Chemosensors, Volume 11, Issue 4 (April 2023) – 55 articles

In this work, MXene/NiO-composite-based formaldehyde (HCHO) sensing materials were successfully synthesized by an in situ precipitation method. The heterostructures between the MXene and NiO nanoparticles were verified by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The HCHO sensing performance of the MXene/NiO-based chemiresistive-type sensors was investigated. Compared to pure MXene and NiO materials, the sensing performance of the MXene/NiO-P2-based sensor to HCHO gas at room temperature was significantly enhanced by the formation of MXene/NiO heterojunctions. The response of the MXene/NiO-P2 sensor to 50 ppm HCHO gas was 8.8, which was much higher than that of the pure MXene and NiO. At room temperature, the detectable HCHO concentration of the MXene/NiO-P2-based sensor was 1 ppm, and the response and recovery time to 2 ppm HCHO was 279 s and 346 s, respectively. The MXene/NiO-P2 sensor also exhibited a good selectivity and a long-term stability to HCHO gas for 56 days. The in situ Fourier transform infrared (FTIR) spectra of the MXene/NiO-P2 sensor, when exposed to HCHO gas at different times, were investigated to verify the adsorption reaction products of HCHO molecules. Full article

This study involved the creation and assessment of a microwave sensor to measure glucose levels in aqueous solutions without invasiveness. The sensor design utilized a planar interdigital capacitor (IDC) loaded with a hexagonal complementary split-ring resonator (HCSRR). The HCSRR was chosen for its ability to generate a highly intense electric field that is capable of detecting variations in the dielectric characteristics of the specimen. A chamber tube was used to fill glucose solutions at the sensor’s sensitive area, and changes in the device’s resonance frequency (F_r) and reflection coefficient (S₁₁) were used to measure glucose levels. Fitting formulas were developed to analyze the data, and laboratory tests showed that the sensor could accurately measure glucose levels within a range of 0–150 mg/dL. At a concentration of 37.5 mg/dL, the sensitivity based on S₁₁ and F_r reached maximum values of 10.023 dB per mg/dL and 1.73 MHz per mg/dL, respectively. This implies that the sensor put forward has the possibility of being utilized in medical settings for the monitoring of glucose levels. Full article

The Tesla valve (TV), a valvular conduit invented by Nicola Tesla over a century ago, has recently acquired significant attention and application in various fields because of the growing interest in microfluidics and nanofluidics. The unique architecture of TV characterized by an asymmetrical design and an arc-shaped channel has long been an intriguing yet underrated design for building a passive component in a microfluidic system. While previously regarded as a technology without significant use, TV structures have been implemented in thermal manipulation fluidics, micromixers and micropumps, benefitting the advancement of urgently demanding technology in various areas, such as in biomedical diagnostics through wearable electronics and medical instruments, lab on a chip, chemosensors and in application toward sustainable technology manifested in fuel cell devices. This article presents the first comprehensive review of TV structures in the literature, which has seen significant growth in the last two years. The review discusses typical TV structures, including single-stage TV (STV), multistage TV (MSTV), and TV derivatives (TVD), along with their characteristics and potential applications. The designs of these structures vary based on their intended applications, but all are constructed based on the fundamental principle of the TV structure. Finally, future trends and potential applications of TV structures are summarized and discussed. This topical review provides a valuable reference for students, early-career scientists, and practitioners in fluidic devices, particularly those interested in using TV structures as passive components. Full article

Lateral flow assay (LFA) is emerging as one of the most popular paper-based biosensors in the field of the diagnostic industry. LFA fills all the gaps between diagnosis and treatment as it provides beneficial qualities to users such as quick response, Point-of-care appeal, early detection, low cost, and effective and sensitive detections of various infectious diseases. These benefits increase LFA’s dependability for disease management because rapid and accurate disease diagnosis is a prerequisite for effective medication. Only 2% of overall healthcare expenditures, according to Roche Molecular Diagnostics, are spent on in vitro diagnostics, even though 60% of treatment choices are based on this data. To make LFA more innovative, futuristic plans have been outlined in many reports. Thus, this review reports on very knowledgeable literature discussing LFA and its development along with recent futuristic plans for LFA-based biosensors that cover all the novel features of the improvement of LFA. LFA might therefore pose a very significant economic success and have a significant influence on medical diagnosis. Full article

