comparative Summary of Types of Carbon Nanotubes (CNTs) and Their Applications.

Type of CNTs	Structure Features	Applications	References
Single-Walled CNTs (SWCNTs)	Single graphene sheet rolled into a cylinder (diameter ~0.4–3 nm)	Sensors, drug delivery, nanoelectronics, energy storage devices	[185,186]
Multi-Walled CNTs (MWCNTs)	Multiple concentric graphene cylinders (diameter ~2–100 nm)	Biosensors, structural composites, supercapacitors, emi shielding	[187,188]
Double-Walled CNTs (DWCNTs)	Two concentric graphene cylinders	Biomedical imaging, gas sensing, flexible electronics	[189]
Functionalized CNTs	Chemically or physically modified CNTs with functional groups	Biosensors, environmental monitoring, targeted drug delivery	[190,191,192]
Doped CNTs	CNTs doped with heteroatoms (N, B, P, S) to enhance properties	Gas sensing, catalysis, energy storage	[193]
CNT Composites	CNTs embedded in polymers, metals, or ceramics	Smart textiles, flexible electronics, antibacterial coatings	[194]

Table 3. Comparative Summary of Types of Carbon Nanotubes (CNTs) and Their Applications.

Type of CNTs	Structure Features	Applications	References
Single-Walled CNTs (SWCNTs)	Single graphene sheet rolled into a cylinder (diameter ~0.4–3 nm)	Sensors, drug delivery, nanoelectronics, energy storage devices	[185,186]
Multi-Walled CNTs (MWCNTs)	Multiple concentric graphene cylinders (diameter ~2–100 nm)	Biosensors, structural composites, supercapacitors, emi shielding	[187,188]
Double-Walled CNTs (DWCNTs)	Two concentric graphene cylinders	Biomedical imaging, gas sensing, flexible electronics	[189]
Functionalized CNTs	Chemically or physically modified CNTs with functional groups	Biosensors, environmental monitoring, targeted drug delivery	[190,191,192]
Doped CNTs	CNTs doped with heteroatoms (N, B, P, S) to enhance properties	Gas sensing, catalysis, energy storage	[193]
CNT Composites	CNTs embedded in polymers, metals, or ceramics	Smart textiles, flexible electronics, antibacterial coatings	[194]

Carbon nanotubes (CNTs) exist in several structural forms, each having unique characteristics influencing their suitability for specific applications. The Table 3 provides a comparative overview of major types of CNTs, highlighting their structural features and practical uses. SWCNTs consist of a single graphene sheet seamlessly rolled into a cylindrical structure with a small diameter range (~0.4 to 3 nm). These nanotubes exhibit remarkable electrical conductivity, high surface area, and excellent mechanical strength. Due to their high aspect ratio and quantum confinement effects, SWCNTs are widely employed in nanosensors, drug delivery systems, and nanoelectronics, where sensitivity and miniaturization are critical [185,186]. MWCNTs comprise multiple concentric graphene cylinders with diameters ranging from 2 to 100 nm. They possess greater mechanical robustness compared to SWCNTs but often exhibit reduced electrical performance due to interlayer interactions. MWCNTs are extensively utilized in structural reinforcement materials, biosensors, electromagnetic interference (EMI) shielding, and supercapacitor electrodes because of their high mechanical strength and conductivity [187,188]. DWCNTs represent an intermediate structure between SWCNTs and MWCNTs, consisting of two concentric graphene cylinders. These structures combine the electrical properties of SWCNTs with the mechanical stability of MWCNTs. DWCNTs are particularly attractive in biomedical imaging, gas sensing, and flexible electronic applications due to their enhanced stability and tunable properties [189]. Functionalization involves chemical or physical modifications on the surface of CNTs to introduce various functional groups (carboxyl, hydroxyl, amine, etc.). This process improves their solubility, dispersibility, and biocompatibility, making them ideal for sensing applications, especially in biosensors, environmental monitoring, and targeted drug delivery systems. Functionalization enhances the interaction of CNTs with specific analytes or biological molecules, thereby improving sensor selectivity and sensitivity [190,191]. Doping refers to the intentional incorporation of heteroatoms such as nitrogen (N), boron (B), phosphorus (P), or sulfur (S) into the CNT lattice to tailor their electronic, catalytic, and sensing properties. Doped CNTs display improved electrical conductivity, chemical reactivity, and selectivity towards various gases and biomolecules. These attributes make them suitable candidates for applications in gas sensing, electrocatalysis, and energy storage devices [193]. CNT composites are formed by embedding CNTs into polymeric, metallic, or ceramic matrices to create multifunctional materials with synergistic properties. Such composites retain the mechanical strength and electrical conductivity of CNTs while benefiting from the flexibility or process ability of the host material. They find applications in smart textiles, flexible electronics, antibacterial coatings, and advanced structural materials [194]. The comparative analysis of various CNT types underscores their versatile potential in sensing and advanced technological applications. The choice of CNT type for a specific application largely depends on the desired properties such as electrical conductivity, mechanical strength, chemical stability, or biocompatibility. Functionalization, doping, and composite formation further expand the applicability of CNTs by enhancing their performance in targeted applications. Future research should focus on optimizing CNT synthesis, functionalization, and integration techniques to enable scalable and cost-effective production for commercial sensing technologies.

4. CNT-Based Sensors

4.1. Gas Sensors

The detection of toxic gases using carbon nanotube (CNT)-based sensors has gained significant momentum due to the unique physicochemical characteristics of CNTs, which include their high surface-to-volume ratio, remarkable electrical conductivity, and chemical tunability. Toxic gases such as carbon monoxide (CO), nitrogen dioxide (NO₂), ammonia (NH₃), hydrogen sulfide (H₂S), and sulfur dioxide (SO₂) pose substantial threats to human health and environmental safety, even at very low concentrations. Traditional sensing technologies, although effective, often suffer from limitations such as poor selectivity, slow response times, and high power consumption. CNT-based sensors offer a promising alternative for the development of portable, sensitive, and selective toxic gas detectors [195]. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have both been employed as active sensing elements for toxic gas detection. Their quasi one-dimensional structures facilitate ballistic transport of electrons along the tube axis, and any gas-induced perturbation on the CNT surface leads to a significant alteration in their electronic properties. These changes are detectable as variations in electrical resistance or current, thereby enabling real-time gas detection [196]. SWCNTs are particularly sensitive to charge transfer interactions due to their semiconducting nature, making them highly effective in detecting electron-donating and electron-withdrawing gases [197]. The detection mechanism of CNT-based gas sensors generally relies on the interaction between the target gas molecules and the π-conjugated surface of the nanotube. These interactions can include physisorption, where the gas molecules are weakly bound through van der Waals forces, or chemisorption, involving stronger covalent or ionic bonding. Upon exposure to toxic gases, the electronic structure of the CNT is perturbed, leading to a detectable signal change. For instance, NO₂ acts as an electron acceptor and decreases the electron density of the CNTs, resulting in increased resistance, while NH₃, an electron donor, increases the electron density and reduces the resistance [198]. To enhance sensitivity and selectivity, CNTs are frequently functionalized with metal nanoparticles, metal oxides, or organic molecules. For example, functionalization with palladium (Pd), platinum (Pt), or gold (Au) nanoparticles significantly improves sensor performance by catalyzing gas adsorption and enhancing charge transfer processes [199]. A study demonstrated that Pd-decorated SWCNTs exhibited rapid response and high sensitivity to H₂S gas at parts-per-billion (ppb) levels, attributed to the spill-over effect of Pd facilitating hydrogen dissociation and transfer to CNTs [200]. Similarly, CNTs modified with tin dioxide (SnO₂) or zinc oxide (ZnO) nanoparticles have shown improved selectivity towards NO₂ and NH₃ detection [201]. Moreover, doping CNTs with heteroatoms such as nitrogen, boron, or sulfur is another strategy to enhance their reactivity towards specific toxic gases. Nitrogen-doped CNTs, for example, possess electron-rich sites that enhance interactions with electron-withdrawing gases like NO₂, resulting in amplified sensing signals [202]. In one study, nitrogen-doped MWCNTs showed nearly a tenfold increase in sensitivity towards NO₂ compared to pristine MWCNTs, attributed to the increased density of active sites and improved charge carrier mobility [203]. The operating temperature of CNT-based sensors plays a crucial role in their performance. Unlike metal oxide-based sensors that require high operational temperatures (often >200 °C), CNT-based sensors can operate efficiently at room temperature. This feature not only reduces power consumption but also makes them more suitable for wearable and portable applications [204]. Room-temperature operation has been successfully demonstrated in the detection of CO using SWCNTs functionalized with cobalt phthalocyanine, where high sensitivity and a low detection limit of 50 ppb were achieved [205]. Response time and recovery time are vital parameters in gas sensing. CNT-based sensors generally exhibit fast response and recovery due to the rapid adsorption and desorption dynamics of gas molecules on the nanotube surface. However, the reversibility of adsorption, particularly in the case of strong chemisorption, can be a limiting factor. To address this, UV or thermal desorption techniques are often employed to regenerate the sensor surface for subsequent use [206]. One of the main challenges in toxic gas sensing using CNTs is the cross-sensitivity to interfering gases, especially in complex environments. Functionalization strategies, as mentioned earlier, help mitigate this by tailoring the surface chemistry of CNTs to favor interactions with target gases. Moreover, integrating CNT sensors into sensor arrays or “electronic noses” and using machine learning algorithms for pattern recognition has shown promising results in distinguishing between multiple gases with high accuracy [207]. Another concern is sensor stability and reproducibility over time. Pristine CNTs may suffer from drift due to environmental factors such as humidity and temperature. To address this, protective coatings and encapsulation techniques have been explored, as well as calibration methods to maintain consistent performance. Furthermore, the scalability of CNT sensor fabrication remains a technical hurdle. While laboratory-scale production and testing have shown excellent results, large-scale, reproducible manufacturing of CNT sensors is still under development [208]. Despite these challenges, several CNT-based toxic gas sensors have progressed towards commercialization. For example, CNT gas sensors for NH₃ detection have been integrated into smart textiles for wearable environmental monitoring, while others have been employed in industrial safety systems for early warning detection of CO and NO₂ leaks [4]. The ongoing research into hybrid nanomaterials, flexible electronics, and Internet of Things (IoT)-based sensor platforms continues to push the boundaries of CNT gas sensing technologies. In conclusion, CNT-based toxic gas sensors offer significant advantages over conventional materials due to their high sensitivity, rapid response, room-temperature operation, and tunable surface chemistry. The ability to detect trace concentrations of hazardous gases in real time makes them indispensable for environmental safety, industrial monitoring, and public health applications. Future developments are expected to focus on improving selectivity, long-term stability, and integration into smart, connected sensor systems. Figure 5 illustrates the fundamental working mechanism of a differential microwave-based gas sensor, which operates by detecting variations in the dielectric properties of the surrounding medium upon gas exposure. The sensor typically comprises two microwave resonators one serving as a reference and the other coated with a sensitive layer such as carbon nanotubes (CNTs) [209]. When target gas molecules interact with the sensing layer, they induce changes in the permittivity and conductivity of the material, thereby shifting the resonant frequency or amplitude of the transmitted microwave signal. By comparing the response of the sensing channel to the reference, high sensitivity and selectivity can be achieved, enabling real-time detection of specific analytes even at low concentrations.

4.2. Environmental and Industrial Monitoring

The application of carbon nanotube (CNT)-based gas sensors in environmental and industrial monitoring has attracted increasing research interest due to the demand for real-time, accurate, and sensitive detection systems in these fields. Environmental and industrial sectors face challenges related to the release of hazardous gases, volatile organic compounds (VOCs), and pollutants into the atmosphere, which pose serious threats to human health, ecosystems, and industrial safety standards. Conventional gas sensing materials, although widely used, often suffer from limitations such as low sensitivity, slow response times, and poor selectivity under variable environmental conditions. CNTs, with their outstanding electrical, chemical, and mechanical properties, provide a promising alternative for the development of advanced sensing platforms tailored for environmental and industrial monitoring applications [210]. CNT-based gas sensors operate on the principle that gas molecules interact with the surface of CNTs, leading to charge transfer or changes in the local electronic structure, which subsequently alters their electrical conductivity or resistance. This interaction is particularly advantageous in environmental monitoring, where detecting trace concentrations of hazardous gases such as nitrogen oxides (NOx), sulfur dioxide (SO₂), ammonia (NH₃), carbon monoxide (CO), and VOCs is critical. The high aspect ratio, large surface area, and hollow tubular structure of CNTs facilitate the adsorption of gas molecules, improving sensitivity even at low gas concentrations [211]. In the context of environmental monitoring, the detection of air pollutants has become a significant focus due to increasing air quality concerns globally. CNT-based sensors have demonstrated excellent performance in detecting air pollutants, particularly NO₂ and NH₃, due to their strong electron-accepting and -donating properties. For example, single-walled CNTs (SWCNTs) have been shown to exhibit enhanced sensitivity to NO₂ due to charge transfer interactions, leading to pronounced changes in resistance upon exposure to the gas [212]. Multi-walled CNTs (MWCNTs), on the other hand, offer structural stability and mechanical robustness, making them suitable for deployment in harsh industrial environments [213]. To further improve selectivity and sensitivity, researchers have functionalized CNTs with metal nanoparticles (e.g., Au, Pt, and Pd) or metal oxides (e.g., SnO₂ and ZnO) to create hybrid sensing platforms capable of detecting specific target gases in complex environmental matrices. For instance, ZnO-decorated CNT sensors have demonstrated remarkable sensitivity towards NH₃ at room temperature, making them viable candidates for continuous environmental monitoring [214]. Moreover, the integration of CNT-based sensors into portable devices and wireless sensor networks has enabled the real-time monitoring of air quality parameters in urban and industrial areas, facilitating proactive environmental management strategies [215]. Industrial monitoring applications also benefit significantly from the use of CNT-based gas sensors. Industrial processes often involve the use of toxic and flammable gases, necessitating accurate and rapid detection systems to ensure workplace safety and regulatory compliance. CNT sensors have shown excellent performance in detecting industrial gases such as CO, H₂S, CH4, and VOCs. For example, CNT sensors functionalized with polyaniline or conducting polymers have demonstrated enhanced sensitivity and selectivity for detecting CO and H₂S, which are commonly present in industrial emissions [216]. Moreover, the ability of CNT-based sensors to operate at low power consumption and at room temperature is particularly advantageous in industrial settings, where energy efficiency and cost-effectiveness are critical considerations. The miniaturization of CNT sensors has enabled their integration into wearable devices for industrial safety monitoring, allowing workers to detect hazardous gases in real-time and alerting them to potential risks [217]. Additionally, CNT sensors have been embedded into smart industrial systems for continuous monitoring of emissions and leak detection, contributing to the implementation of Industry 4.0 and smart manufacturing paradigms [218]. Despite their promising attributes, certain challenges remain in the practical deployment of CNT-based sensors for environmental and industrial monitoring. Issues such as sensor drift, long-term stability, reproducibility, and selectivity under varying humidity and temperature conditions need to be addressed through advanced functionalization techniques and sensor calibration methods. Furthermore, the scalability and cost-effectiveness of manufacturing CNT sensors remain important factors for their commercialization and widespread adoption. In summary, CNT-based gas sensors have emerged as powerful tools for environmental and industrial monitoring due to their superior sensitivity, fast response, low detection limits, and adaptability to complex sensing environments. Advances in functionalization strategies and the integration of CNT sensors into smart systems are paving the way for the development of next-generation monitoring platforms that align with the growing demand for sustainable environmental management and industrial safety compliance.

