Advances and Prospects of Nanomaterial Coatings in Optical Fiber Sensors

Abstract

This review summarizes the recent advances in the application of nanomaterial coatings in optical fiber sensors, with a particular focus on deposition techniques and the research progress over the past five years in humidity sensing, gas detection, and biosensing. Benefiting from the high specific surface area, abundant surface active sites, and quantum confinement effects of nanomaterials, advanced thin-film fabrication techniques—including spin coating, dip coating, self-assembly, physical/chemical vapor deposition, atomic layer deposition (ALD), electrochemical deposition (ECD), electron beam evaporation (E-beam evaporation), pulsed laser deposition (PLD) and electrospinning, and other techniques—have been widely employed in the construction of functional layers for optical fiber sensors, significantly enhancing their sensitivity, response speed, and environmental stability. Studies have demonstrated that nanocoatings can achieve high-sensitivity detection of targets such as humidity, volatile organic compounds (VOCs), and biomarkers by enhancing evanescent field coupling and enabling optical effects such as surface plasmon resonance (SPR), localized surface plasmon resonance (LSPR), and lossy mode resonance (LMR). This paper first analyzes the principles and optimization strategies of nanocoating fabrication techniques, then explores the mechanisms by which nanomaterials enhance sensor performance across various application domains, and finally presents future research directions in material performance optimization, cost control, and the development of novel nanocomposites. These insights provide a theoretical foundation for the functional design and practical implementation of nanomaterial-based optical fiber sensors.

1. Introduction

Nanomaterials are defined as materials with at least one dimension in the nanoscale range (typically 1–100 nm) that exhibit unique size-dependent properties. Due to their exceptionally high specific surface area, abundant surface active sites, and quantum confinement effects, nanomaterials have demonstrated remarkable performance and vast application potential in areas such as sensors, photocatalysts, nanoreactors, cosmetics, energy, and transportation [1,2,3,4,5,6]. For instance, silver nanomaterials, known for their excellent antimicrobial properties, are widely used in cosmetics, water purification systems, and medical products, with an estimated annual production exceeding 500 tons. Meanwhile, traditional nanomaterials such as carbon black, silica (SiO₂), and titanium dioxide (TiO₂) continue to dominate the market and remain among the most widely utilized nanomaterials in industrial applications [7]. According to the classification system jointly established by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC), nanomaterials can be categorized as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), or three-dimensional (3D) structures [8]. Each type of nanomaterial exhibits distinct functional characteristics and application directions: 0D nanoparticles (e.g., quantum dots (QDs), gold nanoparticles) possess unique optical properties and are commonly integrated with fluorescent sensors or used in bioimaging [9,10,11,12]; 1D materials (e.g., carbon nanotubes, nanowires) offer excellent electrical conductivity and mechanical properties, making them suitable for flexible strain sensors [13,14,15]; 2D materials (e.g., graphene) are widely used in catalysis and biosensing due to their ultrahigh surface area [16,17]; and 3D nanomaterials (e.g., metal–organic frameworks (MOFs), porous carbon) are favored in substance detection, separation, and environmental remediation because of their tunable structures and high porosity [18,19].

Optical fiber sensors have garnered significant attention for their inherent advantages, including immunity to electromagnetic interference, high sensitivity, corrosion resistance, and capability for distributed or quasi-distributed sensing. They have found widespread applications in environmental monitoring, biomedicine, structural health monitoring, and chemical gas detection [20,21,22,23]. However, as application scenarios grow more complex—ranging from high temperatures and humidity to corrosive, high-vibration, or bioactive environments—higher demands are placed on the functionality and customization of these sensors. For example, aerospace engine compartments, gas turbines, and nuclear reactors require sensors that can operate reliably under temperatures of several hundred degrees Celsius [24,25]; cold-chain logistics, greenhouse agriculture, and tissue monitoring call for sensors with superior sealing and anti-condensation capabilities in high-humidity conditions [26,27], and in corrosive environments such as deep-sea exploration, chemical pipelines, and oil wells, sensors must exhibit excellent chemical stability and corrosion resistance [28,29]. Furthermore, in high-vibration environments such as high-speed trains, space launch systems, and automotive engines, sensor structures must maintain high mechanical strength and impact resistance [30,31]. In biomedical applications like implantable sensors, tumor microenvironment monitoring, and brain–machine interfaces, additional challenges arise concerning biocompatibility, long-term stability, and miniaturization [32,33]. Coating technology, as an effective approach for functional enhancement, enables optical fiber sensors to adapt to various complex environmental requirements. Functional coatings can significantly improve mechanical robustness, such as wear and corrosion resistance, thereby offering physical protection. Moreover, coatings enhance the interaction between the evanescent field and the surrounding environment—particularly in sensing configurations with highly exposed optical fields (e.g., cladding modes, tilted fiber Bragg gratings (TFBGs), or cladding-stripped structures). The refractive index, thickness, and chemical functionality of the coating material directly affect the optical modulation behavior of the sensor. As a result, functional coatings not only improve sensor sensitivity but also enhance selectivity, stability, and reproducibility [34,35,36,37].

By integrating nanotechnology, nanostructures can be directly grown or deposited on the surface of optical fiber sensors to further boost performance beyond that of conventional coatings. These nanostructures, characterized by high surface area, tunable dimensions, and excellent optical, electrical, or chemical responsiveness, play a critical role in enhancing sensing functionality [38,39]. Specifically, nanomaterials improve the near-field interaction between the sensing interface and target analytes at the molecular scale, thereby increasing capture efficiency and enabling various surface wave-based optical resonance phenomena. For instance, metal nanostructures can excite SPR [40,41], while nanoparticles or island-like nanostructures support LSPR [42,43], both of which significantly enhance sensor sensitivity and detection limits. In addition, nanofilms made of high-index or lossy dielectric materials can trigger LMR, enabling enhanced responses to specific spectral bands or environmental variables such as pH, ion concentration, or chemical gases [44,45]. These resonant effects are highly sensitive to changes in the surrounding refractive index, making it essential to precisely control nanostructure geometry, spatial arrangement, and material composition to tune the sensor’s wavelength response, intensity, and selectivity.

Therefore, this review focuses on recent progress over the past five years in the application of nanomaterial coatings in optical fiber sensors, highlighting three representative domains: humidity sensing, gas detection, and biological recognition. We systematically summarize the functional advantages of different nanomaterials, their deposition techniques, and corresponding sensing performance. It is hoped that this review will provide theoretical guidance and practical insights for the functional design and process optimization of nanomaterial-enhanced optical fiber sensors, thereby promoting their integration and real-world deployment across diverse fields.

2. Nanocoating Deposition Technology

As a critical subfield of nanotechnology, nanomaterial coatings technology is rapidly emerging as one of the core enabling techniques for the application of nanomaterials. By leveraging the unique properties of nanomaterials, nanocoating techniques utilize a variety of advanced deposition methods to fabricate functional nanoscale coatings on substrate surfaces. Despite their typical thickness ranging from just a few tens to several hundred nanometers, these nanocoatings can significantly enhance surface properties—such as hardness, wear resistance, corrosion resistance, and biocompatibility—thereby endowing the base materials with novel functionalities and expanded application potential [46,47,48,49]. From mechanical coating techniques such as spraying, spin coating, and dip coating, to physical and chemical deposition methods—including electron beam evaporation, pulsed laser deposition, chemical vapor deposition, atomic layer deposition, and electrochemical deposition—and further to molecular-scale approaches like self-assembly and electrospinning, each method offers distinct possibilities and advantages for nanomaterial coating. These techniques exhibit diverse developmental pathways in the regulation of film structure, functional integration, and interfacial performance optimization. Each of these techniques offers distinct advantages and design possibilities for the functional integration of nanomaterials [50,51,52,53]. In the following sections, we will provide an in-depth discussion of the underlying principles, advantages and limitations, and representative application cases of these nanocoating techniques, aiming to present readers with a comprehensive and systematic overview of the current landscape of nanocoating technologies.