A novel electrochemical sensor based on Cu-loaded carbon nanospheres (Cu–CNSs) was designed and fabricated. Initially, the CNSs were synthesized using a natural or inexpensive carbon source (dark brown sugar), and Cu was loaded to enhance the electrocatalytic properties of the material. Subsequently, the synthesized Cu–CNSs were modified onto a screen-printed carbon electrode (SPCE), termed Cu–CNS/SPCE, to simultaneously detect the biomarkers dopamine (DA) and melatonin (MT) through differential pulse voltammetry. The surface characterization of the Cu–CNSs confirmed the formation of carbon spheres and Cu nanoparticles covering the spheres. Electrochemical studies showed that the Cu–CNS/SPCE had a high selectivity and sensitivity toward DA and MT, with a significant peak separation of 0.502 V. The two linear ranges of DA were 0.125–20 μM and 20–100 μM and the linear range of MT was 1.0–100 μM, with corresponding detection limits of 0.34 μM and 0.33 μM (S/N = 3), respectively. The quantification limits for DA and MT were 2.19 and 1.09 μM (S/N = 10), respectively. The sensor performance is attributed to the high conductivity and large, electrochemically active surface area of the Cu–CNS. In human serum samples, the Cu–CNS/SPCE exhibited good selectivity and satisfactory reproducibility for the simultaneous determination of DA and MT. Full article

This work presents a new architecture concept for microfluidic devices, which combines the conventional 3D printing fabrication process with the stable and precise integration of polymeric functional materials in small footprints within the microchannels in well-defined locations. The approach solves the assembly errors that normally occur during the integration of functional and/or sensing materials in hybrid microfluidic devices. The method was demonstrated by embedding four pH-sensitive ionogel microstructures along the main microfluidic channel of a complex 3D printed microfluidic device. The results showed that this microfluidic architecture, comprising the internal integration of sensing microstructures of diverse chemical compositions, highly enhanced the adhesion force between the microstructures and the 3D printed microfluidic device that contains them. In addition, the performance of this novel 3D printed pH sensor device was investigated using image analysis of the pH colour variations obtained from photos taken with a conventional camera. The device presented accurate and repetitive pH responses in the 2 to 12 pH range without showing any type of device deterioration or lack of performance over time. Full article

Colloidal quantum dots (CQDs) are gaining increasing attention for gas sensing applications due to their large surface area and abundant active sites. However, traditional resistor-type gas sensors using CQDs to realize molecule recognition and signal transduction at the same time are associated with the trade-off between sensitivity and conductivity. This limitation has restricted their range of practical applications. In this study, we propose and demonstrate a monolithically integrated field-effect transistor (FET) gas sensor. This novel FET-type gas sensor utilizes the capacitance coupling effect of the CQD sensing film based on a floating gate, and the quantum capacitance plays a role in the capacitance response of the CQD sensing film. By effectively separating the gate sensing film from the two-dimensional electron gas (2DEG) conduction channel, the lead sulfide (PbS) CQD gate-sensitized FET gas sensor offers high sensitivity, a high signal-to-noise ratio, and a wide range, with a real-time response of sub-ppb NO₂. This work highlights the potential of quantum dot-sensitized FET gas sensors as a practical solution for integrated gas sensor chip applications using CQDs. Full article

Lung cancer is the most prevalent severe illness in both sexes and all ages and the leading cause of cancer-related deaths globally. Late-stage diagnosis is the primary cause of its high mortality rate. Therefore, the management of lung cancer needs early-stage screening. Breath analysis is a non-invasive, low-cost, and user-friendly approach to diagnosing lung cancer. Among the various types of breath sensors, MOS gas sensors are preferred due to their high gas responses, fast response times, robustness, and lower price. This review focuses on the critical role of MOS gas sensors in detecting VOCs in lung cancer patients’ exhaled breath. It introduces the basic working mechanism of MOS gas-sensitive materials, summarizes some high-performance MOS materials suitable for detecting potential lung cancer biomarkers and provides performance enhancement strategies. The review also briefly introduces the sensor array and its pattern recognition algorithm. Finally, we discuss the challenges in developing MOS gas sensors for lung cancer screening and present the prospect of using the e-nose for large-scale early lung cancer screening. Full article