4.3. Chemical Sensors

4.3.1. Sensing of Chemical Pollutants

Carbon nanotubes (CNTs) have emerged as highly promising materials for the development of chemical sensors due to their superior sensitivity, selectivity, and fast response times. Among various applications, the detection of chemical pollutants has gained significant attention because of the increasing environmental concerns and need for real-time monitoring of hazardous chemicals. Chemical pollutants, including volatile organic compounds (VOCs), heavy metals, and pesticides, pose severe risks to human health and ecosystems. CNT-based sensors have demonstrated remarkable capabilities in detecting these pollutants at very low concentrations owing to their large surface area, high aspect ratio, and ability to be functionalized with specific chemical groups for enhanced selectivity [219]. One of the critical advantages of CNTs in sensing chemical pollutants is their ability to interact with target molecules through various mechanisms, such as physisorption, chemisorption, or charge transfer processes. These interactions lead to changes in the electrical properties of CNTs, such as conductance or resistance, which can be easily measured and correlated with the concentration of pollutants [220]. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have been widely explored for chemical sensing applications due to their high sensitivity towards small molecules and chemical vapors. In particular, the detection of volatile organic compounds (VOCs) like benzene, toluene, xylene, formaldehyde, and acetone has been extensively studied using CNT-based sensors. Functionalization of CNTs with metal nanoparticles (such as Pt, Au, or Pd) or organic molecules enhances the sensitivity and selectivity towards specific VOCs. For instance, Pd-decorated SWCNTs have shown superior sensitivity to hydrogen gas and other VOCs due to the catalytic properties of Pd, which facilitates dissociation and adsorption of the target molecules [221]. Additionally, polymer-functionalized CNTs have been utilized for the selective detection of specific organic vapors by leveraging the chemical affinity between the polymer and the target molecule [222]. Another significant class of chemical pollutants includes heavy metals, which are toxic even at trace levels. CNT-based sensors have demonstrated excellent performance in detecting metal ions like lead (Pb²⁺), cadmium (Cd²⁺), mercury (Hg²⁺), and arsenic (As³⁺). Functionalization of CNTs with chelating agents or ionophores enhances their binding affinity towards metal ions, thereby improving the sensitivity and detection limits of the sensors. Electrochemical detection using CNT-modified electrodes has become a popular approach for heavy metal sensing due to the high electrical conductivity and surface area of CNTs, which provide efficient electron transfer pathways [223]. Pesticide detection is another crucial application of CNT-based chemical sensors, particularly for agricultural and food safety monitoring. Functionalization of CNTs with enzymes, antibodies, or molecularly imprinted polymers (MIPs) enables the selective detection of pesticides like organophosphates and carbamates. For example, acetylcholinesterase (AChE)-immobilized CNTs have been used to detect organophosphate pesticides based on the inhibition of enzymatic activity upon pesticide exposure, leading to measurable electrical changes [224]. Moreover, the integration of CNT-based chemical sensors with portable and wireless platforms has opened new avenues for real-time and on-site detection of chemical pollutants. Advances in flexible electronics, microfluidic systems, and wearable sensors have facilitated the development of compact CNT-based devices capable of continuous monitoring in various environments [225].

Figure 6 illustrates the structural configurations commonly employed in carbon nanotube (CNT)-based chemiresistive sensors. These sensors operate on the principle that the electrical resistance of CNTs changes upon exposure to target analytes due to charge transfer or adsorption-induced modulation of the conduction pathways. The schematic highlights various arrangements, including individual CNTs or networks deposited between interdigitated electrodes, allowing for efficient current flow and sensitive detection. Such designs leverage the high aspect ratio and surface reactivity of CNTs to maximize interaction with gaseous or chemical species [197]. The simplicity, scalability, and low power requirements of these configurations make CNT chemiresistive sensors attractive for integration into compact, real-time monitoring systems in environmental, industrial, and biomedical applications. However, challenges such as sensor stability, selectivity in complex matrices, reproducibility, and scalability need to be addressed for the widespread commercialization of these sensors. Recent studies have also explored hybrid sensing materials combining CNTs with other nanomaterials such as graphene, metal oxides, and conducting polymers to enhance the performance of chemical sensors. These hybrid materials leverage the synergistic effects of their components, providing improved sensitivity, faster response times, and better selectivity towards specific chemical pollutants [226]. The tunability of CNT surfaces through covalent and non-covalent functionalization strategies enables the design of highly customized sensors for targeted applications. In conclusion, CNT-based sensors hold great promise for the detection of chemical pollutants due to their unique structural and electronic properties. Continued research and development in material functionalization, device integration, and performance optimization are expected to advance the practical applications of CNT-based chemical sensors in environmental monitoring, public health, and industrial safety [227].

4.3.2. Chemical Reaction Monitoring

Carbon nanotubes (CNTs) have emerged as promising materials for chemical reaction monitoring due to their exceptional structural, electrical, and chemical properties. The capability of CNTs to interact with various chemical species, along with their high electrical conductivity and large surface area, enables their effective utilization in real-time monitoring of chemical reactions. Chemical reaction monitoring involves the detection, quantification, and analysis of reaction intermediates, reactants, or products, essential for controlling industrial processes, pharmaceutical manufacturing, environmental protection, and chemical research [228]. CNT-based sensors, integrated into reaction monitoring systems, offer enhanced sensitivity, selectivity, and rapid response, facilitating precise analysis of dynamic chemical environments. CNTs are particularly suitable for monitoring chemical reactions due to their ability to undergo surface modification and functionalization, allowing selective interaction with target molecules. The functionalization of CNTs with specific recognition elements, such as metal nanoparticles, enzymes, or polymers, provides them with selective sensing capabilities towards particular reaction species. For example, metal nanoparticle-decorated CNT sensors have been widely used to monitor redox reactions, where electron transfer processes are crucial. These functionalized CNTs can detect changes in the local chemical environment, offering real-time insights into reaction kinetics and mechanisms [229]. One of the most important applications of CNT-based chemical reaction monitoring is in electrochemical sensing platforms. Electrochemical sensors based on CNTs have demonstrated excellent performance in monitoring various chemical processes, including oxidation–reduction reactions, catalytic transformations, and environmental pollutant degradation. The high electrical conductivity and low detection limits of CNT-based electrodes enable the detection of trace amounts of chemical species, which is critical in monitoring slow or low-yielding reactions [230]. Additionally, their fast electron transfer kinetics improve sensor response times, allowing near real-time monitoring of chemical changes. CNTs have also been employed in optical sensors for chemical reaction monitoring. Functionalized CNTs can interact with reaction species, resulting in changes in their optical properties such as fluorescence, absorbance, or Raman scattering. These optical changes provide valuable information about reaction progress and molecular transformations. For instance, single-walled carbon nanotubes (SWCNTs) exhibit intrinsic fluorescence in the near-infrared (NIR) region, which is sensitive to their chemical environment. Changes in the local chemical environment during a reaction can modulate the fluorescence intensity or wavelength of SWCNTs, offering a non-invasive approach for reaction monitoring [231]. Moreover, CNT-based field-effect transistor (FET) sensors are gaining attention for real-time monitoring of chemical reactions. CNT–FET sensors operate based on changes in the electrical characteristics (e.g., current and conductance) upon interaction with target analytes. These devices have been utilized to monitor chemical reactions in liquid and gaseous phases with high sensitivity and selectivity. The small size and flexibility of CNT–FET sensors make them suitable for integration into microreactors or lab-on-a-chip systems for in situ chemical reaction monitoring [232]. In pharmaceutical industries, CNT-based sensors play a critical role in monitoring chemical reactions involved in drug synthesis and formulation. Real-time monitoring of reaction intermediates and products ensures high-quality drug production while minimizing the presence of undesirable by-products. For example, CNT-based electrochemical sensors have been employed to monitor the synthesis of active pharmaceutical ingredients (APIs) by detecting key intermediates or side products during the reaction process [233]. This enhances the control and optimization of reaction conditions, improving overall process efficiency and product quality. Environmental applications of CNT-based reaction monitoring involve detecting and tracking chemical species involved in pollutant degradation and waste treatment processes. CNT sensors have been utilized to monitor chemical oxidation processes, photocatalytic degradation of organic pollutants, and advanced oxidation processes (AOPs) for wastewater treatment. Monitoring the formation and degradation of chemical species during such reactions provides valuable information about the efficiency and safety of the environmental remediation processes [154]. Challenges in the development of CNT-based sensors for chemical reaction monitoring include maintaining sensor stability under harsh reaction conditions, preventing fouling or degradation of the sensor surface, and achieving long-term reproducibility. The integration of CNT sensors with advanced signal processing techniques and data analysis algorithms is crucial to overcome these challenges and improve sensor performance in complex chemical environments [234]. Future perspectives for CNT-based chemical reaction monitoring focus on developing multi-functional sensors capable of simultaneously detecting multiple reaction species, enabling comprehensive analysis of complex reactions. Furthermore, the integration of CNT sensors with wireless communication technologies and Internet of Things (IoT) platforms will facilitate remote and continuous monitoring of chemical processes in industrial and environmental settings. The advancement of CNT-based nanocomposite materials and hybrid sensing platforms is expected to further enhance the sensitivity, selectivity, and durability of chemical reaction monitoring systems [235]. In conclusion, carbon nanotube-based sensors have demonstrated significant potential in chemical reaction monitoring due to their superior physicochemical properties and versatility in functionalization. Their application spans diverse fields, including pharmaceuticals, environmental monitoring, and industrial process control. Continuous research and development efforts are expected to address current challenges, paving the way for the commercialization of advanced CNT-based chemical reaction monitoring systems capable of real-time, sensitive, and selective detection of complex chemical transformations.

4.4. Biosensors

Carbon nanotubes (CNTs), particularly due to their extraordinary physicochemical properties, have demonstrated tremendous potential in the detection of biomolecules, which is a critical area in biosensor development. Biomolecule detection plays a vital role in diverse fields such as medical diagnostics, food safety, environmental monitoring, and pharmaceutical industries. The superior electrical conductivity, high surface area-to-volume ratio, chemical stability, and tunable surface chemistry of CNTs make them highly suitable for immobilizing biological recognition elements and enhancing signal transduction mechanisms in biosensing platforms [236]. CNT-based biosensors can detect a wide range of biomolecules, including proteins, DNA, glucose, enzymes, hormones, and nucleic acids, due to their ability to facilitate effective electron transfer between the biomolecule and the transducer element. This property significantly improves sensitivity and selectivity, enabling ultra-low detection limits, which are essential for early disease diagnosis and real-time biomolecular monitoring [237]. To improve the interaction of CNTs with specific biomolecules, various functionalization strategies have been employed, broadly categorized as covalent and non-covalent methods. Covalent functionalization involves the formation of chemical bonds between CNTs and functional groups, which improves dispersibility in aqueous solutions and enhances stability. For example, carboxylation (-COOH), amidation (-NH2), and hydroxylation (-OH) groups are commonly introduced to CNTs to facilitate further biomolecule immobilization [238]. In contrast, non-covalent functionalization preserves the intrinsic electrical properties of CNTs and involves π–π stacking, hydrophobic interactions, and electrostatic forces to attach biomolecules or linker molecules onto the CNT surface [62]. These functionalization techniques are critical for constructing biosensors capable of selective recognition of target biomolecules. Additionally, functionalization allows the attachment of antibodies, enzymes, aptamers, and nucleic acids, which act as recognition elements, directly onto the CNT surface, thereby enhancing the specificity of the biosensor [166].

Several types of biosensors have been developed using CNTs for biomolecule detection. These include:

Electrochemical Biosensors: Electrochemical biosensors based on CNTs are widely used due to their high sensitivity, fast response time, and compatibility with miniaturization techniques. CNT-modified electrodes enhance the electron transfer kinetics between the biomolecule and the electrode surface. For example, glucose biosensors utilizing glucose oxidase immobilized on CNT-modified electrodes have demonstrated exceptional sensitivity for glucose detection in blood samples [239].

Optical Biosensors: CNTs have been integrated into optical biosensors, such as fluorescence and surface plasmon resonance (SPR) sensors. In these systems, CNTs are used to quench or enhance fluorescence signals upon binding to target biomolecules. DNA and protein detection using CNT-based optical sensors have been reported with high sensitivity and selectivity, providing valuable tools for genetic and proteomic studies [130].

Field-Effect Transistor (FET)-Based Biosensors: CNT-based FET biosensors represent a promising platform for real-time biomolecule detection. In these devices, the adsorption or binding of biomolecules onto the CNT channel alters the electrical properties (such as conductance or resistance), producing a measurable signal. CNT–FET biosensors have been employed for the detection of specific DNA sequences, cancer biomarkers, and pathogenic microorganisms with excellent sensitivity [240].