2.1. Spin Coating

Spin coating is a widely used deposition technique in which a liquid precursor is dropped onto the surface of a substrate and spread evenly by centrifugal force through high-speed rotation. The resulting thin film is subsequently dried to obtain a uniform and stable coating. Key parameters such as film thickness can be precisely controlled by adjusting the spin speed and drying conditions [54,55]. This method offers advantages such as low cost and excellent film uniformity. However, it also presents certain limitations—particularly for substrates with complex geometries such as curved, uneven, or three-dimensional surfaces. For example, on the cylindrical surface of an optical fiber, spin coating often results in poor uniformity due to the inability of the liquid to spread evenly, making it more suitable for flat substrates such as silicon wafers and glass slides [56].

As early as 1958, Emslie et al. developed an idealized mechanical model to describe the dynamics of the spin coating process [57], and subsequent studies have refined these models with systematic estimations of various parameters involved [58,59,60,61,62]. In 2019, K. Sonker employed spin coating to fabricate cadmium sulfide (CdS) thin films of different thicknesses for use in NO₂ gas sensors [63]. In 2021, Karimi et al. used spin coating to deposit magnesium oxide (MgO) and aluminum oxide (Al₂O₃) metal oxide films, significantly enhancing barrier performance. By finely tuning key parameters such as spin speed, they achieved optimal performance for the MgO films [64]. In 2024, Zheng et al. deposited zinc oxide (ZnO) thin films via controlled spin coating, effectively overcoming the issue of surface defects often associated with solution-processed ZnO films [65]. Most recently, in 2025, Giribaldi et al. synthesized Al-doped ZnO (AZO) thin films embedded with amorphous carbon nanoinclusions using a novel spin-coating strategy, and studied how spin parameters influenced the thermoelectric properties of the AZO coatings [66].

2.2. Dip Coating

Dip coating involves immersing a substrate into a nanomaterial-based solution, allowing sufficient interaction time, and then withdrawing the substrate followed by drying or curing. This method is widely used in both industrial and laboratory settings due to its low cost and ability to produce high-quality films [67]. Its advantages include simplicity, flexibility, and high adaptability. By adjusting key parameters such as withdrawal speed, solution concentration, viscosity, temperature, and ambient humidity, dip coating allows precise control over film thickness and morphology. It is particularly suitable for pore filling and for depositing nanocomposite materials [68]. However, for multilayer structures, multiple immersion and drying steps are often required, making the overall process time-consuming. Moreover, it remains challenging to predict the final performance of dip-coated films theoretically, which limits the efficiency of large-scale or high-throughput applications [69].

In 2021, Wang et al. deposited gold and polyvinyl alcohol (PVA) layers on optical fibers using magnetron sputtering and dip coating, respectively, to fabricate a SPR-based humidity sensor. The uniform PVA film produced by dip coating exhibited excellent hygroscopic expansion characteristics, which improved the sensor’s sensitivity [70]. In the same year, Nagoor Meeran et al. developed a hydrogen sensor by dip coating MOFs onto an optical fiber surface. The Zn-doped MOFs sensor demonstrated excellent sensitivity with response and recovery times of 20 and 27 s, respectively [71]. In 2022, Zaremba et al. systematically investigated the uniformity and reproducibility of single-walled carbon nanotube (SWCNT) films prepared by dip coating under various processing conditions [72].

2.3. Spray Coating

Spray coating involves delivering a coating material to the substrate surface via a spray gun or nozzle, and encompasses methods such as cold spraying, thermal spraying, plasma spraying, and flame spraying. This technique is known for its fast deposition rate and the resulting coatings generally exhibit good wear and corrosion resistance. However, the coatings often contain larger pores, and the high temperatures and mechanical stresses involved in the process can compromise coating integrity [73,74,75].

In 2022, Lin et al. addressed membrane wetting and scaling issues in membrane distillation by spraying hydrophobic SiO₂ nanoparticles onto porous polyvinylidene fluoride (PVDF) substrates [76]. In 2023, Xue et al. investigated the surface roughness, thickness, and adhesion of coatings deposited on two different substrates using solid-state cold spray under varying process parameters [77].

2.4. Physical Vapor Deposition (PVD)

In addition to spray techniques, PVD is a prominent method for fabricating high-performance functional thin films. Coatings produced by PVD generally exhibit excellent wear resistance and corrosion protection. PVD involves the transformation of solid or liquid materials into the gas phase under high-vacuum conditions, followed by condensation onto a substrate surface to form a thin film. Common PVD methods include magnetron sputtering, thermal evaporation, and arc ion plating. Compared with spray coating, PVD yields denser films with fewer impurities [78,79]. Furthermore, PVD is well-suited for creating multilayer coatings due to its ability to produce thin and precise layers. However, during the deposition process, high-energy particle bombardment or plasma effects may cause microstructural damage to the substrate, which could adversely affect the corrosion resistance of the resulting film [80].

In 2021, Pang et al. deposited manganese dioxide (MnO₂) films onto polydimethylsiloxane (PDMS) substrates using magnetron sputtering. They investigated the film morphology and structure under fixed process parameters and fabricated electrodes that exhibited excellent sensitivity and repeatability in sensing applications [81]. In 2015, M. Marques et al. employed DC/pulsed magnetron sputtering to coat sliver (Ag)–Sn_X and Ti_1−XAg_X films onto poly(vinylidene fluoride) (PVDF) substrates, evaluating the antimicrobial performance of these electrodes formed via sputtering [82]. In 2024, Aslanidis et al. used PVD to deposit platinum nanoparticles on flexible substrates for the development of strain sensors. Their study systematically examined the dynamic behavior of nanoparticle-based metal-resistive strain sensors grown under vacuum conditions [83].

2.5. Chemical Vapor Deposition (CVD)

CVD is a technique in which gaseous precursors are introduced into a reaction chamber, where they undergo chemical reactions on a substrate surface to form solid thin films. CVD is characterized by its relatively simple equipment, ease of operation, and material selectivity [84]. It enables the deposition of high-quality films on a wide variety of substrates. However, by-products generated during the reaction may remain in the chamber, requiring proper system design and post-processing [85]. In recent years, CVD has become a widely used method for the fabrication of noble metal films and graphene oxide (GO) structures [86,87,88,89].

In 2020, Shekhirev et al. developed an electronic nose system based on graphene nanoribbons fabricated via CVD, which showed excellent sensitivity to a range of volatile organic compounds (VOCs). They also demonstrated that uniform graphene nanoribbon films could be deposited on arbitrary substrates using the CVD process [90]. In the same year, A. Andrés et al. applied CVD to coat porous poly(p-xylylene) (Parylene C) films on MOFs nanoparticles, significantly enhancing the sensor’s sensitivity and selectivity toward water vapor [91].

2.6. Hydrothermal/Solvothermal Methods

Hydrothermal and solvothermal methods involve chemical reactions conducted in aqueous or organic solvents under elevated temperature and pressure conditions, typically within a sealed environment. These conditions are conducive to the controlled doping of synthesized materials and often lead to the formation of metastable phases or unique morphologies under isothermal and isobaric conditions. As such, these methods are particularly suitable for fabricating nanocoatings with specialized structures [92,93]. Hydrothermal synthesis, conducted in water rather than organic solvents, is considered safer and more environmentally friendly, and has become a common approach for preparing metal oxide nanomaterials. However, both hydrothermal and solvothermal techniques are generally considered slow processes [94,95].