Monitoring the concentration of hydrogen is very important as it is a flammable and explosive gas. Non-Nernstian potentiometric hydrogen sensors hold promising potentials for the sensitive detection of hydrogen. This paper reports the improved H₂-sensing performance of a mixed oxide ion-electron conducting (MIEC) Pr_0.1Ce_0.9O_2−δ (PCO) electrode using Fe₂O₃ surface modification. The Fe₂O₃-modified PCO exhibited a high response of −184.29 mV to 1000 ppm H₂ at 450 °C. The response values exhibited a linear or logarithmic dependence on the H₂ concentration for below or above 20 ppm, respectively. A sensitivity of −74.9 mV/decade in the concentration range of 20–1000 ppm was achieved, and the theoretical limit of detection was calculated to be 343 ppb. Moreover, a power-law relationship between the response time and the concentration value was also found. Electrochemical impedance analyses revealed that the excellent H₂-sensing performance may be attributed to the large ratio of the electrochemical activity of the hydrogen oxidation reaction (HOR) over the oxygen exchange reaction (OER). In addition, the distribution of relaxation time (DRT) results reveal that the enhanced electrochemical kinetics caused by H₂ presence in air is mainly related to acceleration of the electrode surface processes. Full article

An optical sensor based on pyranine immobilized on aminopropyl-modified mesoporous silica films was developed for paraquat detection in aqueous solutions. An electrochemically assisted self-assembly method was used to deposit mesoporous silica film on fluorine-doped tin oxide glass. The obtained films were modified with various concentrations of 3-aminopropyl triethoxysilane (APTES) before the immobilization of pyranine. Cyclic voltammetry, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and fluorescence spectroscopy were used to characterize the films. Pyranine-immobilized films gave an emission at 506 nm with an excitation at 450 nm. The fluorescence signal was quenched in the presence of paraquat. The films modified with 3% APTES provided the optimum response to paraquat. The developed films had a linear response to paraquat in the concentration range of 1 to 10 ppm at the optimum conditions, with a detection limit of 0.80 ppm. The developed method was used to quantify paraquat in sugarcane peel and tap water samples with satisfactory results. Full article

The flavor of foods and beverages is generally composed of a mixture of volatile compounds, however not all the molecules that form an aroma are sensorially relevant. The odor-active compounds present in a mixture are different for each subject, both in quantitative and qualitative terms. This means that the ability of the human nose to act as a chemical sensor varies among individuals. In this study, we used the headspace of roasted coffee beans as a complex olfactory stimulus and, by means of the coupled Gas Chromatography-Olfactometry (GC-O) technique, the single components of coffee flavor were separated. Each subject, previously classified for his/her olfactory status (normosmic, hyposmic or anosmic) by means of the Sniffin’ Sticks battery (composed of Threshold, Discrimination and Identification subtests), had to identify and evaluate each smelled molecule. The results show that the individual ability to detect individual compounds during the GC-O experiments and the odor intensity reported during the sniffing of pen #10 (the pen of the identification test) containing coffee aroma were related to TDI olfactory status (based on the score obtained from the sum composed of Threshold, Discrimination and Identification scores). We also found that the number of total molecules and of molecules smelling of coffee is linearly related to the TDI olfactory score. Finally, the odor intensity reported when sniffing pen #10 containing coffee aroma is positively correlated with the number of molecules detected and the average intensity reported. In conclusion, our findings show that the human perception of both individual compounds and complex odors is strongly conditioned by the olfactory function of subjects. Full article

The present study reports on the fabrication and performance of ammonia sensors based on single-walled carbon nanotubes (SWCNTs) coated with gold nanoparticles (AuNPs). The AuNPs were incorporated onto the SWCNTs using two different methods: sputtering and chemical deposition. The sensors were exposed to controlled concentrations of ammonia at two temperatures, namely, 25 °C and 140 °C, and their response was monitored through successive cycles of ammonia exposure (0.5 ppm and 1.0 ppm) and nitrogen purging. The results demonstrate that the sputtering-based deposition of the AuNPs on SWCNTs led to the best sensor performance, characterized by a rapid increase in resistance values (t_resp = 12 s) upon exposure to ammonia and an efficient recovery at 140 °C (t_rec = 52 s). By contrast, the sensor with chemically impregnated AuNPs exhibited a slower response time (t_resp = 25 s) and the same recovery time (t_rec = 52 s). Additionally, a novel device was developed that combined MoS₂-AuNPs (sputtering)-SWCNTs. This sensor was obtained by impregnating nanosheets of MoS₂ onto AuNPs (sputtering)-SWCNTs showing improved sensor performance compared to the devices with only AuNPs. In this case, the sensor exhibited a better behavior with a faster recovery of resistance values, even at room temperature. Overall, the study provides valuable insights into the fabrication and optimization of SWCNT-based ammonia sensors for various applications, particularly in detecting and quantifying small amounts of ammonia (concentrations below 1 ppm). Full article