Applications in Biomolecule Detection

CNT-based biosensors have been successfully applied in various biomolecular detection scenarios:

• Detection of glucose for diabetes management.
• Monitoring of DNA hybridization events for genetic screening.
• Detection of specific proteins and enzymes as disease biomarkers.
• Identification of pathogens (bacteria and viruses) for infection control.
• Detection of hormones and neurotransmitters for physiological monitoring [241].

CNT-based biosensors offer several advantages, including high sensitivity, fast response time, miniaturization capability, and suitability for real-time monitoring. Their biocompatibility and ease of functionalization further enhance their applicability in biological systems. However, challenges such as reproducibility, long-term stability, selectivity in complex biological matrices, and potential cytotoxicity need to be addressed for their widespread commercialization. Efforts are being directed towards improving the fabrication processes, surface modification techniques, and integration of CNT biosensors with microfluidic and lab-on-chip systems to overcome these challenges. Additionally, the combination of CNTs with other nanomaterials, such as metal nanoparticles, graphene, and polymers, is being explored to enhance the sensing performance [242].

4.4.1. Role of CNTs in Medical Diagnostics

The integration of carbon nanotubes (CNTs) in the field of medical diagnostics has revolutionized the landscape of biosensing technologies by enabling high sensitivity, rapid detection, and multiplexed analysis of diverse biomolecular targets. The unique electrical, optical, and structural properties of CNTs have paved the way for the development of novel diagnostic platforms capable of detecting biomarkers at ultra-low concentrations, making them ideal candidates for early disease detection, point-of-care (POC) diagnostics, and real-time monitoring of physiological conditions [243]. Medical diagnostics primarily rely on the detection of biomolecules such as proteins, nucleic acids, glucose, hormones, and pathogens, where the sensitivity, specificity, and speed of the sensing platform are critical parameters. The extraordinary surface area-to-volume ratio of CNTs provides abundant active sites for the immobilization of bioreceptors like antibodies, aptamers, enzymes, and DNA probes, enhancing the sensitivity of biosensors [244]. Moreover, their superior electrical conductivity facilitates efficient transduction of binding events into measurable signals, thereby amplifying sensor performance [245]. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) have both been extensively explored for their application in medical diagnostics. SWCNTs are often favored for electronic-based biosensors due to their semiconducting properties, while MWCNTs are commonly utilized for their mechanical robustness and high loading capacity for biomolecules [246]. The functionalization of CNTs with specific recognition elements ensures selective detection of target analytes, reducing cross-reactivity and enhancing the diagnostic accuracy.

4.4.2. CNT-Based Electrochemical Biosensors in Medical Diagnostics

Electrochemical biosensors leveraging CNT-based platforms have demonstrated exceptional performance in detecting various disease biomarkers. For instance, glucose biosensors employing CNT-modified electrodes have shown rapid response times, high sensitivity, and excellent stability for glucose monitoring in diabetic patients [247]. Similarly, CNT-based DNA sensors have been developed for the detection of genetic mutations and infectious agents, providing valuable tools for personalized medicine and molecular diagnostics [248]. Cancer biomarker detection is another area where CNT-based biosensors have exhibited promising results. Sensors incorporating CNTs functionalized with aptamers or antibodies have been employed for the ultrasensitive detection of cancer-related proteins such as carcinoembryonic antigen (CEA), prostate-specific antigen (PSA), and alpha-fetoprotein (AFP) [249]. These platforms facilitate early cancer detection, which is crucial for effective treatment and patient survival.

4.4.3. CNT-Based Optical Biosensors for Medical Diagnostics

Optical biosensors utilizing the intrinsic photoluminescence and fluorescence quenching properties of CNTs have been widely applied in the detection of biomolecules. SWCNTs, in particular, exhibit near-infrared (NIR) fluorescence, enabling their use in deep-tissue imaging and in vivo diagnostics with minimal background interference [250]. Fluorescence-based CNT biosensors have been designed for the detection of nucleic acids, small molecules, and proteins, offering high sensitivity and real-time detection capabilities [134]. Additionally, CNT-based surface-enhanced Raman scattering (SERS) biosensors have gained attention for their ability to provide highly specific molecular fingerprints of target analytes, facilitating the detection of pathogens and disease biomarkers at ultra-low concentrations [18].

4.4.4. CNT-Based Field-Effect Transistor (FET) Biosensors in Medical Diagnostics

Field-effect transistor (FET) biosensors integrating CNTs as the conductive channel have emerged as powerful tools for medical diagnostics due to their label-free detection capability and high sensitivity. CNT–FET biosensors have been successfully applied for the detection of various biomarkers, including DNA, microRNA, proteins, and viruses [251]. These platforms offer rapid response times and are suitable for miniaturized, portable diagnostic devices, making them highly applicable in point-of-care settings. For example, a CNT–FET biosensor developed for COVID-19 detection demonstrated rapid and highly sensitive detection of the SARS-CoV-2 spike protein, highlighting the potential of CNT-based devices in managing emerging infectious diseases [252]. Despite the promising advancements, several challenges remain in the widespread adoption of CNT-based biosensors in medical diagnostics. Issues related to the reproducibility of CNT synthesis, biocompatibility, long-term stability, and potential toxicity need to be addressed for their clinical translation [253]. Additionally, the integration of CNT-based biosensors with microfluidics, lab-on-a-chip systems, and wireless communication technologies will play a pivotal role in advancing next-generation diagnostic platforms. Future research is expected to focus on the development of multifunctional CNT-based biosensors capable of simultaneous detection of multiple biomarkers, real-time monitoring of disease progression, and integration with wearable healthcare devices [254]. The convergence of nanotechnology, biotechnology, and information technology will further accelerate the application of CNTs in personalized medicine, early disease detection, and continuous health monitoring.

4.5. Pressure and Strain Sensors

Carbon nanotubes (CNTs) have emerged as promising nanomaterials in the development of pressure and strain sensors due to their exceptional electrical, mechanical, and structural properties. Their ability to convert mechanical deformation into electrical signals has paved the way for their application in advanced sensing platforms. Pressure and strain sensors are essential in numerous domains, including aerospace, structural health monitoring, robotics, flexible electronics, and wearable devices. CNT-based pressure and strain sensors exhibit remarkable sensitivity, durability, flexibility, and high response rates, surpassing conventional materials in terms of performance and application diversity. Their inherent mechanical robustness, coupled with tunable electrical conductivity and piezoresistive behavior, enable them to detect minute deformations, making them ideal for real-time sensing applications [255]. The sensing mechanism in CNT-based pressure and strain sensors largely relies on the piezoresistive effect, wherein the electrical resistance changes proportionally with the applied mechanical stress or strain. Additionally, the high aspect ratio and large surface area of CNTs facilitate efficient load transfer and deformation sensing even under low-pressure conditions [192]. Depending on the fabrication technique, CNTs can be integrated into various matrix materials such as polymers, elastomers, or composites, enhancing the mechanical flexibility and sensitivity of the sensors. The alignment and dispersion of CNTs within these matrices play a crucial role in optimizing their electromechanical performance [193]. Recent advancements have seen the development of CNT networks, buckypapers, CNT–polymer composites, and CNT-coated fabrics for diverse strain-sensing applications [194]. Furthermore, the scalability and customization potential of CNT-based pressure and strain sensors have broadened their usability in industries requiring precise structural monitoring, wearable devices, and next-generation flexible electronics. These sensors not only offer high sensitivity but also maintain stability under repeated mechanical loading and unloading cycles, addressing the durability challenges faced by conventional sensors [195].

4.5.1. Structural Monitoring in Aerospace and Engineering

Structural health monitoring (SHM) is a critical aspect of aerospace and civil engineering applications to ensure the safety and integrity of complex structures. The ability of CNT-based sensors to detect minute deformations, cracks, or stress variations within materials makes them highly valuable for SHM systems. Traditional strain gauges and piezoelectric sensors often face limitations in sensitivity, flexibility, and real-time monitoring, whereas CNT-based sensors provide superior performance in harsh environments, including extreme temperatures and mechanical loads [196]. In aerospace structures, the need for lightweight, durable, and highly sensitive sensors has driven the integration of CNT-based materials into composite structures for in situ monitoring. For instance, CNT networks embedded within aircraft wings, fuselage panels, or turbine blades enable real-time monitoring of structural stress, fatigue, and potential damage without adding significant weight to the system [197]. These sensors are particularly beneficial in detecting early-stage damage, such as microcracks, which could otherwise lead to catastrophic failures if undetected. Moreover, CNT-based strain sensors are highly adaptable for use in civil engineering structures such as bridges, buildings, and pipelines. Their ability to conform to curved or irregular surfaces, coupled with their resistance to environmental degradation, positions them as a reliable solution for long-term monitoring of infrastructure [198]. Various studies have reported the use of CNT-embedded composites for monitoring load distribution, crack propagation, and strain variations in real-world structural applications [199]. The multifunctionality of CNT-based sensors, which can simultaneously detect strain, pressure, temperature, and even chemical changes, enhances their utility in integrated SHM systems. Recent research has also explored the use of wireless CNT-based sensing networks that facilitate remote monitoring and data transmission, further advancing the capabilities of smart structural systems [200].

4.5.2. Flexible Electronics

Flexible electronics represent a revolutionary domain wherein electronic devices are designed to maintain functionality under mechanical deformation such as bending, stretching, or twisting. The integration of CNT-based pressure and strain sensors into flexible electronic systems has garnered substantial attention due to their superior mechanical resilience and electrical performance. These sensors form the backbone of emerging technologies, including wearable devices, electronic skin (e-skin), soft robotics, and human–machine interfaces [201]. The inherent flexibility and stretchability of CNT-based sensors arise from their ability to form conductive networks that maintain electrical connectivity under deformation. CNTs, when dispersed within elastomeric matrices or deposited onto flexible substrates, exhibit excellent electromechanical properties, enabling them to detect subtle pressure changes and strains associated with human motion or external forces [202]. Wearable health monitoring devices, for instance, utilize CNT-based strain sensors to track physiological parameters such as pulse, respiration, body motion, and joint movement in real time. These sensors provide valuable data for personalized healthcare, rehabilitation, and fitness monitoring [203]. Furthermore, the development of CNT-based e-skin has revolutionized tactile sensing, allowing devices to mimic human skin’s sensitivity to pressure, strain, and temperature variations. In soft robotics, CNT-based pressure sensors enable robots to interact safely with their environment by sensing contact forces and adapting their movement accordingly. Their lightweight nature and mechanical durability make them ideal for integration into artificial muscles, soft grippers, and wearable robotic exoskeletons [204]. Flexible electronic systems also benefit from CNT-based sensors in the field of human–machine interfaces (HMI), where touch-sensitive or gesture-recognition devices rely on high-performance strain sensors for real-time response. Recent advancements in CNT patterning, 3D printing, and scalable fabrication techniques have further expanded the potential for commercializing CNT-based flexible sensors [205]. Despite their numerous advantages, challenges remain in optimizing CNT dispersion, achieving uniform sensor performance, and ensuring long-term stability in flexible electronic systems. Nevertheless, continuous research and innovation in CNT synthesis, functionalization, and sensor design are expected to overcome these limitations, paving the way for widespread adoption in next-generation wearable and flexible electronics [206].

Table 4 provides a comprehensive comparison of the different categories of CNT-based sensors, summarizing their sensing mechanisms, target analytes, advantages, and relevant literature references. Carbon nanotubes (CNTs) have been integrated into various sensor platforms owing to their exceptional electrical, chemical, and mechanical properties, enabling ultra-sensitive and selective detection in diverse environments. Electrochemical sensors based on CNTs utilize their ability to enhance electron transfer and redox activity. These sensors are particularly effective in detecting biological molecules like glucose and various metal ions with high sensitivity and low detection limits due to the large surface area and superior electrical conductivity of CNTs [1,2]. Gas sensors exploiting CNTs are highly effective because of their large surface-to-volume ratio, which facilitates significant adsorption of gas molecules. Upon gas adsorption, there is a notable change in the electrical conductivity of CNTs, enabling the detection of gases like ammonia (NH₃), hydrogen (H₂), carbon dioxide (CO₂), and volatile organic compounds (VOCs) even at room temperature [3,4]. Optical sensors based on CNTs leverage mechanisms such as fluorescence quenching or enhancement and Raman scattering for sensitive detection of biomolecules or ions. These sensors offer advantages like real-time detection and label-free operation, making them suitable for biomedical and environmental monitoring applications [5]. CNT-based FET sensors operate by modulating electrical conductivity when target analytes interact with the CNT channel under an electric field. These sensors are highly selective, consume low power, and are suitable for detecting pathogens, gases, or biomolecules in complex environments [6,7]. Although CNTs themselves are not inherently piezoelectric, their integration with piezoelectric materials enables the development of flexible sensors that can detect mechanical stimuli like pressure, strain, or vibration. Such sensors are essential for wearable electronics and smart materials due to their mechanical robustness and flexibility [8]. Functionalization of CNTs with specific chemical or biological recognition elements (e.g., enzymes, antibodies, and DNA) improves their selectivity and biocompatibility. These biosensors are widely used in clinical diagnostics for the detection of glucose, nucleic acids, or antigen-antibody interactions, providing high specificity and sensitivity [9,10]. The comparative analysis presented in Table 4 highlights the versatility of CNT-based sensors across various sensing platforms. Each sensor type leverages specific properties of CNTs such as high electrical conductivity, large surface area, mechanical strength, and chemical tunability to achieve efficient detection of target analytes. The selection of a particular CNT-based sensor depends on the desired application, analyte characteristics, operational environment, and sensitivity requirements. The development of hybrid CNT sensors, functionalized materials, and integrated sensor systems is expected to drive further innovations in fields like healthcare, environmental monitoring, food safety, and wearable electronics.

Table 4. Comparative Summary of CNT-Based Sensors, Sensing Mechanisms, Target Analytes, and Applications.