In 2021, Ling et al. fabricated calcium phosphate coatings on magnesium and its alloys via hydrothermal treatment, which significantly improved the corrosion resistance and antibacterial properties of the substrate [96]. In 2025, Khizir et al. successfully synthesized well-aligned and uniform TiO₂ nanorods using the solvothermal method. Based on these structures, they developed a gas sensor that demonstrated enhanced ethanol detection capabilities, confirming the effectiveness of TiO₂ nanorods as a sensitive sensing material [97].

2.7. Nanomaterial Self-Assembly Techniques

In recent years, nanomaterial synthesis has gradually shifted from traditional "top-down" approaches (e.g., mechanical milling, lithography) to "bottom-up" strategies, which involve the construction of functional nanostructures from molecular or atomic building blocks through controlled assembly processes [98]. Self-assembly refers to the spontaneous organization of nanoscale units into ordered and functional systems via non-covalent interactions such as hydrogen bonding, van der Waals forces, electrostatic interactions, and coordination bonding [99]. Self-assembly of nanomaterials can be categorized by their driving forces into chemically driven, physically driven, and bio-inspired self-assembly. Among these, chemically driven self-assembly (CDSA) has been widely applied in the fabrication of functional coatings for sensors. CDSA typically involves the pre-functionalization of substrate surfaces with reactive groups (e.g., –SH, –NH₂) using chemical crosslinkers. These functionalized surfaces can then bind target materials via non-covalent forces, such as hydrogen bonds, electrostatic interactions, or coordination bonds, enabling the formation of ordered structures with high selectivity.

For example, thiol (–SH) groups exhibit strong coordination affinity to gold nanoparticles (AuNPs), allowing for the spontaneous and uniform assembly of Au films through the formation of Au–S bonds. This type of self-assembly not only simplifies surface modification procedures but also ensures uniform dispersion and precise control of nanomaterial arrangement, significantly enhancing the sensitivity and functional integration of sensors [100,101,102]. This approach is demonstrated in a case study discussed later in this review, where it was applied to design a glucose sensor coating for optical fiber sensors, validating its effectiveness in constructing high-performance biosensing interfaces. However, this method imposes strict requirements on the substrate’s surface chemistry. Pre-functionalization is often necessary to enhance interfacial interactions, and self-assembly on low-surface-energy materials typically exhibits reduced efficiency. Moreover, large-area film fabrication and performance-specific nanostructure design via self-assembly remain technically challenging [103,104].

Given that sensor surfaces often require the integration of multiple functional components, layer-by-layer (LbL) self-assembly has attracted increasing attention. This technique has been widely employed in material science, nanotechnology, biomedicine, and energy storage [105,106,107,108]. The LbL method involves alternating deposition of oppositely charged species or complementary functional groups, driven by various interactions such as electrostatic attraction, hydrogen bonding, and coordination forces. It enables the precise control of film composition, architecture, thickness, and properties at the molecular level—offering unparalleled flexibility and modularity. Importantly, the LbL process can be conducted at room temperature without the need for harsh chemical or thermal conditions, thereby reducing fabrication costs and simplifying operation [109,110,111].

Since its first demonstration by Iler in 1966 using oppositely charged colloidal particles [112], electrostatic LbL assembly has become one of the most widely used thin-film deposition techniques [113]. It allows the incorporation of functional macromolecules and reactive groups, although the use of electrostatic interactions may limit the diversity of compatible materials. Electrostatic LbL has been successfully employed to deposit metal nanoparticles and enhance sensor performance. For example, noble metal nanoparticles (e.g., Au, Ag) have been widely used for their high stability, electrical conductivity, and biocompatibility [114], while magnetic nanoparticles (e.g., iron (Fe) and its oxides) offer cost-effectiveness and ease of synthesis [115].

In 2010, Guo et al. fabricated three-dimensional AuNPs assemblies via LbL for plasmonic biosensing. The resulting structures offered high surface area and abundant binding sites, significantly improving sensor sensitivity [116]. In 2021, Teeparuksapun et al. developed an IgG biosensor probe based on the LbL assembly of thiourea (TU) and hydrogen tetrachloroaurate (HAuCl₄) followed by chemical reduction. The probe exhibited improved IgG immobilization and enhanced sensitivity [117]. In 2022, Nangare et al. constructed a beta-amyloid1–42 (Aβ1–42) biosensor by assembling a GO–chitosan (CS) multilayer on AgNPs via LbL. The LbL-modified interface enhanced both sensitivity and selectivity for Aβ peptide detection [118]. In 2025, Zhu et al. immobilized glucose oxidase in a multilayer film containing AuNPs via the LbL technique to develop a glucose biosensor. The device exhibited excellent accuracy in detecting glucose levels in human serum [119].

2.8. Atomic Layer Deposition

ALD is a vapor-phase thin-film deposition technique that enables precise atomic-level control by sequentially depositing materials in a layer-by-layer manner. In this method, volatile precursor gases are introduced into a vacuum reaction chamber in a pulsed fashion, where they react on the surface of a substrate. After each precursor pulse, an inert gas purge is employed to remove any unreacted precursor molecules and reaction by-products, thereby preventing undesired cross-reactions. Due to its self-limiting surface reactions, ALD offers exceptional control over film composition and thickness, allowing for the deposition of uniform and conformal films with atomic-scale (angstrom-level) precision. However, this technique imposes strict requirements on the volatility, thermal stability, and reactivity of precursor materials, and the deposition process must be carried out in a controlled vacuum environment, which to some extent limits its material applicability and industrial scalability [120]. In 2022, Si et al. employed ALD to fabricate indium oxide thin films, enabling the construction of transistors with reduced dimensions and lower contact resistance. The ALD process yielded indium oxide films with atomically smooth surfaces and excellent uniformity, effectively mitigating performance degradation caused by surface roughness and thus improving overall device performance [121]. In 2023, Niazi et al. conducted a life cycle assessment to systematically compare the environmental impacts of conventional ALD and spatial ALD (SALD) during the deposition of 20 nm aluminum oxide films. Among 16 environmental indicators, SALD outperformed ALD in 14 categories, owing to its higher deposition rate and lower process energy consumption. However, SALD exhibited a slightly higher "non-carcinogenic human toxicity" due to excessive precursor usage [122]. In 2025, Li et al. proposed the use of ALD to introduce nanoscale oxide interfacial layers within porous structures, aiming to synergistically regulate carrier and phonon transport. This strategy facilitated the development of low-cost, high-performance thermoelectric materials [123].

2.9. Pulsed Laser Deposition

PLD is a physical vapor deposition technique that utilizes high-energy pulsed laser ablation of a solid target to grow thin films at relatively high rates with minimal contamination. It is particularly well-suited for the deposition of multilayer thin-film structures. In a typical PLD process, a high-power pulsed laser is focused onto the surface of a target material placed in a vacuum or controlled gas atmosphere. The laser energy rapidly heats and evaporates the target, generating a high-energy plasma plume that expands toward the substrate. Upon reaching the substrate, the ablated species condense and form a thin film. This technique offers a unique and effective approach for the deposition of metals and metal oxides. However, only the laser-focused region of the target actively participates in the ablation process, resulting in low target utilization. Moreover, the highly directional nature of the plasma plume poses challenges for achieving uniform deposition over large-area substrates [124,125,126]. In 2022, Haider et al. systematically reviewed the influence of key PLD parameters—including laser wavelength, laser fluence, and the distance between the substrate and the target—on the formation of the desired materials [127]. In 2025, Mouloua et al. investigated the structural and optoelectronic properties of MoS₂ thin films deposited by PLD at various substrate temperatures (25–700 °C), and fabricated photodetectors under the optimized condition of 500 °C [128]. In the same year, Cremer et al. deposited GeTe–Sb₂Te₃ heterostructures with both periodic and aperiodic configurations at room temperature via PLD, and systematically analyzed the effects of deposition parameters on microstructure, interfacial diffusion, and crystallinity, providing structural insights for performance optimization [129].

2.10. Electrochemical Deposition

ECD is a technique in which an external electric field drives the reduction in metal or alloy ions from an electrolyte solution onto the surface of a conductive substrate. This method offers advantages such as high deposition rate, low cost, and operational simplicity, enabling the fabrication of dense, non-porous thin films. It has been widely applied for the deposition of various metals and polymers. However, ECD requires the substrate to be electrically conductive, which imposes certain limitations on material selectivity [130,131,132]. In 2024, Falcón et al. achieved compact, low-porosity, and corrosion-resistant Zn–Co alloy coatings by tuning the deposition potential and bath concentration, providing an experimental basis for replacing traditional cadmium-based coatings [133]. In 2025, Wang et al. developed a high-infrared-emissivity Ni-based composite coating with an emissivity of up to 0.94 by employing magnetic-field-assisted jet electrodeposition in combination with SiC particles, significantly enhancing the infrared radiative performance of the coating [134].

2.11. Electron Beam Evaporation

E-beam evaporation works by focusing a high-energy electron beam onto the surface of a target material in the evaporation source, causing rapid localized heating, melting, and evaporation. The resulting vapor is transported to the substrate surface and condenses to form a thin film. This technique is suitable for depositing metals or inorganic materials with high melting points and strong thermal stability, and offers advantages such as high purity, high deposition rate, and good film uniformity. However, during the evaporation of multicomponent materials, significant differences in the evaporation rates of different components can easily lead to discrepancies between the composition of the deposited film and that of the target, making accurate stoichiometric control difficult. In addition, the E-beam evaporation process is highly directional, with vapor traveling along straight-line paths, which can result in thickness non-uniformity on large-area or non-rotating substrates. This problem is especially prominent in the absence of substrate heating [135,136,137]. In 2024, Singh et al. studied the crystallinity and semiconductor properties of tin oxide (SnO_x) deposited by electron beam evaporation and found that increasing the substrate temperature changed the film from amorphous to polycrystalline, and its semiconductor type also shifted from n-type to p-type [138]. In 2025, Kant et al. investigated the influence of deposition parameters on HfO₂ thin films prepared by electron beam evaporation, and explored their potential as protective coatings in antireflection and marine applications [139].

2.12. Electrospinning

Electrospinning is a technique that utilizes a high-voltage electrostatic field to stretch a polymer solution or melt into continuous nanofibers. In a typical electrospinning setup, the precursor solution is loaded into a syringe, and under a high-voltage electric field between the nozzle and the collector, Coulombic forces overcome the surface tension of the liquid, forming a Taylor cone at the nozzle tip and ejecting a jet that is stretched into fine nanofibers. The method features a simple setup and high process flexibility, and the fiber diameter can be effectively controlled by tuning parameters such as solution viscosity and conductivity. However, the resulting nanofibers usually possess high surface area and porous structures, which lead to relatively poor mechanical strength and often require post-treatment [140,141,142]. Electrospinning has been employed to fabricate functional nanofiber membranes that can be integrated onto optical fiber surfaces as sensing layers, thereby enhancing sensor responsiveness. In 2024, Zhao et al. synthesized Mo₂C/CoO bimetallic/nitrogen-doped carbon nanofibers via electrospinning, followed by pre-oxidation and carbonization, and immobilized them onto a glassy carbon electrode to construct an electrochemiluminescence (ECL) sensor for the detection of azithromycin. The sensor exhibited high sensitivity, good selectivity, and strong anti-interference capability [143]. In 2025, Luo et al. prepared SnC nanofibers using electrospinning, followed by calcination under nitrogen atmosphere, and in situ reduction in AuNPs on the fiber surface. Subsequently, tetracycline-specific aptamers (Apta) were immobilized onto the SnC@Au surface via Au–S bonds to construct a SnC@Au@Apta sensor for the rapid and highly sensitive detection of tetracycline [144].

2.13. Optical Tweezer Effect

The optical tweezer effect refers to the use of tightly focused laser beams to generate gradient and scattering forces at the micro/nanoscale, enabling non-contact trapping and manipulation of microparticles. When laser light is coupled into an optical fiber, a strong gradient field is formed around the fiber surface, especially in the tapered region, which can stably trap suspended nanomaterials (such as MoS₂, WS₂, and graphene oxide) and guide their gradual deposition onto the fiber surface. As these nanosheets are primarily adsorbed within the evanescent field region excited by the fiber, tapering the fiber can significantly enhance the evanescent field strength, thereby improving the trapping efficiency. However, the types of nanomaterials that can be effectively deposited using this method remain limited, indicating certain constraints in material applicability [145,146,147]. In 2022, Wang et al. utilized the optical tweezer effect to deposit graphene oxide onto the surface of a tapered long-period fiber grating, thereby constructing a refractive index chemical sensor with high sensitivity and accurate response [148]. In 2024, Shi et al. reported the in situ and dynamic manipulation of few-layer materials on solid substrates via the optical tweezer effect, and successfully constructed both homostructures of monolayer/multilayer MoS₂ and heterostructures of monolayer MoS₂ with graphene [149].

Each of the aforementioned nanocoating strategies presents unique advantages and limitations in terms of substrate adaptability, film uniformity, and functional integration. Spin coating stands out for its cost-effectiveness and excellent film uniformity, making it ideal for planar substrates such as silicon wafers and glass. However, it is less effective for substrates with complex geometries. Dip coating is highly versatile and equipment-friendly, allowing for controlled film thickness through parameters like withdrawal speed and solution viscosity, though the process becomes time-consuming for multilayer coatings and is difficult to model theoretically. Spray coating (e.g., cold spray, plasma spray) enables rapid deposition and good mechanical durability but may result in high porosity and thermal damage to the substrate. PVD techniques, such as magnetron sputtering and thermal evaporation, offer dense and multilayer coatings with low impurity levels yet may induce microscopic damage due to high-energy particle bombardment. CVD, known for its universal substrate compatibility and straightforward setup, allows high-quality film formation via chemical reactions, although by-product residues must be managed carefully. Hydrothermal and solvothermal methods excel at fabricating complex nanostructures under environmentally friendly, closed-system conditions. However, the extended reaction time limits throughput. In contrast, self-assembly technologies, especially LbL assembly, offer molecular-level precision and multi-functionality. ALD offers unparalleled advantages in the fabrication of ultrathin and dense films due to its atomic-level thickness control. It is particularly well-suited for high aspect ratio or three-dimensional structures; however, its relatively low deposition rate and stringent equipment requirements limit its throughput. ECD features low cost, mild processing conditions, and the ability to form compact films, making it suitable for large-area deposition of metals or composite materials on conductive substrates. Nevertheless, it is restricted by the conductivity requirements of the substrate. E-beam evaporation enables the deposition of high-purity, high-melting-point materials, making it ideal for the rapid fabrication of inorganic functional films. However, precise compositional control and uniform film thickness remain challenging, especially on complex structures. PLD can effectively retain the stoichiometry of the target material and is suitable for the deposition of complex multicomponent thin films, but its limitations include poor target utilization and difficulties in large-area coating. Electrospinning allows for the fabrication of nanofiber membranes with high surface area and porous structure, which are advantageous for surface functionalization and sensing applications. Nonetheless, the resulting fibers typically have low mechanical strength and limited production throughput, requiring post-treatment and further process optimization. The optical tweezer effect, as a laser-induced non-contact nanomanipulation technique, enables precise deposition of nanosheets on microstructured surfaces such as optical fibers. It is well-suited for spatially selective surface functionalization, but its deposition efficiency is relatively low and its applicability is constrained by material specificity and optical system requirements.