Type of CNT-Based Sensor	Sensing Mechanism	Target Analyte/Parameter	Key Advantages	References
Electrochemical CNT Sensor	Electron transfer, redox reaction enhancement	Glucose, heavy metals, biomolecules	High sensitivity, fast response, low detection limit	[1,2]
Gas CNT Sensor	Adsorption-induced conductivity change	NH3, H2, CO2, NO2, VOCs	High surface area, room temperature operation	[3,4]
Optical CNT Sensor	Fluorescence quenching/enhancement, Raman scattering	DNA, proteins, metal ions	Label-free detection, real-time monitoring	[5]
Field-Effect Transistor (FET) CNT Sensor	Modulation of electrical conductivity via field effect	Biomolecules, gases, pathogens	High selectivity, low power consumption	[6,7]
Piezoelectric CNT Sensor	Strain-induced charge generation	Pressure, vibration, motion	Flexibility, mechanical robustness	[8]
Biosensor with Functionalized CNTs	Specific bioreceptor–analyte interaction	Glucose, DNA, antigens, enzymes	High specificity, enhanced biocompatibility	[9,10]

Table 4. Comparative Summary of CNT-Based Sensors, Sensing Mechanisms, Target Analytes, and Applications.

Type of CNT-Based Sensor	Sensing Mechanism	Target Analyte/Parameter	Key Advantages	References
Electrochemical CNT Sensor	Electron transfer, redox reaction enhancement	Glucose, heavy metals, biomolecules	High sensitivity, fast response, low detection limit	[1,2]
Gas CNT Sensor	Adsorption-induced conductivity change	NH3, H2, CO2, NO2, VOCs	High surface area, room temperature operation	[3,4]
Optical CNT Sensor	Fluorescence quenching/enhancement, Raman scattering	DNA, proteins, metal ions	Label-free detection, real-time monitoring	[5]
Field-Effect Transistor (FET) CNT Sensor	Modulation of electrical conductivity via field effect	Biomolecules, gases, pathogens	High selectivity, low power consumption	[6,7]
Piezoelectric CNT Sensor	Strain-induced charge generation	Pressure, vibration, motion	Flexibility, mechanical robustness	[8]
Biosensor with Functionalized CNTs	Specific bioreceptor–analyte interaction	Glucose, DNA, antigens, enzymes	High specificity, enhanced biocompatibility	[9,10]

5. Mechanisms of Sensing with CNTs

5.1. Electrical Conductivity Changes

Carbon nanotubes (CNTs) possess extraordinary electrical properties, which form the fundamental basis for their application in sensing technologies. One of the most prominent mechanisms of sensing with CNTs is based on changes in their electrical conductivity when exposed to external stimuli such as gases, chemicals, or biomolecules. The intrinsic conductivity of CNTs can vary dramatically upon the adsorption of analytes, making them highly effective for sensitive and selective detection applications [203]. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) exhibit different behaviors in terms of electrical conductivity modulation. In pristine CNTs, the electrical properties are governed by their structural configuration, such as chirality and diameter. For instance, metallic CNTs exhibit ballistic transport behavior, while semiconducting CNTs demonstrate tunable conductivity based on doping or interaction with external species [204]. When analytes interact with the surface of CNTs, they can either donate electrons (n-type behavior) or withdraw electrons (p-type behavior), leading to a measurable change in resistance or conductance. This mechanism is particularly effective in gas sensors, where toxic gases like NH₃, NO₂, CO, or H₂S can be detected due to their electron transfer interactions with the CNT surface [205]. For example, NO₂ acts as an electron acceptor, leading to a decrease in CNT resistance, while NH₃ donates electrons, increasing resistance [206]. Moreover, functionalization of CNTs with metallic nanoparticles or polymer coatings can further enhance sensitivity by creating more active sites for interaction. The formation of heterojunctions between CNTs and other nanomaterials often improves charge transfer mechanisms, facilitating higher sensor performance [207]. In addition to gas sensing, CNT-based conductivity modulation is extensively applied in biosensors for detecting biomolecules such as glucose, proteins, and DNA. The immobilization of enzymes or antibodies on the CNT surface leads to specific biorecognition events, altering charge density and resulting in detectable electrical signals [208]. Another critical aspect is the percolation network in CNT-based composites. The presence of analytes can disrupt or enhance the conductive pathways in the CNT network, leading to significant changes in electrical properties. This percolation threshold behavior has been exploited in pressure sensors, strain sensors, and chemical sensors for enhanced detection performance [209]. The main advantage of conductivity-based sensing mechanisms in CNTs lies in their ultra-fast response time, high sensitivity, and low detection limits. However, challenges such as baseline drift, selectivity issues, and environmental interferences remain areas for further research and development [210].

5.2. Surface Interaction and Adsorption

Carbon nanotubes (CNTs) exhibit exceptional surface interaction and adsorption properties, which make them highly effective in a broad spectrum of sensing applications. These properties stem primarily from their large specific surface area, high aspect ratio, and unique electronic structure, allowing them to interact strongly with various chemical species. In the context of sensing technologies, surface interaction and adsorption mechanisms are vital, as they directly influence the sensitivity, selectivity, and overall performance of CNT-based sensors. Adsorption of target analytes onto the surface of CNTs often induces changes in their electronic properties, such as electrical conductivity or work function, forming the basis of signal transduction in many sensors. The high surface-to-volume ratio of CNTs is one of the most critical factors enhancing their adsorption capabilities. Single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) provide extensive adsorption sites, both externally and within their inner cavities. These adsorption sites enable interaction with diverse analytes, ranging from gases and chemical pollutants to biomolecules and heavy metals. Moreover, the π–π interactions between the conjugated π-electron system of CNTs and aromatic or unsaturated molecules enhance adsorption capacity, leading to efficient capture and detection of specific targets. The Van der Waals forces and electrostatic interactions also contribute significantly to the adsorption mechanisms of CNTs. The weak Van der Waals interactions facilitate reversible adsorption, which is crucial for sensor reusability, whereas stronger covalent or electrostatic interactions may provide stable, long-term binding, improving detection stability for certain analytes. The high density of defect sites and functional groups on the CNT surface, either naturally present or introduced via chemical functionalization, further promotes adsorption efficiency by providing active binding sites. Chemical modification or functionalization of CNTs is a widely employed strategy to enhance their adsorption characteristics. Functional groups such as carboxyl (-COOH), hydroxyl (-OH), amine (-NH₂), or sulfonic (-SO₃H) groups can be introduced onto the CNT surface, improving dispersibility in aqueous media and increasing the number of reactive sites available for adsorption. These functionalized CNTs can selectively interact with specific analytes through hydrogen bonding, electrostatic interactions, or covalent bonding, leading to highly selective and sensitive detection systems. For example, carboxyl-functionalized CNTs have shown excellent adsorption capacities for heavy metal ions like Pb²⁺, Cd²⁺, and Hg²⁺, making them suitable for environmental monitoring applications. Similarly, amine-functionalized CNTs exhibit enhanced adsorption of acidic gases such as CO₂ and SO₂ due to acid-base interactions. The ability to tailor the surface chemistry of CNTs enables their use in a wide range of sensing applications, where the adsorption behavior can be optimized for the target analyte. The adsorption of molecules on CNT surfaces is often described using adsorption isotherms such as the Langmuir and Freundlich models. The Langmuir isotherm assumes monolayer adsorption on a homogeneous surface with a finite number of adsorption sites, while the Freundlich isotherm is applicable to heterogeneous surfaces with varying adsorption energies. Studies have demonstrated that CNTs exhibit adsorption behavior that fits both models, depending on the nature of the analyte and the surface modification of the CNTs. Kinetic studies reveal that the adsorption of molecules onto CNT surfaces generally follows pseudo-second-order kinetics, indicating that chemisorption processes involving valence forces through sharing or exchange of electrons play a dominant role. Fast adsorption kinetics are desirable in sensing applications, enabling rapid response times, which are critical for real-time monitoring systems. Several environmental factors affect the adsorption behavior of CNTs, including temperature, pH, humidity, and the presence of competing species. For instance, an increase in temperature can enhance the diffusion rate of analytes towards the CNT surface, improving adsorption efficiency; however, excessive temperatures may desorb weakly bound molecules, reducing sensitivity. The pH of the environment also influences the ionization state of functional groups on CNTs and the analyte, thereby affecting the adsorption process. Humidity is a critical factor in gas sensing applications, as water molecules can compete with target gases for adsorption sites on CNT surfaces, potentially leading to signal interference. Therefore, strategies such as surface hydrophobic modification or selective functionalization are employed to mitigate the effects of humidity and improve the reliability of CNT-based sensors. CNT-based sensors exploiting surface interaction and adsorption mechanisms have been developed for various applications, including gas sensors, chemical sensors, and biosensors. For example, gas sensors based on pristine or functionalized CNTs can detect toxic gases like NH₃, NO₂, and H₂S at parts-per-billion (ppb) levels due to their excellent adsorption capabilities. Similarly, chemical sensors utilize the adsorption properties of CNTs to detect organic solvents, volatile organic compounds (VOCs), and chemical warfare agents with high sensitivity and selectivity. In biosensing applications, CNTs have been utilized to immobilize biomolecules such as enzymes, antibodies, and DNA strands on their surface, enabling the selective adsorption and detection of biomolecular targets like glucose, proteins, and nucleic acids. The immobilization of biomolecules onto CNTs through adsorption not only preserves their biological activity but also enhances sensor stability and performance. Despite the significant progress in utilizing the surface interaction and adsorption mechanisms of CNTs in sensing applications, challenges remain, particularly in understanding the adsorption dynamics at the molecular level and achieving reproducible sensor fabrication. Advanced characterization techniques, computational modeling, and surface engineering approaches are expected to provide deeper insights into the adsorption phenomena on CNT surfaces, leading to the development of next-generation CNT-based sensors with superior performance.

5.3. Optical Properties in Sensing

Carbon nanotubes (CNTs), particularly single-walled carbon nanotubes (SWCNTs), exhibit unique optical properties that have garnered substantial attention for their application in sensing technologies. These optical characteristics, including photoluminescence (PL), Raman scattering, and absorption in the near-infrared (NIR) region, are highly sensitive to changes in the local environment of CNTs. This sensitivity provides an effective transduction mechanism for detecting chemical, biological, and physical stimuli, making CNT-based optical sensors a promising platform for next-generation sensing technologies [216].

SWCNTs possess direct band gaps that enable them to exhibit fluorescence in the NIR region (900–1600 nm). This feature allows for the development of highly sensitive optical sensors because NIR light offers minimal background interference from biological tissues or environmental matrices [217]. The photoluminescence of CNTs is extremely sensitive to chemical doping, local dielectric environment changes, surface functionalization, and mechanical deformation. This sensitivity allows for the detection of target analytes, where the binding of specific molecules to CNT surfaces leads to measurable shifts in the emission wavelength or intensity [218]. In biosensing applications, functionalized SWCNTs have demonstrated the ability to detect proteins, DNA, and other biomolecules based on PL quenching or enhancement mechanisms [219]. Additionally, the high stability of CNT fluorescence under continuous excitation is beneficial for long-term monitoring applications in real-time sensing platforms [220]. Raman spectroscopy is a powerful technique used to study the vibrational properties of CNTs. The unique radial breathing mode (RBM) and G-band features of CNTs are sensitive to changes in their chemical environment, making Raman scattering an essential tool for sensing applications [221]. For instance, the adsorption of chemical species or the functionalization of CNTs often leads to noticeable shifts in Raman spectra, enabling their use in chemical and gas sensors [222]. Furthermore, CNTs have been used as substrates for surface-enhanced Raman scattering (SERS). When combined with metallic nanoparticles such as gold (Au) or silver (Ag), CNT-based hybrid structures exhibit strong electromagnetic field enhancements, boosting the SERS signal intensity for trace-level detection of analytes [223]. This synergistic effect enhances the capability of CNT-based sensors in detecting low concentrations of pollutants, toxins, or biomolecules with high sensitivity and specificity [224]. CNTs display broad optical absorption characteristics across the visible and NIR regions, with distinct van Hove singularities arising from their one-dimensional structure [225]. The absorption peaks are sensitive to environmental perturbations such as doping, strain, or the presence of adsorbed molecules. This optical behavior is exploited in various sensing applications, particularly for chemical sensing and biosensing in complex environments [226]. For example, CNT-based optical sensors have been developed to detect volatile organic compounds (VOCs), toxic gases, and biomolecules by monitoring changes in their NIR absorption spectra upon analyte interaction [227]. Additionally, the high NIR transparency of biological tissues allows CNTs to function effectively in biomedical sensing applications without interference from the surrounding matrix, facilitating non-invasive diagnostic approaches [228]. CNTs can act as energy donors or acceptors in Förster resonance energy transfer (FRET)-based sensing platforms. This phenomenon occurs when an excited donor fluorophore transfers energy to an acceptor molecule in close proximity, resulting in changes in fluorescence emission [229]. Functionalized CNTs integrated with fluorophores or fluorescent biomolecules have been utilized to detect various analytes, including metal ions, nucleic acids, and proteins [230]. For instance, the binding of target molecules to the CNT surface can induce conformational changes in the attached fluorophores, leading to modulation of the FRET efficiency and a measurable change in fluorescence signal [231]. This approach offers excellent sensitivity and specificity, especially when combined with aptamer-based recognition elements for selective analyte detection [256]. The integration of optical properties in CNT-based sensing provides numerous advantages, including high sensitivity, rapid response, non-invasive detection, and compatibility with remote sensing applications. The inherent NIR fluorescence of SWCNTs enables bioimaging and sensing in living systems without significant photobleaching or background noise [257]. Moreover, the flexibility of surface functionalization allows for the creation of tailored CNT sensors targeting specific analytes across chemical, biological, and environmental domains [258]. However, several challenges remain in the widespread implementation of CNT-based optical sensors. The heterogeneity of CNT samples, particularly in terms of chirality and diameter, can lead to variability in optical responses. Additionally, achieving stable and reproducible functionalization while preserving the intrinsic optical properties of CNTs is a critical hurdle [259]. The potential cytotoxicity of CNTs in biomedical applications also necessitates thorough evaluation and optimization of biocompatibility [260]. The future development of CNT-based optical sensors is likely to benefit from advances in synthesis techniques, enabling the production of chirality-pure CNTs with uniform optical characteristics. The combination of CNTs with other nanomaterials, such as quantum dots, metallic nanoparticles, and two-dimensional materials, is expected to create hybrid sensing platforms with enhanced sensitivity and multifunctionality [261]. Moreover, the integration of CNT optical sensors into wearable and implantable devices holds significant promise for real-time health monitoring, environmental surveillance, and industrial safety applications. The continued research into scalable fabrication processes and biocompatibility optimization will pave the way for the commercialization of CNT-based optical sensing technologies in diverse sectors [262].