These methods are highly promising in biosensing and nanointerface engineering, although challenges remain in large-scale uniform coating and performance-oriented structure design. Taken together, these techniques are complementary and collectively drive the advancement of thin-film fabrication toward greater precision and functionality. Therefore, in practical applications, the selection of the most suitable coating method—or a hybrid integration of multiple techniques—should be based on specific requirements, including substrate geometry, coating thickness control, compatibility, and functional performance demands. Such strategic selection is crucial for enhancing the efficiency and overall performance of functional interface materials in optical fiber sensors and beyond.

3. Optic Fiber Sensor

Optical fiber sensors function by modulating light waves propagating through the fiber and detecting the resulting changes in the modulated optical signals to measure and perceive various physical parameters. Compared with traditional sensors, optical fiber sensors offer several advantages, including immunity to electromagnetic interference, resistance to chemical corrosion, low cost, suitability for distributed measurement, and compatibility with remote monitoring and control systems [150,151]. Based on the modulation characteristics of the detected optical signals, optical fiber sensors can be broadly classified into three categories: intensity-modulated sensors (based on variations in optical power), phase-modulated sensors (based on phase changes), and wavelength-modulated sensors (based on wavelength shifts). Each type has its own unique modulation mechanism and is suited to specific application scenarios. In the following sections, the working principles, advantages, and disadvantages of these sensor types are introduced and discussed in detail [152,153].

3.1. Intensity-Modulated Optical Fiber Sensors

Intensity-modulated optical fiber sensors achieve the measurement of target parameters by detecting variations in the transmitted optical power within the fiber caused by external physical stimuli. This type of sensor can be further categorized into several forms, including reflective intensity modulation, transmissive intensity modulation, mode-based intensity modulation, and absorption coefficient-based intensity modulation. The primary advantages of intensity-modulated sensors lie in their simple structure, low cost, and ease of implementation. As a result, they have been widely applied in the detection of various physical quantities, such as force, refractive index, and health monitoring. However, since these sensors rely on the measurement of optical intensity, they are susceptible to fluctuations in light source power, fiber bending losses, connection losses, and other environmental disturbances, which can affect the stability and accuracy of the sensing system [154,155,156].

3.2. Phase-Modulated Optical Fiber Sensors

Phase-modulated optical fiber sensors detect target parameters by measuring changes in the phase of light waves propagating through the fiber, which are induced by external physical quantities. Since phase variations cannot be directly detected by photodetectors, interferometric techniques are typically employed to convert phase changes into variations in interference fringes or optical intensity, thereby enabling indirect measurement of the measurand.

Common interferometric configurations include the Fabry–Pérot (F–P) interferometer, Sagnac interferometer, Mach–Zehnder interferometer (MZI), and Michelson interferometer. Among them, the F–P interferometric fiber sensor consists of a cavity formed by two parallel partially reflective mirrors, where multiple internal reflections generate interference. In optical fiber sensing, F–P cavities can be constructed as either intrinsic (between fiber end faces) or extrinsic (using thin films or microstructures), and are widely applied in high-resolution measurement of strain, temperature, and pressure [157,158]. The Sagnac fiber-optic sensor is based on the phase difference generated by light traveling in clockwise and counterclockwise directions along a looped path, making it particularly sensitive to rotation and angular velocity. Its core structure features a closed-loop optical path and is widely used in fiber-optic gyroscopes and rotational sensing applications [159,160]. The MZI sensor operates by splitting light into two paths—a measurement arm and a reference arm—and then recombining them to detect the relative phase difference. It enables high-sensitivity detection of multiple physical parameters such as strain, temperature, and pressure [161,162]. The Michelson fiber interferometric sensor splits light into two arms, reflects them back, and then recombines the beams to form interference. Similar in concept to the MZ configuration but differing in reflection geometry, it is particularly suited for multi-point and distributed sensing systems [163,164]. These interferometric configurations provide a solid foundation for phase-modulated optical fiber sensors and offer excellent sensitivity and measurement resolution. However, such sensors typically require high structural precision and involve complex fabrication processes. Improving the consistency and repeatability of these sensors during large-scale production remains a significant challenge [165,166].

3.3. Wavelength-Modulated Optical Fiber Sensors

Wavelength-modulated optical fiber sensors operate by detecting shifts in the spectral wavelength position caused by changes in external physical parameters, thereby enabling the measurement and perception of target quantities. These sensors offer advantages such as compact structure, strong immunity to electromagnetic interference, and high stability. Moreover, they are well suited for multi-point multiplexing and distributed sensing, making them widely used in fields such as structural health monitoring, chemical sensing, and biosensing [167,168,169].

The Fiber Bragg Grating (FBG) sensor is a typical representative of wavelength-modulated optical fiber sensors. Its operating principle is based on the fact that external perturbations, such as strain or temperature, lead to shifts in the central Bragg wavelength. The Bragg wavelength can be expressed as:

$${\lambda }_B=2n_{eff}\Lambda$$

(1)

where ${\lambda }_B$ is the Bragg central wavelength, n_eff is the effective refractive index, and Λ is the grating period [170,171,172]. In addition to standard FBGs, several structural variations in fiber gratings have been widely adopted to enhance sensing performance in specific applications. These include Long-Period Fiber Gratings (LPFGs), which have larger grating periods and are sensitive to refractive index and bending [173,174]; TFBGs, in which the grating planes are tilted at an angle with respect to the fiber axis, allowing coupling to cladding modes and enabling high-sensitivity refractive index or polarization sensing [175,176]; and Chirped Fiber Bragg Gratings (CFBGs), in which the grating period varies gradually along the fiber axis, allowing for broadband response and distributed parameter detection [177].

4. Applications in Various Fields

Nanomaterials, owing to their high specific surface area, excellent physicochemical properties, and superior surface functionalization capability, offer a novel material foundation and technical pathway for enhancing the performance of optical fiber sensors. In recent years, with continuous advancements in fabrication and coating techniques, a wide range of nanomaterials have been employed for functional modification of optical fiber surfaces. These modifications have effectively improved the overall sensing performance, including sensitivity, selectivity, response time, and environmental stability [178,179,180]. This section focuses on recent progress in the application of nanomaterial-based coatings for optical fiber sensors, with an emphasis on three representative application areas: humidity sensing, gas detection, and biosensing. We systematically review the types of nanomaterials used (e.g., metal oxides, carbon-based materials, polymer composites), the primary deposition and assembly methods (e.g., spin coating, dip coating, self-assembly, vapor-phase deposition), as well as their sensing performance and working mechanisms in various application scenarios. By summarizing and comparing these three representative directions, this review aims to provide theoretical guidance and reference for the rational design of functional materials and the multi-scenario expansion of optical fiber sensors.

4.1. Relative Humidity (RH)

In numerous fields where precise humidity control is essential, RH sensors play an indispensable role, exerting significant influence on both daily life and industrial production. From food storage and pharmaceutical manufacturing to the controlled environments of electronic device production, accurate humidity monitoring and regulation directly affect product quality, device performance, and operational safety [181,182,183]. Optical fiber sensors, with their unique advantages, have emerged as prominent candidates in the field of humidity sensing. Compared to conventional sensors, fiber-optic sensors offer a compact structure, ease of integration, and high sensitivity, enabling rapid detection of subtle humidity variations. In addition, they exhibit strong corrosion resistance, allowing stable operation in chemically harsh environments, and possess excellent immunity to electromagnetic interference—ensuring precise and reliable measurements even in complex industrial settings. Nanomaterials, due to their high specific surface area, greatly enhance interactions with surrounding environments, significantly improving detection sensitivity. They are often employed as sensitive coating layers in humidity sensing applications. In such coatings, the refractive index changes with the adsorption/desorption of water molecules, modulating the optical signal accordingly.