5.4. Molecular Recognition and Specificity

Molecular recognition and specificity are vital mechanisms that enhance the sensitivity, selectivity, and functional efficiency of carbon nanotube (CNT)-based sensors in detecting various target analytes. These properties enable CNT-based sensors to identify specific molecules from complex chemical or biological environments, which is essential for applications in biosensing, chemical detection, environmental monitoring, and medical diagnostics. The inherent properties of CNTs, including high aspect ratio, tunable surface chemistry, and unique electronic characteristics, provide an excellent platform for designing sensors with molecular recognition capabilities [208]. One of the key strategies to achieve molecular recognition in CNT-based sensors is surface functionalization. This process involves the covalent or non-covalent attachment of specific functional groups, biomolecules, or polymers onto the surface of CNTs to enhance their interaction with target analytes [263]. Covalent functionalization typically alters the electronic structure of CNTs but provides strong and stable bonding with recognition elements, such as antibodies, DNA aptamers, or enzymes [133]. Non-covalent functionalization preserves the intrinsic properties of CNTs while enabling π–π stacking, van der Waals interactions, or electrostatic forces to immobilize recognition elements [264]. Functional groups such as carboxyl (-COOH), hydroxyl (-OH), and amine (-NH₂) groups introduced onto CNTs facilitate specific interactions with biomolecules and chemical species, improving selectivity and reducing non-specific adsorption [85]. For example, carboxylated CNTs have been utilized for immobilizing proteins and antibodies to enable selective detection of cancer biomarkers and pathogens [265]. Moreover, polymers such as polyethylene glycol (PEG) and polyaniline (PANI) have been integrated onto CNT surfaces to enhance molecular recognition and biocompatibility [109]. The incorporation of biomolecular probes such as antibodies, aptamers, peptides, and molecularly imprinted polymers (MIPs) onto CNTs further enhances the specificity of sensing platforms [266]. Antibody-functionalized CNTs are widely used in immunosensors due to their ability to bind selectively with antigens, enabling the detection of pathogens, toxins, or disease-related proteins at ultra-low concentrations. Aptamers, which are short oligonucleotide or peptide sequences, exhibit high binding affinity and specificity towards target molecules such as small drugs, proteins, or metal ions. CNT-based aptasensors have shown remarkable sensitivity and selectivity in detecting biomolecules like thrombin, adenosine, and glucose in complex biological samples [267]. Similarly, molecularly imprinted polymers (MIPs), synthesized with specific cavities complementary to target analytes, are used to impart molecular recognition capabilities to CNT-based chemical sensors. Molecular recognition in CNT-based sensors operates through several mechanisms, depending on the nature of the interaction between the functionalized CNT surface and the target analyte. These mechanisms include hydrogen bonding, electrostatic interaction, π–π stacking, hydrophobic interaction, and covalent bonding [268]. The selective binding of target molecules on the CNT surface alters the local electronic environment, leading to changes in electrical conductivity, resistance, or optical properties, which form the basis of signal transduction in sensing devices. For instance, in electrochemical sensors, the binding of specific biomolecules to the functionalized CNT surface induces charge transfer or modulation of the Fermi level, resulting in measurable electrical signals [15]. In fluorescence-based sensors, molecular recognition leads to quenching or enhancement of fluorescence intensity due to energy transfer between the CNTs and the analyte. Molecular recognition in CNT-based sensors has enabled diverse applications across various fields. In medical diagnostics, CNT sensors functionalized with antibodies or aptamers have demonstrated high specificity in detecting disease biomarkers such as cardiac troponin, prostate-specific antigen (PSA), and glucose [269]. In environmental monitoring, CNT sensors equipped with MIPs or functionalized polymers have been used for selective detection of pesticides, heavy metals, and toxic gases. Moreover, CNT-based biosensors capable of detecting DNA sequences, proteins, or viruses have shown potential for early disease diagnosis and personalized healthcare. The ability of CNTs to differentiate structurally similar molecules, such as glucose from fructose or specific bacterial strains from complex microbial populations, highlights their superior specificity compared to conventional sensors. Despite significant progress, certain challenges remain in enhancing molecular recognition and specificity in CNT-based sensors. Non-specific adsorption, biofouling, and signal interference from complex sample matrices can limit sensor performance. Advanced functionalization techniques, development of novel recognition elements, and integration of nanocomposites or hybrid materials are being explored to overcome these limitations. Future research is focusing on multiplexed sensing platforms, where CNT-based sensors can detect multiple analytes simultaneously with high specificity. The combination of CNTs with emerging materials such as graphene, metal–organic frameworks (MOFs), and quantum dots is anticipated to further improve the molecular recognition capabilities of next-generation sensors [270]. Additionally, machine learning and data analysis techniques are being integrated with CNT-based sensing systems to enhance selectivity and reduce false-positive results. In summary, molecular recognition and specificity are fundamental to the successful application of CNT-based sensors in real-world environments. Through innovative functionalization strategies and integration with biomolecular probes, CNT-based sensing platforms continue to advance towards highly selective, sensitive, and reliable detection systems for healthcare, environmental monitoring, and industrial applications.

Table 5. Comparative Summary of Mechanisms of Sensing with Carbon Nanotubes (CNTs).

Sensing Mechanism	Working Principle	Key Advantages	Limitations	References
Chemiresistive Mechanism	Change in electrical resistance due to analyte adsorption on CNT surface	Simple design, fast response, low cost	Poor selectivity, environmental sensitivity	[271]
Field-Effect Transistor (FET) Mechanism	Modulation of current flow in CNT channel under applied electric field after analyte interaction	High sensitivity, low power consumption	Complex fabrication, limited stability	[272]
Electrochemical Mechanism	Electron transfer between analyte and CNT-modified electrode surface	High sensitivity, real-time monitoring	Need for electrolyte, possible fouling	[273]
Optical Sensing Mechanism	Change in optical properties (fluorescence, absorbance, Raman scattering) upon analyte binding	Non-invasive, label-free detection	Optical signal instability, expensive equipment	[274]
Piezoelectric/Strain Sensing Mechanism	Mechanical deformation induces electrical signals in CNT composites	High flexibility, suitable for wearable sensors	Limited to mechanical stimuli sensing	[275]

Table 5. Comparative Summary of Mechanisms of Sensing with Carbon Nanotubes (CNTs).

Sensing Mechanism	Working Principle	Key Advantages	Limitations	References
Chemiresistive Mechanism	Change in electrical resistance due to analyte adsorption on CNT surface	Simple design, fast response, low cost	Poor selectivity, environmental sensitivity	[271]
Field-Effect Transistor (FET) Mechanism	Modulation of current flow in CNT channel under applied electric field after analyte interaction	High sensitivity, low power consumption	Complex fabrication, limited stability	[272]
Electrochemical Mechanism	Electron transfer between analyte and CNT-modified electrode surface	High sensitivity, real-time monitoring	Need for electrolyte, possible fouling	[273]
Optical Sensing Mechanism	Change in optical properties (fluorescence, absorbance, Raman scattering) upon analyte binding	Non-invasive, label-free detection	Optical signal instability, expensive equipment	[274]
Piezoelectric/Strain Sensing Mechanism	Mechanical deformation induces electrical signals in CNT composites	High flexibility, suitable for wearable sensors	Limited to mechanical stimuli sensing	[275]

Table 5 illustrates the diverse mechanisms through which CNT-based sensors operate for detecting various chemical, biological, or physical analytes. Each sensing mechanism utilizes the inherent unique properties of CNTs such as their high electrical conductivity, large surface area, and remarkable mechanical strength. The chemiresistive sensing mechanism is one of the simplest and most widely used in CNT-based sensors. It involves the adsorption of target analytes (gases or vapors) on the surface of CNTs, leading to a measurable change in electrical resistance. The major advantages include ease of fabrication, rapid response time, and cost-effectiveness [247]). However, the mechanism often suffers from poor selectivity and high sensitivity to environmental factors like humidity and temperature. CNT–FET sensors work by modulating the current flowing through a semiconducting CNT channel in response to an external electric field altered by analyte binding. This mechanism offers excellent sensitivity and low power consumption, making it suitable for biosensing applications. However, device fabrication can be complex, and stability under varying conditions remains challenging [272]. Electrochemical sensing with CNTs relies on facilitating electron transfer reactions between the analyte and the electrode surface modified with CNTs. This mechanism provides a high sensitivity platform suitable for detecting biomolecules, heavy metals, and environmental pollutants [249]). The limitations include the requirement for an electrolyte medium and potential electrode fouling during long-term use. Optical sensors based on CNTs utilize changes in fluorescence intensity, absorbance, or Raman scattering properties when analytes interact with CNT surfaces. These sensors provide non-invasive and label-free detection with real-time monitoring capability. Nevertheless, they require sophisticated optical equipment and suffer from signal instability under varying environmental conditions [250]). In this mechanism, CNT-based composites generate an electrical signal when subjected to mechanical deformation like pressure, strain, or vibration. It is highly suitable for flexible and wearable sensors used in healthcare monitoring and structural health detection [251]). However, it is generally limited to physical stimuli sensing rather than chemical or biological detection.

6. Advancements in CNT-Based Sensing

6.1. Nanostructured CNT Sensors for Enhanced Sensitivity

The advancement of nanostructured carbon nanotube (CNT)-based sensors has revolutionized the field of sensing technologies, particularly in enhancing sensitivity and detection limits. The intrinsic one-dimensional (1D) structure of CNTs, coupled with their nanoscale diameter, large surface area, and outstanding electrical conductivity, positions them as superior candidates for high-performance sensing devices. However, to further optimize their sensing performance, extensive research has been devoted to engineering nanostructured configurations of CNTs, such as aligned arrays, hierarchical architectures, and hybrid nanocomposites, that significantly augment their sensitivity and selectivity in various applications [21].

6.1.1. Strategies for Nanostructured CNT Sensors

One of the critical strategies for enhancing sensitivity is the fabrication of vertically aligned CNT (VACNT) arrays. These structures provide a highly accessible surface area for analyte adsorption, facilitating rapid electron transport and signal transduction. VACNTs act as effective sensing scaffolds due to their high aspect ratio and reduced contact resistance, which amplify the detection of low-concentration analytes [276]. For instance, CNT forests have been shown to detect trace levels of gases and biomolecules with improved sensitivity compared to randomly dispersed CNT networks. Another significant approach involves the development of hierarchical CNT-based structures, where CNTs are integrated with other nanostructures such as nanoparticles, nanowires, or porous materials to create multi-level architectures. These hierarchical systems offer synergistic effects by increasing the density of active sites, enhancing electron mobility, and promoting selective adsorption of target molecules. Metal-decorated CNTs, such as gold (Au), platinum (Pt), and palladium (Pd) nanoparticles, have demonstrated superior sensitivity for gas sensing due to the catalytic properties of the metal nanoparticles facilitating analyte dissociation and interaction [277]. Furthermore, the modification of CNT surfaces with functional groups or polymers can enhance sensitivity by improving the affinity towards specific analytes. Functional groups such as carboxyl (-COOH), hydroxyl (-OH), and amine (-NH2) have been extensively utilized to tailor the chemical environment of CNTs, thereby promoting selective binding and interaction with target species.

Figure 7 illustrates the stepwise fabrication and underlying electronic behavior of a hybrid sensor platform composed of multi-walled carbon nanotubes (MWCNTs) and tin dioxide (SnO₂) nanowires (NWs) [219]. This heterojunction architecture combines the high surface area, chemical stability, and electrical conductivity of MWCNTs with the semiconducting properties and gas-sensitivity of SnO₂ NWs to produce a highly responsive and selective sensing interface. The process begins with the deposition of platinum (Pt) electrodes, followed by the selective growth of SnO₂ NWs, which provide a crystalline, high-sensitivity matrix for charge transfer. Subsequent deposition of MWCNTs results in the formation of a p–n heterojunction, where the interaction between the p-type CNTs and n-type SnO₂ creates a depletion layer at the interface, modulating the charge carrier dynamics. The band diagram reveals a potential barrier that is sensitive to gas adsorption, thereby influencing the electron flow across the junction. This hybrid interface significantly enhances gas detection capabilities, as the resistance changes upon analyte exposure due to band bending effects and carrier modulation. The equivalent circuit model helps to conceptualize the overall charge transport and sensor response mechanisms, underlining the synergistic behavior of the hybrid nanostructure in enabling rapid, sensitive, and stable sensing performance under ambient conditions.

6.1.2. Enhanced Electrical and Electrochemical Properties

Nanostructuring CNTs not only improves surface interaction but also optimizes their electrical and electrochemical properties, which are essential for signal amplification in sensors. For example, doping CNTs with heteroatoms such as nitrogen (N), boron (B), or sulfur (S) can modulate their bandgap, increase carrier density, and enhance conductivity, all of which contribute to higher sensitivity [278]. Nitrogen-doped CNTs (N-CNTs) have shown exceptional performance in electrochemical sensors for detecting biomolecules like glucose, dopamine, and uric acid due to their increased active sites and improved electron transfer kinetics. Additionally, CNT-based field-effect transistors (CNT–FETs) have emerged as a promising platform for highly sensitive detection. The semiconducting properties of single-walled CNTs (SWCNTs) make them suitable for FET sensor devices, where changes in the surface potential induced by analyte adsorption modulate the channel conductance, resulting in detectable signals at ultra-low analyte concentrations [64].

6.1.3. Nanocomposites and Hybrid Materials

The integration of CNTs with other nanomaterials has led to the development of CNT-based nanocomposites with enhanced sensing properties. Incorporating metal oxides (e.g., ZnO, SnO₂, and TiO₂), conducting polymers (e.g., polyaniline and polypyrrole), or graphene into CNT networks provides multifunctionality, improves stability, and increases sensitivity [185]. These nanocomposites exhibit improved charge transfer, higher surface area, and enhanced interaction with target analytes. For example, CNT–graphene hybrids combine the high surface area of CNTs with the excellent electrical conductivity of graphene, resulting in superior performance for gas sensing and biosensing applications. Nanostructured CNT/polymer composites have also gained attention for flexible and wearable sensors. Polymers impart mechanical flexibility and processability, while CNTs provide electrical conductivity and sensitivity. These hybrid systems are particularly useful for strain sensors, pressure sensors, and wearable health monitoring devices [279].