In 2020, Tsai et al. innovatively proposed a GO-coated base-S type long-period fiber grating (LPFG) sensor. achieving an average transmission loss sensitivity of −0.1824 dB/%RH over the 20–80% RH range. The sensor exhibited fully reversible behavior at room temperature and demonstrated excellent long-term repeatability, confirming its stability and reliability for practical applications. [184]. That same year, Al-Hayali et al. developed a humidity sensor by coating a balloon-shaped single-mode fiber with a 20 nm gold nanolayer, reaching a sensitivity of −0.571 nm/%RH [185]. In 2021, Li et al. reported a humidity sensor by sequentially coating polyimide (PI) and graphene thin films on a fiber Bragg grating (FBG), which showed a sensitivity 1.8 times higher than that of a PI-only coated sensor [186]. In 2024, Ahmad et al. fabricated a Mach–Zehnder interferometric humidity sensor by coating polyvinylpyrrolidone (PVP)/SiO₂ nanofilms onto etched-coreless fibers connected between two single-mode fibers (SMFs), achieving a sensitivity of −0.2718 dB/%RH [187]. In 2025, Salim et al. introduced a sensor using rose bengal-functionalized silver ellipsoidal nanoparticles as a sensing layer, which demonstrated an excellent sensitivity of 0.16 dB/%RH [188].

Beyond single-parameter humidity sensors, dual-parameter measurement—such as simultaneous monitoring of temperature and humidity—has gained increasing attention in practical applications like food processing, environmental engineering, and pharmaceutical production. This approach provides more comprehensive environmental information while reducing sensor count and system complexity, thereby enhancing measurement efficiency and reliability. In 2022, Cheng et al. developed a dual-parameter SPR fiber sensor using multimode fibers connected to two coreless fibers, with Ag/carboxymethyl cellulose (CMC) and Au/PDMS coatings on each, respectively. The CMC layer detected RH through water-induced RI changes, while the PDMS layer monitored temperature via its temperature-dependent RI. The humidity sensitivities across 50–80% RH were −1.230, −2.932, and −0.431 nm/RH%, and the temperature sensitivity was −2.213 nm/°C over 10–50 °C [189]. In 2023, Zhang et al. proposed a novel dual-parameter sensor by cascading two LPGs (see Figure 1), coating LPG1 with a PI/graphene QDs hybrid film for humidity sensing (achieving 78 pm/%RH over 45–75% RH), and LPG2 with dimethyl silicone oil (DSO) for temperature sensitivity, reaching 445 pm/°C. This simple yet effective structure achieved high sensitivity and good stability in simultaneous humidity and temperature monitoring [190].

Whether based on graphene, SiO₂ nanospheres, metallic nanoparticles, or polymer-based functional coatings, these materials with high specific surface areas and high response activity play a critical role in enhancing the evanescent field interaction and improving the moisture absorption characteristics of the sensing films. They significantly improve the sensitivity, selectivity, and response speed of the sensors. In addition, approaches such as dual-grating configurations and composite material coatings have enabled simultaneous detection of multiple parameters, such as temperature and humidity, thereby enhancing overall measurement efficiency and addressing the need for comprehensive environmental information in complex application scenarios. As a result, humidity sensors that integrate nanomaterials with optical fiber structural design are evolving toward higher sensitivity, multi-parameter integration, structural simplification, and enhanced practical reliability.

4.2. Gas Detection

Gas sensors play a vital role in various fields such as environmental monitoring, industrial production, healthcare, and agriculture. For instance, the detection of pollutant gases is essential for the effective and real-time control of harmful emissions [191,192]; in industrial settings, timely gas detection enables rapid response to prevent potentially catastrophic accidents caused by leaks [193]; in medical diagnostics, gas-based biomarkers facilitate early disease detection and assessment [194,195]; and in agriculture, monitoring gases such as ethylene and nitric oxide can provide valuable insights into the health status of crops and algae [196,197].

VOCs are a group of harmful chemicals widely present in the environment. Major sources of VOCs include vehicular exhaust emissions, industrial processes, evaporation of petroleum products, and the use of various organic solvents [198]. These compounds are not only chemically diverse but also highly volatile and diffusive, enabling them to spread rapidly through the atmosphere. Many VOCs have been identified as hazardous air pollutants; aromatic hydrocarbons (e.g., benzene, toluene, xylene) and halogenated hydrocarbons (e.g., trichloroethylene, carbon tetrachloride) are classified as potential carcinogens. Long-term exposure to VOCs poses serious health risks, including respiratory diseases, neurological damage, and even cancer [199]. As a highly efficient and sensitive detection platform, fiber-optic gas sensors play an indispensable role in VOC monitoring. Through the integration of highly selective sensing materials and advanced optical interrogation techniques, these sensors can achieve real-time and rapid detection of VOC concentration changes in the environment. Such capabilities offer reliable data support for applications including environmental monitoring, pollution source tracing, industrial emission control, and indoor air quality assessment [200]. To clearly illustrate recent progress in the application of nanomaterials in VOC gas sensing using optical fiber platforms,

Table 1. Applications of nanomaterial-based coatings in VOC optical fiber gas sensors.

Target Gas	Nanomaterial Coating	Deposition Method	Sensitivity	Response Time	Recovery Time	Reference
Ammonia, ethanol, and methanol vapors	Copper(II) Oxide (CuO)	Dip Coating	26 counts/ppm (Ammonia)	59 s (Ammonia)	53 s (Ammonia)	[201]
Acetone	MIL-101 (Chromium)	PVD	---	---	----	[202]
Acetone, isopropyl alcohol (IPA) and ethanol	ZnO	Spray Coating	Acetone: 0.16 nm/% IPA: 0.08 nm/% Ethanol: 0.07 nm/%	Acetone: 10 min IPA: 9 min Ethanol: 8 min	---	[203]
IPA	ZnO	Hydrothermal	0.053 nm/%IPA vapor	9 min	---	[204]
Acetone, acetophenone, IPA, and ethanol	ZnO/AZO/ tin dioxide (SnO2)	PVD	---	17 s (SnO2 thin-film sensor monitors 250 ppm IPA)	21 s (SnO2 thin-film sensor monitors 250 ppm IPA)	[205]
Acetone, ethanol, and methanol	Au	LbL	Acetone: 13.7 nm/% Ethanol: 15.5 nm/% Methanol: 6.7 nm/%	Acetone: 9.3 5 min Ethanol: 5.35 min Methanol: 2.39 min	Acetone: 3.85 min Ethanol: 2.12 min Methanol: 1.44 min	[206]
Acetone, methanol, ethanol, isopropanol, toluene, xylene	ZnO	Spray Coating	0.116 nm/ppm (Acetone)	26 s (Acetone)	32 s (Acetone)	[207]
Ammonia, benzene, acetone, nitrogen oxides, ethanol, methanol, triethylamine, trimethylamine, toluene, formaldehyde	Palladium (Pd)-loaded tungsten trioxide (WO3)	Dip Coating	80 normalized response/ppm (Acetone)	---	---	[208]
Ethanol	Magnesium (Mg)-doping ZnO	Spray Coating	---	---	---	[209]
Isopropanol	SnO2	Dip Coating	22 counts/ppm	---	---	[210]
Ethylene	Copper (Cu) Complex-1	CDSA	60 pm/ppm	---	---	[211]
Acetone, ethyl acetate, cyclohexane, isopropanol	Molybdenum disulfide (MoS2)	Optical Tweezer Effect	Acetone: 0.0195 nm/ppm Ethyl acetate: 0.0143 nm/ppm Cyclohexane: 0.0072 nm/ppm Isopropanol: 0.0058 nm/ppm	15 min	5 min	[212]
Ethanol	TiO2	Dip Coating	3.85 pm/ppm	20 s	60 s	[213]

summarizes key studies by listing the target gases, types of nanomaterials used, deposition methods, reported sensitivities, and corresponding response and recovery times.