6.1.4. Applications in Ultra-Sensitive Detection

Nanostructured CNT sensors have been successfully employed in diverse applications requiring ultra-sensitive detection. Gas sensors based on VACNTs or metal-decorated CNTs exhibit detection limits down to parts-per-billion (ppb) or even parts-per-trillion (ppt) levels for toxic gases such as NO₂, NH₃, and H₂S [280]. Similarly, biosensors utilizing CNT-based nanostructures have demonstrated femtomolar (fM) sensitivity for biomarker detection in medical diagnostics, offering rapid and accurate analysis of biological samples. Electrochemical sensors using CNT nanocomposites have shown significant improvements in detecting heavy metals, pesticides, and pharmaceutical residues in environmental monitoring due to enhanced electron transfer and analyte adsorption [281]. The continuous development of nanostructured CNT sensors is anticipated to address existing challenges in sensitivity, selectivity, stability, and reproducibility. Emerging fabrication techniques such as 3D printing, electrospinning, and atomic layer deposition offer new opportunities for constructing precisely engineered CNT architectures. Moreover, combining CNTs with machine learning algorithms for data analysis can further improve sensor performance and enable real-time monitoring in complex environments. In conclusion, the advancement of nanostructured CNT-based sensors holds great promise for next-generation sensing technologies across environmental, medical, and industrial domains. Their superior sensitivity, combined with tailored functionalization and integration with other nanomaterials, paves the way for highly efficient and reliable sensing platforms.

6.2. Integration of CNTs with Other Nanomaterials

The integration of carbon nanotubes (CNTs) with other nanomaterials has emerged as a transformative approach to overcome certain limitations of pristine CNT-based sensors and to enhance their overall performance characteristics, including sensitivity, selectivity, response time, and stability [31]. This synergistic strategy leverages the unique properties of CNTs such as their extraordinary electrical conductivity, high mechanical strength, and large surface area with complementary features provided by other nanomaterials, such as metal nanoparticles, metal oxides, polymers, and two-dimensional (2D) materials like graphene. This section explores the recent advancements and methodologies employed in integrating CNTs with various nanomaterials for developing high-performance sensing platforms across diverse applications.

6.2.1. Metal Nanoparticles and CNT Hybrids

The decoration of CNTs with metal nanoparticles (NPs) such as gold (Au), platinum (Pt), silver (Ag), palladium (Pd), and copper (Cu) has been extensively investigated to improve sensing capabilities, particularly in gas and chemical sensors. Metal NPs act as catalytic centers or electron donors/acceptors, facilitating enhanced interaction between the analyte molecules and the CNT surface. For example, Pd-decorated CNTs have shown remarkable sensitivity towards hydrogen (H₂) detection due to the high affinity of Pd for hydrogen adsorption, leading to changes in the electronic structure of the CNTs and measurable electrical responses [282]. Moreover, noble metals such as Au and Pt enhance the biocompatibility of CNTs, making them suitable for biosensing applications. Gold nanoparticle (AuNP)–CNT hybrids have been used for the detection of glucose, DNA, and proteins, where the AuNPs serve as electrochemical signal amplifiers or bio-recognition sites.

6.2.2. Metal Oxide–CNT Nanocomposites

The combination of CNTs with semiconducting metal oxides, including zinc oxide (ZnO), tin oxide (SnO₂), titanium dioxide (TiO₂), and iron oxide (Fe₂O₃), has led to the development of highly sensitive chemical and gas sensors. Metal oxides offer superior chemical activity and selectivity, while CNTs provide excellent charge transport properties. The integration of these materials results in sensors with lower detection limits, enhanced stability, and faster response-recovery behavior [283]. For instance, ZnO–CNT nanocomposites have demonstrated improved sensing performance for volatile organic compounds (VOCs) and toxic gases like nitrogen dioxide (NO₂) and ammonia (NH₃) due to the synergistic interaction between ZnO and CNTs that facilitates charge transfer and enhances adsorption sites.

6.2.3. Polymer–CNT Hybrid Sensors

The incorporation of conductive polymers such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) with CNTs has opened new avenues for flexible and wearable sensing devices. Polymers can be functionalized to introduce selective binding sites, improve dispersion of CNTs in solution, and enhance the mechanical flexibility of the sensor platform [80]. PANI–CNT composites, for example, have been employed for gas sensing due to the redox-active nature of PANI, which changes conductivity upon exposure to gases like ammonia and hydrogen sulfide. Similarly, PPy–CNT hybrids have shown potential in biosensors for detecting glucose and cholesterol, where the polymer matrix enhances biocompatibility while CNTs provide the necessary electrical pathways.

6.2.4. CNT–Graphene and 2D Nanomaterial Hybrids

The integration of CNTs with two-dimensional (2D) nanomaterials, particularly graphene, has been a significant breakthrough in sensor technology. Both CNTs and graphene possess exceptional electrical, mechanical, and thermal properties, and their hybridization results in unique conductive networks with high surface area and tunable electronic properties [284,285]. CNT–graphene hybrids have demonstrated superior sensing performance in detecting gases, biomolecules, and environmental pollutants. For example, a graphene–CNT hybrid sensor has been reported for ultra-sensitive detection of NO₂ with a detection limit in the parts-per-billion (ppb) range due to enhanced adsorption and charge transfer processes [286]. Additionally, other 2D materials like molybdenum disulfide (MoS₂), black phosphorus (BP), and hexagonal boron nitride (h-BN) have been combined with CNTs to fabricate multifunctional sensing devices with improved selectivity and multi-analyte detection capabilities. Future research is anticipated to focus on scalable and cost-effective fabrication techniques for CNT-nanomaterial hybrids, the development of multi-functional sensing platforms capable of detecting multiple analytes simultaneously, and the integration of these sensors into smart systems for real-time monitoring through wireless communication and Internet of Things (IoT) frameworks [287]. Additionally, addressing issues such as sensor reproducibility, long-term stability, and environmental safety will be critical to the commercialization of CNT-based hybrid sensors.

6.3. Smart Sensors and Internet of Things (IoT) Applications

The rapid evolution of the Internet of Things (IoT) has revolutionized the sensing landscape, integrating advanced materials like carbon nanotubes (CNTs) into smart sensors that are capable of real-time monitoring, wireless communication, and intelligent data processing. CNT-based smart sensors, due to their exceptional electrical, chemical, and mechanical properties, are increasingly being employed in IoT-driven applications across healthcare, environmental monitoring, smart cities, industrial automation, and agriculture [288]. The integration of CNT sensors within the IoT framework provides new avenues for the development of highly sensitive, miniaturized, energy-efficient, and multifunctional devices. CNTs offer several intrinsic advantages that align well with the requirements of IoT-enabled sensors. Their large surface area, high aspect ratio, excellent electrical conductivity, and chemical tunability enable the detection of a wide variety of analytes at ultra-low concentrations. Additionally, their ability to undergo surface functionalization enhances their specificity towards targeted molecules or environmental conditions, a critical feature for smart sensing systems. In IoT environments, CNT-based sensors are designed to operate autonomously with minimal energy consumption. These sensors are capable of interfacing with wireless modules such as Bluetooth, Zigbee, or LoRa, enabling seamless data transmission to cloud-based platforms for real-time analysis and decision making [289]. CNT-based smart sensors have shown significant promise in wearable devices for personalized healthcare monitoring. Sensors integrated into wearable fabrics or flexible substrates can detect physiological parameters such as sweat biomarkers, heart rate, respiration, and body temperature. For example, CNT-based electrochemical sensors have been incorporated into smart wristbands for continuous glucose monitoring in diabetic patients, providing real-time data that can be transmitted to smartphones or healthcare providers [290]. IoT-based environmental monitoring systems utilizing CNT sensors offer real-time detection of air quality parameters, including toxic gases (NO₂, CO, SO₂), volatile organic compounds (VOCs), humidity, and temperature. These systems contribute to the development of smart cities by enabling the deployment of dense sensor networks for continuous air quality monitoring, thus facilitating rapid response to pollution incidents and environmental hazards. In industrial settings, CNT-based IoT sensors are employed for predictive maintenance, monitoring of hazardous gases, and ensuring workplace safety. In smart agriculture, CNT sensors are used to monitor soil moisture, nutrient levels, and pest presence, optimizing irrigation and fertilizer use, which enhances crop yield while reducing environmental impact [173]. Despite the significant progress, several challenges remain in the deployment of CNT-based smart sensors for IoT applications. Key issues include the large-scale manufacturing of uniform CNT materials, sensor calibration, long-term stability, and integration with energy-harvesting systems to enable self-powered devices. Future research is focusing on developing hybrid CNT materials with self-healing properties, multi-functional sensing capabilities, and machine learning algorithms for intelligent signal processing and anomaly detection. In conclusion, the integration of CNT-based sensors within the IoT ecosystem represents a paradigm shift in sensing technology, offering transformative solutions for real-time, wireless, and smart monitoring systems across various domains. The continued advancement in nanomaterials engineering, coupled with innovations in data analytics and wireless communication, will undoubtedly accelerate the commercialization and widespread adoption of CNT-enabled IoT sensing technologies.

6.4. Wearable and Portable CNT-Based Sensors

The growing demand for wearable and portable sensing devices has stimulated significant research into the development of advanced materials that offer flexibility, sensitivity, miniaturization, and biocompatibility. Carbon nanotubes (CNTs), due to their outstanding mechanical strength, flexibility, electrical conductivity, and high surface-to-volume ratio, have emerged as leading candidates for next-generation wearable and portable sensor technologies. These sensors are designed to monitor physiological, environmental, and chemical parameters in real-time, enabling applications in personalized healthcare, fitness tracking, human–machine interfaces, and environmental monitoring [208]. The unique structural features of CNTs allow for their integration into flexible and stretchable substrates without compromising their electrical performance. CNTs exhibit high Young’s modulus, excellent tensile strength, and the ability to maintain conductivity under mechanical deformation, making them ideal for applications in wearable sensors subjected to frequent bending, stretching, or twisting [291]. Furthermore, their ability to form conductive networks within polymer matrices or textiles facilitates the fabrication of lightweight, breathable, and comfortable sensing platforms suitable for continuous use on the human body. CNT-based wearable sensors have demonstrated significant utility in non-invasive health monitoring systems. These devices can detect physiological signals such as pulse rate, blood pressure, respiration, body temperature, and even specific biomarkers present in sweat or interstitial fluids. For example, CNT-modified electrodes have been incorporated into flexible patches for electrocardiogram (ECG) monitoring, providing high-fidelity signal detection with minimal skin irritation [292]. Additionally, CNT-based electrochemical sensors have been utilized for the detection of biomarkers like glucose, lactate, and cortisol, enabling real-time metabolic monitoring in wearable health devices [293].

Table 6. Comparative Summary of Wearable and Portable CNT-Based Sensors.

Type of Sensor Device	Sensing Target	Device Configuration	Key Features	References
CNT-Based Sweat Sensor	Electrolytes (Na+, K+), glucose, lactate	Flexible CNT electrode on patch or textile	Non-invasive monitoring, real-time analysis	[53]
CNT-Based Strain/Pressure Sensor	Body motion, pulse, respiration	CNT/polymer composite films or fibers	High flexibility, stretchability, skin-conformability	[294,295]
CNT-Based Gas Sensor Wearable	Volatile organic compounds (VOCs), NH3, CO	CNT-coated flexible substrates or masks	Lightweight, low power, room temperature sensing	[204]
CNT-Based Temperature Sensor	Body temperature monitoring	CNT-integrated fabric or tattoo sensors	Continuous monitoring, fast response	[296]
CNT-Based Biosensor Patch	Biomolecules (glucose, DNA, uric acid)	Functionalized CNT arrays on skin patches	High sensitivity, biocompatible, portable	[297]
CNT-Integrated Smart Textiles	Multiple parameters (pressure, strain, moisture)	CNT yarns, CNT-coated fibers in fabric	Washable, durable, multiplexed sensing	[298]

Table 6. Comparative Summary of Wearable and Portable CNT-Based Sensors.

Type of Sensor Device	Sensing Target	Device Configuration	Key Features	References
CNT-Based Sweat Sensor	Electrolytes (Na+, K+), glucose, lactate	Flexible CNT electrode on patch or textile	Non-invasive monitoring, real-time analysis	[53]
CNT-Based Strain/Pressure Sensor	Body motion, pulse, respiration	CNT/polymer composite films or fibers	High flexibility, stretchability, skin-conformability	[294,295]
CNT-Based Gas Sensor Wearable	Volatile organic compounds (VOCs), NH3, CO	CNT-coated flexible substrates or masks	Lightweight, low power, room temperature sensing	[204]
CNT-Based Temperature Sensor	Body temperature monitoring	CNT-integrated fabric or tattoo sensors	Continuous monitoring, fast response	[296]
CNT-Based Biosensor Patch	Biomolecules (glucose, DNA, uric acid)	Functionalized CNT arrays on skin patches	High sensitivity, biocompatible, portable	[297]
CNT-Integrated Smart Textiles	Multiple parameters (pressure, strain, moisture)	CNT yarns, CNT-coated fibers in fabric	Washable, durable, multiplexed sensing	[298]

Table 6 provides a comparative analysis of different types of wearable and portable sensors based on carbon nanotubes (CNTs), highlighting their sensing targets, device configurations, and essential performance features. Wearable CNT-based sensors represent a rapidly growing class of devices designed for continuous health monitoring, environmental sensing, and human motion detection. Their flexibility, lightweight structure, and excellent mechanical strength enable integration with fabrics, skin patches, or flexible substrates for non-invasive, real-time sensing. Sweat sensors based on CNTs have gained significant attention for monitoring electrolytes (Na⁺, K⁺), metabolites (glucose, lactate), and pH in human sweat [53]. Their flexibility and skin conformity allow comfortable wear during daily activities or sports. Similarly, CNT-based strain and pressure sensors enable the detection of subtle body movements, pulse monitoring, or respiratory rates due to the piezoresistive nature of CNT–polymer composites [294]. Wearable CNT-based gas sensors have been developed for environmental monitoring and detecting hazardous gases (NH₃, CO, VOCs) in real-time [204]. Their operation at room temperature and low power consumption make them ideal for portable use in masks or clothing. Temperature sensors based on CNTs offer rapid response and are suitable for continuous body temperature monitoring through flexible tattoos or CNT-coated fabrics [296]. Moreover, wearable biosensor patches incorporating functionalized CNTs enable the detection of specific biomolecules such as glucose, DNA, or uric acid with high sensitivity and specificity [297]. Finally, CNT-integrated smart textiles provide a platform for multi-functional sensing capable of monitoring pressure, strain, moisture, or temperature within a single fabric-based device [10]. These smart textiles exhibit durability, washability, and scalability, making them highly suitable for next-generation wearable technologies. Overall, wearable and portable CNT-based sensors offer remarkable potential for personalized healthcare, sports monitoring, smart clothing, and environmental surveillance, paving the way for future advancements in wearable electronics and Internet of Things (IoT)-enabled health monitoring systems.

Portable sensors based on CNT technology have been developed for environmental monitoring, capable of detecting pollutants such as volatile organic compounds (VOCs), toxic gases, and heavy metals in real time. These portable devices, often coupled with wireless communication modules, facilitate rapid field-deployable sensing with high sensitivity and selectivity [266]. For example, CNT-based gas sensors integrated into portable devices have shown excellent performance in detecting hazardous gases such as ammonia, nitrogen dioxide, and carbon monoxide at trace levels, contributing to environmental safety and public health [298]. The integration of CNTs into smart textiles has opened new avenues for multifunctional wearable electronics. Conductive CNT-based yarns and fabrics have been developed for sensing applications, enabling the creation of garments that can monitor motion, posture, or physiological signals. These smart textiles can also act as energy storage devices or energy harvesters, providing self-powered sensing capabilities for wearable systems [299]. Despite the significant advancements, several challenges remain in the development of wearable and portable CNT-based sensors. Issues such as long-term stability, biocompatibility, large-scale fabrication, and cost-effectiveness need to be addressed for commercial translation. Furthermore, ensuring reliable sensor performance under dynamic conditions, including mechanical stress, humidity, and temperature variations, is critical for practical applications. Future research is expected to focus on hybridizing CNTs with other nanomaterials, such as graphene, metal nanoparticles, and conductive polymers, to enhance sensor performance and multifunctionality. Additionally, the integration of artificial intelligence (AI) and machine learning algorithms with CNT-based wearable devices will enable advanced data analysis, pattern recognition, and predictive diagnostics, revolutionizing the landscape of personalized healthcare and environmental monitoring [300].

6.5. Integration of CNT-Based Sensors with AI, Machine Learning, and IoT Technologies

The emergence of smart technologies has initiated a paradigm shift in sensor development, transforming passive detectors into intelligent sensing platforms. Carbon nanotube (CNT)-based sensors, known for their exceptional sensitivity, rapid response times, and high surface-to-volume ratios, are increasingly being integrated with advanced computational frameworks such as artificial intelligence (AI), machine learning (ML), and artificial neural networks (ANNs) to extract meaningful insights from complex datasets. These integrations are paving the way for real-time, adaptive, and predictive sensing systems that are central to next-generation applications in healthcare, environmental monitoring, industrial diagnostics, and wearable electronics.

Machine Learning (ML) and Artificial Neural Networks (ANNs): The use of ML and ANNs allows for the automated interpretation of signals generated by CNT sensors. Since the electrical response of CNTs can be influenced by multiple overlapping stimuli such as variations in humidity, temperature, or interfering chemical species traditional data processing techniques often fall short in discriminating these interactions. ML algorithms such as support vector machines (SVM), k-nearest neighbors (KNN), and decision trees can be trained on labeled sensor data to classify analytes or detect specific environmental conditions with high precision [301]. Meanwhile, ANNs mimic the human brain’s pattern recognition capabilities and are highly effective for nonlinear data analysis, especially in biosensing platforms where subtle biological interactions must be resolved. For example, CNT-based electronic noses (e-noses) have been developed for volatile organic compound (VOC) detection using ANN-based models that can distinguish complex odor profiles with high sensitivity and specificity. These systems are instrumental in medical diagnostics, such as breath analysis for disease detection, where nuanced chemical patterns must be analyzed rapidly and accurately.

Artificial Intelligence (AI): Beyond conventional ML models, AI enables adaptive learning and decision-making capabilities in CNT-based sensors. AI algorithms, including deep learning frameworks like convolutional neural networks (CNNs) and recurrent neural networks (RNNs), are capable of processing large-scale time-series data and recognizing patterns that evolve dynamically. In structural health monitoring, for instance, AI can interpret signals from CNT-enhanced strain sensors to predict material fatigue or impending failure, supporting preventative maintenance in aerospace and civil engineering infrastructures [302]. The integration of AI also allows for anomaly detection and automated fault diagnosis in complex systems. CNT sensors embedded in wearable devices can provide continuous physiological monitoring, where AI algorithms analyze biosignals to detect irregularities such as arrhythmias or respiratory distress in real time. This transition from static measurement to predictive analytics represents a major leap in personalized medicine and remote healthcare.

Internet of Things (IoT): The synergy between CNT-based sensors and IoT networks creates interconnected sensing platforms that facilitate continuous data acquisition, transmission, and cloud-based analysis. CNT sensors’ miniaturized dimensions and low power requirements make them ideal candidates for IoT deployment. These sensors can be embedded into smart fabrics, environmental stations, or industrial equipment, where they wirelessly communicate with edge devices or cloud servers for centralized monitoring and decision making. In environmental monitoring, for instance, CNT-based gas sensors distributed across a geographic area can form a real-time air quality surveillance network. These sensors transmit their data via IoT protocols such as LoRaWAN or NB-IoT, which are optimized for low-power wide-area networks. The cloud-based infrastructure allows for large-scale data aggregation and visualization, while AI/ML algorithms detect trends and issue early warnings of pollution events or hazardous gas leaks.

Despite the immense promise, challenges remain in ensuring the seamless integration of CNT sensors with AI and IoT ecosystems. Sensor calibration, data standardization, cybersecurity, and energy autonomy are critical issues that need to be addressed. Moreover, the development of universally accessible platforms for sensor data fusion and machine learning model deployment is necessary for broader scalability. Nevertheless, ongoing research is rapidly advancing these areas. The development of self-powered CNT sensors using energy-harvesting technologies, combined with edge AI chips for local data processing, represents a future where smart sensing becomes ubiquitous. These innovations are expected to revolutionize real-time detection systems across disciplines, ultimately leading to more responsive, intelligent, and resilient sensor networks.

So the convergence of CNT-based sensing with AI, ML, and IoT technologies marks a transformative era in sensing science. This multidisciplinary integration not only enhances the analytical power of sensors but also aligns with the vision of smart, autonomous, and interconnected systems capable of addressing complex challenges in real-world environments.

7. Challenges in CNT-Based Sensing Technologies

7.1. Functionalization and Stability Issues

Functionalization involves modifying the surface of CNTs to enhance their interaction with specific analytes, thereby improving sensor performance. This modification can be achieved through covalent or non-covalent methods:

• Covalent Functionalization: This approach entails forming chemical bonds between functional groups and the CNT structure. While it can significantly enhance solubility and provide specific binding sites, it may introduce defects into the CNTs, potentially altering their intrinsic properties such as electrical conductivity and mechanical strength. For instance, oxidation processes can introduce carboxyl groups, but excessive oxidation can compromise the CNT structure.
• Non-Covalent Functionalization: This method relies on physical interactions, such as π–π stacking or van der Waals forces, to attach functional molecules to the CNT surface without altering its inherent structure. While this preserves the CNTs’ original properties, the stability of the attached molecules can be a concern, as desorption may occur over time.

Achieving long-term stability of functionalized CNTs is critical for practical sensor applications. Several factors influence this stability:

• Environmental Factors: Exposure to varying temperatures, humidity levels, and chemical environments can affect the integrity of functionalized CNTs. For example, high humidity can lead to the desorption of non-covalently attached molecules, reducing sensor reliability [198].
• Chemical Stability: Functional groups introduced during covalent functionalization may degrade over time or under specific conditions, leading to a loss of functionality. Ensuring that these groups remain stable throughout the sensor’s operational life is a significant challenge.
• Mechanical Stability: The process of functionalization, especially covalent methods, can introduce defects that compromise the mechanical integrity of CNTs. This can affect the durability and lifespan of the sensor [8].

To address these challenges, researchers have explored various strategies:

• Optimized Functionalization Techniques: Carefully controlling reaction conditions during covalent functionalization can minimize defects. For instance, using milder oxidizing agents or shorter reaction times can reduce damage to the CNT structure [303].
• Protective Coatings: Applying protective polymer coatings can shield functionalized CNTs from environmental factors, enhancing their stability without significantly impacting their sensing capabilities [50].
• Hybrid Functionalization: Combining covalent and non-covalent methods can leverage the advantages of both approaches, achieving stable functionalization while preserving the CNTs’ intrinsic properties [304].

7.2. Sensitivity and Selectivity Limitations

CNT-based sensors are renowned for their high sensitivity, attributed to their large surface area and excellent electrical properties. However, several factors can influence this sensitivity:

• Baseline Drift: Over time, CNT sensors may exhibit changes in baseline resistance or current, affecting the accuracy of measurements. This drift can result from environmental factors or the gradual desorption of functional groups [305].
• Response Time: While CNT sensors often exhibit rapid response times, certain functionalizations or environmental conditions can slow the interaction between the analyte and the sensor surface, delaying detection [195].

Achieving high selectivity—the ability to distinguish a specific analyte in the presence of others—is a significant hurdle:

• Non-Specific Binding: CNTs can interact with a wide range of molecules, leading to non-specific binding and false positives. For instance, gases like NH₃ and NO₂ can both donate or accept electrons, making it challenging to differentiate between them using pristine CNTs [90].
• Environmental Interference: Factors such as humidity and temperature can affect sensor responses. High humidity levels, for example, can lead to water molecule adsorption, altering the sensor’s baseline and response to target analytes.

To overcome these limitations, several approaches have been explored:

• Specific Functionalization: Introducing functional groups or biomolecules that have a high affinity for the target analyte can enhance selectivity. For example, attaching antibodies specific to a biomarker can enable the detection of that biomarker amidst a complex mixture [306].
• Hybrid Nanomaterials: Combining CNTs with other nanomaterials, such as metal nanoparticles or polymers, can create synergistic effects that enhance both sensitivity and selectivity. These hybrids can provide additional binding sites or catalytic properties that improve sensor performance [307].
• Sensor Arrays: Employing arrays of CNT sensors, each functionalized differently, can allow for pattern recognition techniques to distinguish between multiple analytes, improving overall selectivity [308].

Table 7. Comparative Summary of Sensitivity and Selectivity Limitations of CNT-Based Sensing Technologies.

Type of CNT Sensor	Sensitivity Limitations	Selectivity Limitations	Influencing Factors	References
Chemiresistive CNT Sensors	Low response to low-concentration analytes	Poor selectivity towards similar molecules	Surface defects, ambient conditions	[309]
Electrochemical CNT Sensors	Signal interference from non-target species	Cross-reactivity in complex samples	Electrode fouling, electrolyte effects	[310]
Optical CNT Sensors	Weak optical signals at low analyte levels	Overlapping fluorescence or Raman signals	Background noise, optical quenching	[311]
FET-Based CNT Sensors	Drift in signal over time	Non-specific adsorption of analytes	Device instability, surface contamination	[312]
Gas CNT Sensors	Poor detection at ultra-low gas concentrations	Cross-sensitivity to humidity or other gases	Adsorption-desorption kinetics	[313]
Biosensors with Functionalized CNTs	Sensitivity affected by bio-receptor degradation	Limited specificity in complex biological media	Stability of functionalization layer	[314]

Table 7. Comparative Summary of Sensitivity and Selectivity Limitations of CNT-Based Sensing Technologies.

Type of CNT Sensor	Sensitivity Limitations	Selectivity Limitations	Influencing Factors	References
Chemiresistive CNT Sensors	Low response to low-concentration analytes	Poor selectivity towards similar molecules	Surface defects, ambient conditions	[309]
Electrochemical CNT Sensors	Signal interference from non-target species	Cross-reactivity in complex samples	Electrode fouling, electrolyte effects	[310]
Optical CNT Sensors	Weak optical signals at low analyte levels	Overlapping fluorescence or Raman signals	Background noise, optical quenching	[311]
FET-Based CNT Sensors	Drift in signal over time	Non-specific adsorption of analytes	Device instability, surface contamination	[312]
Gas CNT Sensors	Poor detection at ultra-low gas concentrations	Cross-sensitivity to humidity or other gases	Adsorption-desorption kinetics	[313]
Biosensors with Functionalized CNTs	Sensitivity affected by bio-receptor degradation	Limited specificity in complex biological media	Stability of functionalization layer	[314]

Table 7 highlights the critical sensitivity and selectivity challenges commonly encountered in CNT-based sensing systems. Although CNTs possess exceptional physical, chemical, and electrical properties that make them suitable for diverse sensing applications, their performance is often restricted by certain inherent and environmental factors. Chemiresistive CNT sensors, despite their simple configuration, often struggle with low sensitivity when detecting trace-level analytes due to limited adsorption sites or insufficient charge transfer at very low concentrations [309]. Additionally, these sensors frequently exhibit poor selectivity, especially when structurally similar molecules are present in the environment. Electrochemical CNT sensors suffer from signal interference caused by the presence of electroactive species other than the target analyte. This cross-reactivity is particularly challenging in biological fluids or environmental samples where complex matrices are involved [310]. Optical CNT sensors face difficulties in maintaining high sensitivity due to weak optical signals at trace analyte levels. Moreover, the selectivity can be compromised when the optical spectra (fluorescence or Raman signals) of the target analyte overlap with that of interfering species. FET-based CNT sensors are highly sensitive; however, their long-term stability is a major concern due to signal drift and contamination of the sensing surface. Non-specific adsorption of environmental molecules can further compromise the selectivity of these sensors. CNT-based gas sensors show excellent performance at room temperature, but their detection capability often diminishes at ultra-low concentrations. Additionally, cross-sensitivity to humidity and other interfering gases affects their reliability in real-world applications [314]. Functionalized CNT biosensors offer enhanced specificity through biorecognition elements such as enzymes, antibodies, or aptamers. However, the stability of these functional layers can degrade over time, affecting both sensitivity and selectivity, especially in harsh environments or complex biological media [314].