In addition to the detection of VOCs, fiber-optic gas sensors targeting other gases of significant environmental and biological relevance—such as hydrogen sulfide (H₂S), ammonia (NH₃), and nitrogen dioxide (NO₂)—have attracted growing attention in recent years. These gases are often characterized by high toxicity, low concentrations, and rapid response requirements, thus imposing stricter demands on sensor selectivity, sensitivity, and stability. By introducing functional nanomaterial coatings, including metal oxides, noble metal nanoparticles, two-dimensional materials, and porous framework structures, researchers have achieved effective recognition and quantitative detection of these gases.

In 2021, D. Lopez et al. developed two H₂S gas sensors by coating nanomaterials onto U-shaped plastic optical fibers: one with CuO nanoparticles and the other with iron(II) oxide (FeO) nanoparticles. The experimental results demonstrated that both sensors effectively detected H₂S in mixed gas environments, with the CuO-coated sensor exhibiting higher sensitivity. Moreover, the CuO-based sensor showed an irreversible chemical adsorption behavior, whereas the FeO-coated sensor demonstrated reversible physical adsorption characteristics [214]. In the same year, R. Prado et al. achieved efficient detection of H₂S gas in the concentration range of 0.4–2.0 ppm at room temperature by depositing Au nanoparticles on the surface of an optical fiber to induce LSPR [215]. Chen et al. developed a surface plasmon resonance sensor using a composite coating of TiO₂ and multi-walled carbon nanotubes (MWCNTs), achieving an outstanding detection limit of 0.2 ppm by optimizing the composite formulation [216]. In 2023, D. Lopez-Vargas et al. developed and tested two H₂S gas sensors by coating U-shaped plastic optical fibers with magnetite (Fe₃O₄) nanoparticles and a combination of Ag and Fe₃O₄ nanoparticles. The results showed that the Ag/Fe₃O₄-coated sensor exhibited faster response, higher selectivity, and greater sensitivity compared to the Fe₃O₄-only sensor [217]. In the same year, Liu et al. developed a fiber-optic SPR sensor for H₂S detection using Ag/(3-aminopropyl)triethoxysilane (APTES)/Cu-MWCNT as the sensing membrane. This sensor demonstrated a detection limit of approximately 0.146 ppm within the 10–100 ppm concentration range [218]. Also in 2023, Cai et al. developed a NO₂ gas sensor by embedding indium oxide (In₂O₃) nanoparticle-loaded porous Au nanostructures into ZnO nanofibers, as shown in Figure 2. The sensor exhibited a high response at low operating temperatures [219]. In 2025, Khomarloo et al. designed a sensor capable of simultaneously detecting nitric oxide (NO) and NO₂ by doping reduced graphene oxide (rGO) into ZnO nanofibers at various concentrations. The experimental results indicated that the addition of rGO improved the sensor’s response and recovery times by 34% and 54%, respectively, and yielded a maximum response value of 169.5 at 150 °C [220].

In gas sensors, the performance enhancement of fiber-optic gas sensors highly depends on the selection of functional nanomaterials and the optimization of coating deposition techniques. Widely used nanomaterials include metal oxides, noble metal nanoparticles, two-dimensional materials, and metal–organic frameworks. These materials, owing to their high specific surface area, tunable optical/chemical properties, and strong gas sensitivity responses, enable highly sensitive detection of various gases. Regarding the deposition methods for nanocoatings, spraying and PVD are suitable for rapid fabrication of large-area films, while LbL assembly and CDSA are better suited for precise molecular-scale modifications. Overall, fiber-optic gas sensors based on functional nanomaterial coatings have demonstrated sub-ppm level detection capabilities and exhibit great potential in simultaneous multi-gas monitoring, room-temperature low-power operation, high response rates, and selective sensing. These advantages lay a solid foundation for their broad applications in environmental monitoring, industrial emission control, biomedical diagnostics, and public safety warning systems.

4.3. Biosensors

The working principle of traditional biosensors is based on the specific interaction between the biorecognition layer on the fiber-optic surface and the target analyte. This interaction induces changes in the local refractive index surrounding the optical fiber. Since the evanescent field at the fiber–medium interface is highly sensitive to refractive index variations, such changes result in corresponding modifications in the intensity and wavelength of the evanescent field. These variations can be detected by the fiber-optic sensor, enabling high-sensitivity detection of the target analyte. Because biomarker concentrations in biological samples are typically very low, biosensors are required to exhibit ultrahigh sensitivity and extremely low detection limits in highly dilute biological solutions [221,222].

Nanomaterials possess large specific surface areas that provide more binding sites. Consequently, biosensors constructed by depositing various nanomaterials as bioselective layers on optical fibers demonstrate faster detection speeds, higher sensitivity, and improved reliability compared to traditional biosensors.

In 2020, Singh et al. ingeniously developed a high-performance sensor for cancer cell detection. They spliced multicore fibers with single-mode fibers and coated their surfaces with multiple nanomaterials. AuNPs were employed to excite LSPR on the fiber, significantly enhancing the sensor’s sensitivity toward cancer cells. GO and CuO nanoflowers were used to improve biocompatibility. The sensor exhibited extremely low detection limits for multiple cancer cell lines and demonstrated excellent anti-interference capability and reproducibility [223]. In 2021, Wang et al. reported a DNAzyme biosensor based on tilted fiber Bragg grating (TFBG) with SPR for ultra-trace detection of lead ions. The sensor exploited the “hotspot” effect between AuNPs and a gold film to amplify detection signals. Experimental results showed a detection limit as low as 8.56 pM for lead ions, with excellent selectivity and a large dynamic response over the concentration range of 10⁻¹¹ M to 10⁻⁶ M. Furthermore, its practical potential was validated through tests on clinical human serum samples, highlighting its value as a highly efficient, portable, and miniaturizable onsite rapid detection technology [224]. In 2024, Liu et al. developed a sensor for detecting enrofloxacin residues by coating Au nanoparticles, ZnO nanowires, and a composite of Fe₃O₄ and Au on a spliced fiber structure to excite localized surface plasmon resonance. The sensor achieved a sensitivity of 1.89 nm/ln(μg/mL) within the 0–1000 μg/mL concentration range of enrofloxacin solutions [225]. In the same year, Fu et al. developed a fiber-optic sensor for detecting Vibrio parahaemolyticus by depositing multilayer films composed of gold nanoparticles, MoS₂ nanoparticles, and cerium oxide (CeO₂) nanorods on a tapered-in-tapered fiber structure. This sensor exhibited a high sensitivity of 1.61 nm/[colony-forming unit(CFU)/mL] over a linear detection range from 1 to 10⁷ CFU/mL [226]. In 2025, Borjikhani et al. developed a label-free refractive index biosensor for early cancer cell detection by coating hollow gold nanoparticles on a tapered fiber surface, as shown in Figure 3. This sensor demonstrated a sensitivity approximately 2–3 times higher than sensors with similar structures [227].

In the fields of glucose sensing and protein detection, coating with nanomaterials to increase binding sites—thereby enhancing sensor sensitivity and improving biocompatibility—has become a widely adopted and effective strategy. Over the past five years, significant progress has been made in applying nanomaterials to fiber-optic glucose sensors and fiber-optic protein sensors. Specifically,

Table 2. Applications of nanomaterials in coatings for fiber-optic glucose sensors.

Nanocoating Materials	Deposition Method	Measurement Range	Sensitivity/Limit of Detection (LOD)	References
Au	CDSA	0–10 mM	0.9261 nm/mM	[228]
GO, Au	CDSA	0–11 mM	1.06 nm/mM	[229]
GO, Au	CDSA	0–1 mL	11.134 nm/(mg/mL)	[230]
Au	CDSA	1.328–1.393	2032%/RIU	[231]
GO, multi-wall carbon nanotubes, Au	CDSA	0–10 mM	1.04 nm/mM	[232]
Ag, ZnO	PVD	0.5 mM	6.156 nm/mM	[233]
MoS2, graphene	CVD	0–300 mg/dL	6126.25 nm/RIU	[234]
Au	PVD	0–0.5 mg/mL	85.4nm/(mg/mL)	[235]
Aniline functionalized graphene QDs	---	0.05–20 mM	2.1 μM	[236]
Ag	PVD	5–25%	2.882 nm/%	[237]
GO	Dip Coating	0–4 mg/mL	0.77 nm/(mg/mL)	[238]
Au	CDSA	1nM-1mM	2.5nM	[239]

and

Table 3. Applications of nanomaterials in coatings of optical fiber protein detection sensors.

Target Analyte	Nanocoating Material	Deposition Method	Measurement Range	LOD	Reference
Ig-G protein	Au	PVD	0–16 μg/mL	4.6 ng/mL	[240]
Ig-G protein	MoS2-Au	self-assemble	0–1 mg/mL	0.33 μg/mL	[241]
C-reactive protein	Ag-Au	CSDA	0–2.5 mg/L	2.4 × 10−5 mg/L	[242]
SARS-CoV-2 spike protein	Au	CSDA	1 pg/mL–100 μg/mL	---	[243]
Cardiac troponin I protein	GO-Au	CSDA	0–1000 ng/mL	96.2638 ng/mL	[244]
SARS-CoV-2 nucleocapsid protein	Au	CSDA	0.1 ng/mL–100 ng/mL	2.5 ng/mL	[245]
CD44 protein	Au	PVD	0 Nm–100 nM	17 pM	[246]
Tau protein	SnO2−x	PVD	10−3–10 μg/mL	1 pM	[247]
Collagen-IV	Au-ZnO	CSDA	0–40 μg/mL	1.6 μg/mL	[248]
Ig-G protein	MoS2	electrostatic self-assembly	0–22 μg/mL	19.7 ng/mL	[249]
SARS-CoV-2 spike protein	Au	PVD	25–1000 nM	37 nM	[250]
C-reactive protein	Au, Cr, polydopamine	PVD, solution-based self-assembly	0–78.6 μg/mL	0.22 μg/mL	[251]

provide comprehensive summaries and analyses of these applications, covering key information such as types of nanomaterials, coating methods, and sensor performance. These data not only visually demonstrate the diverse applications of nanomaterials in the sensor domain but also offer valuable references for subsequent research and development, facilitating further advancement and innovation of fiber-optic sensor technologies in biomedical detection.

Nanomaterials exhibit unique quantum size effects due to their distinctive dimensional characteristics. Among them, QDs, as typical semiconductor nanocrystals, usually range in size from 2 to 20 nm, which is smaller than the corresponding bulk Bohr diameter. One of the most important advantages of nanoparticle quantum dots is their facile functionalization and the ability to be embedded into or bonded onto various surfaces and matrices [252,253].

In 2024, Lang et al. developed a fiber-optic sensor for the detection of Staphylococcus aureus. By utilizing gold nanoparticles to excite localized surface plasmon resonance (LSPR) and further enhancing sensitivity with chitosan-coated iron(III) oxide nanoparticles and tungsten disulfide quantum dots, antibodies were immobilized on the fiber surface to achieve specific detection of S. aureus. Experimental results demonstrated that the sensor reached a maximum sensitivity of 2.74 nm/lg (CFU/mL) at a bacterial concentration of 1 × 10⁸ CFU/mL, with a detection limit as low as 6.67 CFU/mL. Additionally, the sensor exhibited good stability and reproducibility in real food samples such as chocolate, fish, juice, and milk, indicating broad application potential in disease detection, medical diagnostics, and food safety inspection [254]. In the same year, Azargoshasb et al. developed a ratiometric fluorescent biosensor based on a fiber-optic platform for dopamine detection. This sensor employed blue-emitting carbon quantum dots (CQDs) derived from cabbage and a silica-based molecularly imprinted polymer (MIP) coating. Transmission electron microscopy (TEM) images of the various coatings are shown in Figure 4. Dopamine detection was achieved through monitoring LMR formed on the fiber surface. The sensor demonstrated a dynamic response over the concentration range of 0.3–100 µM with a detection limit as low as 0.027 µM. Moreover, the sensor exhibited excellent selectivity and anti-interference performance in real samples, with negligible cross-reactivity to other potential interferents such as adrenaline, ascorbic acid, and uric acid [255].

In the field of biosensors, green-synthesized nanomaterials offer excellent biocompatibility and stability, along with the advantages of simple synthesis procedures, low cost, and environmental friendliness. As such, nanocoatings prepared via green synthesis methods have been widely applied as functional layers in sensor development [256,257,258]. Among these, green-synthesized metallic nanoparticles (such as AuNPs and AgNPs) and metal oxide nanomaterials like ZnO are currently the focus and hotspot of extensive research.

In 2022, Ashikbayeva et al. utilized tea extract to green-synthesize gold nanoparticles and integrated them onto the surface of a spherical optical resonator for the detection of low concentrations of the cancer biomarker CD44. The experimental results showed a significant enhancement in detection sensitivity for the CD44 antigen, with an intensity change of up to 13.17 dB (corresponding to a 1.52 dB change per 10-fold concentration increase). The detection range spanned from 42.9 aM to 100 nM, with a LOD as low as 0.111 pM. Compared to traditional fiber-optic sensors, this sensor exhibited a 25-fold improvement in sensitivity while maintaining sub-picomolar detection limits and real-time, wide-range sensing capabilities [259]. In 2025, Gangal et al. also successfully synthesized silver nanoparticles through a plant-mediated green synthesis approach, as illustrated in Figure 5. The silver nanoparticles were then coated along with enzyme layers onto the surface of single-mode optical fibers to fabricate a sensor for the detection of ascorbic acid [260].

The incorporation of nanomaterials—particularly metal nanoparticles, oxide nanostructures, and quantum dots with high specific surface areas and favorable surface activity—has significantly enhanced the surface binding capacity and signal amplification of fiber-optic biosensors. This advancement has effectively addressed critical bottlenecks such as low detection limits and slow response times. Notably, the widespread adoption of green synthesis techniques has made the fabrication of nanomaterials more environmentally friendly, straightforward, and biocompatible, offering a promising pathway for the large-scale application of fiber-optic biosensors.

5. Conclusions

This paper presents a comprehensive review of the research progress over the past five years on nanocoating materials in optical fiber sensors, focusing on three representative application areas: humidity sensing, gas detection, and biosensing. Particular attention is given to the key roles and technological advancements of nanomaterials in enhancing sensor performance. Benefiting from their high specific surface area, excellent surface activity, and tunable physicochemical properties, nanomaterials have significantly improved the overall performance of optical fiber sensors in terms of sensitivity, response time, selectivity, and detection limits. Moreover, the introduction of emerging techniques such as green synthesis offers environmentally friendly and facile solutions for the large-scale production and practical application of nanomaterials, further promoting the deployment of optical fiber sensors in various fields, including biomedicine, environmental monitoring, and food safety.

In terms of deposition methods, the evolution from traditional techniques such as spin coating and dip coating to more advanced methods like self-assembly has provided diverse fabrication strategies and precise control for constructing functional coatings. The integration of multiple sensing parameters, structural optimization, and the synergistic design of composite materials are becoming key trends in enhancing the performance and adaptability of sensing systems. Future research should focus on the development of novel nanocomposites, precise control of deposition techniques, and the miniaturization and integration of sensor structures to achieve higher sensitivity, improved robustness, and broader applicability.

In conclusion, the cross-disciplinary integration of nanocoating materials and optical fiber sensing technology is driving the evolution of sensors toward high sensitivity, low power consumption, and intelligent functionality. This advancement offers an innovative technological pathway for accurate sensing in complex environments and holds great potential for broad practical applications.