8. Future Perspectives and Trends

8.1. Emerging Trends in CNT-Based Sensing

The incorporation of CNT-based sensors into wearable devices is a notable trend. Their flexibility and lightweight nature make CNTs ideal for continuous health monitoring applications. For instance, CNTs have been utilized in smart textiles to monitor vital signs such as heart rate, temperature, and glucose levels. These advancements aim to provide real-time health data with minimal discomfort to users [53]. Recent developments in CNT patterning technologies have enabled the creation of sensors with micro- and nanoscale resolution. Techniques such as dielectrophoresis and oxidative etching facilitate precise control over CNT alignment and density, enhancing sensor performance and reproducibility. These advancements are crucial for the practical deployment of CNT-based sensors in various applications [315]. Combining CNTs with other nanomaterials, such as metal nanoparticles or graphene, has led to the development of hybrid sensors that leverage the unique properties of each component. These hybrid systems exhibit enhanced sensitivity and selectivity, broadening the scope of detectable analytes and improving sensor performance. CNT-based sensors are increasingly being applied in environmental monitoring to detect pollutants and hazardous gases. Their high surface area and chemical reactivity enable the detection of low concentrations of environmental toxins, contributing to improved public health and safety [4].

8.2. Potential Applications in Healthcare and Environmental Monitoring

In the medical field, CNT-based sensors offer promising solutions for non-invasive diagnostics and continuous patient monitoring. For example, they have been employed in the development of biosensors capable of detecting biomolecules such as proteins and nucleic acids, facilitating early disease diagnosis. Additionally, CNT-based sensors integrated into wearable devices can monitor physiological parameters, providing valuable data for managing chronic conditions and promoting proactive healthcare [316]. CNT-based sensors are utilized to detect environmental pollutants, including gases like nitrogen dioxide and ammonia. Their high sensitivity allows for the detection of trace amounts of hazardous substances, enabling timely interventions to mitigate environmental risks. Furthermore, advancements in CNT sensor technology have facilitated the development of portable and real-time monitoring devices, enhancing the efficiency of environmental assessments. The future of CNT-based sensing technologies is marked by significant advancements in fabrication techniques, integration with wearable devices, and the development of hybrid systems. These trends are expanding the applications of CNT sensors, particularly in healthcare and environmental monitoring, offering promising solutions for real-time, sensitive, and selective detection of a wide range of analytes.

Table 8 summarizes the current landscape of commercialization and industrial adoption of CNT-based sensing systems across various sectors. While carbon nanotube (CNT) sensors offer exceptional properties such as high sensitivity, flexibility, and miniaturization potential, their translation from laboratory research to commercial products is still limited and sector-specific. In the healthcare and biomedical domain, several early-stage commercial products have emerged, especially wearable biosensors for monitoring glucose, electrolytes, or vital signs. Companies are exploring CNT-based patches and flexible devices, although regulatory hurdles and biocompatibility concerns remain significant barriers to large-scale commercialization [317]. In environmental monitoring, CNT-based sensors for detecting pollutants, toxic gases, and contaminants have seen niche commercial deployment. However, ensuring high sensitivity and selectivity under fluctuating environmental conditions, such as humidity and temperature, poses challenges for widespread industrial adoption [318]. Industrial gas sensing applications, particularly for occupational safety and hazardous gas detection, have witnessed limited commercial prototypes. Here, cross-sensitivity to interfering gases and the requirement for robust sensor performance in harsh conditions limit adoption [4]. In the smart textiles and wearable electronics sector, there is growing commercial interest, especially for sports and fitness devices integrating CNT-based strain or pressure sensors. Yet, challenges such as washability, mechanical durability, and mass-production scalability hinder broader commercialization [319]. Automotive and aerospace industries are exploring CNT-based sensors primarily at the research and prototype level, focusing on structural health monitoring, pressure sensing, and environmental detection. These sectors require ultra-reliable sensors integrated seamlessly with existing systems, a challenge yet to be fully overcome [320,321]. Agriculture and food safety applications have seen some developments in CNT sensor technology for detecting pesticide residues, pathogens, and environmental factors. However, achieving cost-effective and selective sensing in complex biological samples continues to limit market penetration [322,323]. Military and defense sectors have advanced prototypes leveraging CNT-based sensing systems for chemical and biological threat detection. Nevertheless, sensors must withstand extreme operational conditions, maintain security, and offer real-time performance, which are complex to achieve in practical field conditions [324]. In conclusion, despite the significant research advancements in CNT-based sensing technologies, their industrial adoption remains limited due to technical, regulatory, and economic challenges. Future research should focus on enhancing sensor robustness, cost-effectiveness, and regulatory compliance to accelerate commercialization across diverse application sectors.

Table 8. Comparative Summary of Commercialization and Industrial Adoption of CNT-Based Sensing Technologies.

Application Sector	Commercialization Status	Key Challenges for Industrial Adoption	References
Healthcare and Biomedical Sensors	Early-stage commercial products (wearables, glucose sensors)	Biocompatibility, regulatory approval, long-term stability	[317]
Environmental Monitoring Sensors	Pilot-scale and niche commercial products	Sensitivity in real-world conditions, sensor calibration	[318]
Gas Sensing for Industrial Safety	Limited commercial prototypes	Cross-sensitivity, long-term performance, harsh environment tolerance	[4]
Smart Textiles and Wearables	Emerging commercial interest (sports and fitness devices)	Durability, washability, mass production cost	[319]
Automotive and Aerospace Sensors	Research and prototype stage	Reliability, integration with existing systems	[321]
Food Safety and Agricultural Sensors	Limited commercial deployment	Selectivity in complex samples, cost-effectiveness	[322]
Military and Defense Applications	Advanced prototypes in specific projects	Harsh operational conditions, sensor security	[324]

Table 8. Comparative Summary of Commercialization and Industrial Adoption of CNT-Based Sensing Technologies.

Application Sector	Commercialization Status	Key Challenges for Industrial Adoption	References
Healthcare and Biomedical Sensors	Early-stage commercial products (wearables, glucose sensors)	Biocompatibility, regulatory approval, long-term stability	[317]
Environmental Monitoring Sensors	Pilot-scale and niche commercial products	Sensitivity in real-world conditions, sensor calibration	[318]
Gas Sensing for Industrial Safety	Limited commercial prototypes	Cross-sensitivity, long-term performance, harsh environment tolerance	[4]
Smart Textiles and Wearables	Emerging commercial interest (sports and fitness devices)	Durability, washability, mass production cost	[319]
Automotive and Aerospace Sensors	Research and prototype stage	Reliability, integration with existing systems	[321]
Food Safety and Agricultural Sensors	Limited commercial deployment	Selectivity in complex samples, cost-effectiveness	[322]
Military and Defense Applications	Advanced prototypes in specific projects	Harsh operational conditions, sensor security	[324]

9. Conclusions

9.1. Summary of Key Findings

Carbon nanotubes (CNTs) have emerged as one of the most promising nanomaterials in the field of sensing technologies due to their exceptional physicochemical properties, such as high electrical conductivity, mechanical strength, large specific surface area, and tunable surface chemistry. This review has comprehensively discussed the fundamental properties of CNTs, their types, mechanisms of sensing, and wide-ranging applications in different sensor platforms, including gas sensors, chemical sensors, biosensors, pressure and strain sensors. In the initial sections, the review highlighted the structural uniqueness of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), emphasizing their differences in terms of structure, electrical properties, and applicability in various sensing platforms. SWCNTs exhibit remarkable sensitivity due to their single atomic layer structure, while MWCNTs provide enhanced mechanical strength and chemical robustness, making them suitable for industrial-scale applications. The review also addressed the essential mechanisms of CNT-based sensing, including changes in electrical conductivity, surface adsorption interactions, optical responses, and molecular recognition capabilities. The sensitivity of CNTs to various analytes arises primarily from their surface adsorption capacity and their ability to exhibit significant changes in electronic properties upon interaction with target molecules. In terms of applications, CNTs have demonstrated superior performance in gas sensors for detecting toxic gases like NO₂, NH₃, and CO, as well as in chemical sensors for environmental monitoring of pollutants and industrial safety systems. Additionally, CNT-based biosensors have shown excellent potential in the detection of biomolecules such as glucose, DNA, proteins, and various disease biomarkers, positioning them as critical components in medical diagnostics. Furthermore, advancements in nanostructuring techniques, hybrid CNT composites, and integration with other nanomaterials have significantly enhanced the sensitivity, selectivity, and stability of CNT-based sensors. The emerging trends of incorporating CNT sensors into smart, wearable devices and Internet of Things (IoT)-enabled platforms offer exciting prospects for real-time health monitoring and environmental surveillance. However, despite these remarkable advancements, challenges such as functionalization stability, sensitivity limitations under real-world conditions, cost-effectiveness, scalability of fabrication techniques, and potential environmental or toxicological concerns remain significant barriers to widespread commercialization. Future research must focus on addressing these limitations through novel material engineering, sustainable production methods, and thorough biocompatibility assessments. In conclusion, CNT-based sensing technologies are poised to revolutionize next-generation sensor development across healthcare, environmental monitoring, industrial safety, and wearable electronics. With continuous interdisciplinary research efforts and technological advancements, CNTs are expected to play a pivotal role in the future of smart sensing systems, contributing to the development of highly sensitive, selective, stable, and eco-friendly sensors for a broad spectrum of applications.

9.2. Prospects for CNTs in Sensing Technologies

The future of Carbon Nanotube (CNT)-based sensing technologies is poised for remarkable advancement, driven by rapid progress in nanomaterials science, device engineering, and interdisciplinary research. Despite several challenges, the unique characteristics of CNTs—including high surface area, exceptional electrical and mechanical properties, and the ability for chemical functionalization—position them as vital components in the next generation of sensing platforms. One of the most promising prospects lies in the development of highly sensitive and selective sensors for healthcare diagnostics and environmental monitoring. In healthcare, CNT-based biosensors are expected to revolutionize point-of-care diagnostics by enabling real-time, non-invasive detection of biomarkers for diseases such as cancer, cardiovascular disorders, and infectious diseases. The integration of CNTs with bioreceptors like enzymes, antibodies, and DNA sequences offers significant advantages for early disease detection due to their rapid electron transfer capabilities and enhanced sensitivity at the nanoscale. In environmental applications, CNT-based sensors are anticipated to play a critical role in detecting toxic gases, heavy metals, and chemical pollutants at ultra-low concentrations. Their rapid response times, durability under harsh conditions, and miniaturization potential make them ideal candidates for on-site environmental monitoring systems. This is particularly important for air and water quality assessment in urban and industrial areas, contributing to sustainable development and public health protection. The integration of CNTs into wearable and flexible electronics represents another exciting frontier. Advances in material engineering now allow CNTs to be incorporated into stretchable, lightweight substrates, enabling the development of wearable sensors for continuous monitoring of physiological parameters such as heart rate, sweat composition, or body temperature. These systems could significantly impact personalized medicine and remote health monitoring, particularly in an increasingly digital healthcare landscape. Furthermore, the combination of CNT-based sensors with Internet of Things (IoT) platforms is expected to facilitate real-time data acquisition, analysis, and transmission. This will enable the development of smart sensing networks for industrial automation, smart cities, and agricultural monitoring systems. The ability of CNT sensors to operate at low power and their scalability for mass production are essential factors for IoT integration. However, realizing the full potential of CNT-based sensing technologies requires overcoming existing barriers such as cost-effective large-scale synthesis, uniformity of CNT properties, long-term stability, and biocompatibility in biosensing applications. Future research is likely to focus on improving functionalization techniques, developing hybrid nanocomposites, and advancing green synthesis methods to address toxicity and environmental concerns. Additionally, the convergence of CNTs with emerging technologies like artificial intelligence (AI), machine learning (ML), and blockchain-based data security may enable smarter and more secure sensing systems capable of complex data analysis and autonomous decision making. In summary, CNT-based sensing technologies are poised to reshape multiple sectors through innovative applications in healthcare, environmental monitoring, industrial safety, and wearable electronics. Continued multidisciplinary research efforts, supported by technological innovations and regulatory frameworks, will be crucial in transitioning CNT-based sensors from laboratory prototypes to real-world commercial devices. Their future promises not only enhanced sensing performance but also greater integration into the fabric of everyday life, contributing to smarter, safer, and more sustainable technological ecosystems.

9.3. Final Thoughts and Future Directions

The exploration of carbon nanotubes (CNTs) in sensing technologies has undeniably opened a transformative pathway in the field of nanotechnology and sensor development. CNTs, owing to their exceptional physicochemical properties, have demonstrated the capacity to address the growing demands for highly sensitive, selective, and miniaturized sensing devices across diverse sectors including healthcare, environmental monitoring, industrial safety, and smart wearable electronics. The key findings of this review underline the versatility of CNTs in different sensing platforms from gas and chemical sensors to biosensors and pressure/strain sensors. Their ability to undergo surface modification and functionalization allows for tailoring their properties to meet specific sensing requirements, such as enhanced sensitivity, improved stability, and selectivity towards target analytes. Moving forward, the integration of CNT-based sensors with emerging technologies such as artificial intelligence (AI), machine learning (ML), and the Internet of Things (IoT) will play a crucial role in advancing smart sensing systems. These systems will enable real-time data processing, remote monitoring, and predictive analytics, which are essential for modern healthcare diagnostics, smart environmental monitoring, and industrial automation. Moreover, future research must focus on overcoming the existing challenges associated with CNT-based sensors, particularly in the areas of large-scale synthesis, functionalization reproducibility, environmental safety, and cost-effectiveness. Advances in green synthesis methods, hybrid nanocomposites, and scalable fabrication technologies are expected to address these challenges, ensuring that CNT-based sensors move from laboratory-scale innovation to commercial and industrial deployment. In conclusion, CNT-based sensing technologies hold immense potential for shaping the future of sensor systems, driving forward innovations in personalized healthcare, smart environmental monitoring, and next-generation wearable electronics. Collaborative efforts between academia, industry, and regulatory bodies will be essential to realize the full potential of CNTs while ensuring their safe and sustainable application in real-world scenarios.