The development of novel chemical nitro-explosive sensors with high sensitivity, low cost and a compact size is essential for homeland security, environmental protection and addressing military challenges. Polymeric optical waveguides based on refractive index sensing are widely used in biochemical detection due to their advantages of large-scale integration, low cost, high sensitivity and anti-electromagnetic interference. In this study, we designed and fabricated a polymer waveguide Mach–Zehnder interferometer (MZI) sensor to detect 2,4-dinitrotoluene (DNT) in water. One phase shifter of the MZI waveguide was functionalized by coating a thin cladding layer of polycarbonate with dipolar chromophores and used as the sensing arm; the other arm was coated with passive epoxy resin cladding and used as the reference arm. The phase difference between the two arms of the MZI was modulated using the refractive index (RI) change in the polycarbonate cladding when dipolar chromophores interacted with electro-deficient DNT. The theoretical sensitivity of the designed MZI can reach up to 24,696 nm/RIU. When used for explosive detection, our fabricated sensor had a maximum wavelength shift of 4.465 nm and good linear relation, with an R² of 0.96 between the wavelength shift and a concentration ranging from 3.5 × 10⁻⁵ to 6.3 × 10⁻⁴ mol/L. The sensitivity of our device was 6821.6 nm/(mol/L). The design of an unbalanced MZI sensor, together with the sensing material, provides a new approach to using low-cost, compact and highly sensitive devices for in-field explosive detection. Full article

The development and validation of a novel enhanced chemiluminescence enzyme immunoassay (CLEIA) with excellent sensitivity for the quantification of monoclonal antibodies (mAbs) used for immunotherapy of cancer are described in this paper for the first time. The 96-microwell plates were used for the assay procedures, which involved the non-competitive binding reaction to a specific antigen. The immune complex of the antigen-mAb formed on the internal surface of the plate wells was quantified by a novel chemiluminescence (CL)-producing horseradish peroxidase (HRP) reaction. The reaction employed 4-(imidazol-1-yl)phenol (IMP) as a highly potent signal enhancer for the HRP-luminol–hydrogen peroxide (H₂O₂) CL reaction. The proposed CLEIA was developed for bevacizumab (BEV), as a representative example for mAbs. The CLEIA was validated in accordance with the immunoassay validation for bioanalysis standards, and all of the validation criteria were met. The assay’s limit of detection (LOD) and limit of quantitation (LOQ) were 9.3 and 28.2 pg mL⁻¹, respectively, with a working dynamic range of 10–400 pg mL⁻¹. The assay enables the accurate and precise quantitation of mAbs in human plasma samples without any interference from endogenous substances and/or plasma matrix. The novel CLEIA was compared in terms of dynamic range and sensitivity with other pre-validated enzyme-linked immunosorbent assay (ELISA) using HRP/colorimetric substrate as a detection system and the observed differences were explained. The CLEIA protocol’s ease of use, high throughput, and simplicity allows to analyze numerous samples in clinical settings. The proposed CLEIA has a significant benefit in the assessment of mAbs in clinical settings for the evaluation of their pharmacokinetics, pharmacodynamics, therapeutic drug monitoring, and refining their safety profiles, opening a new era for a better understanding of pharmacodynamics at the cellular level. Full article

Sweat analysis by means of minimally invasive wearable sensors is considered a potentially disruptive method for assessing clinical parameters, with exciting applications in early medical diagnostics and high-performance sports. Electrochemical sensors and biosensors are especially attractive because of the possibility of the electronic integration of wearable devices. In this article, we review several aspects regarding the potentialities and present limitations of electrochemical sweat (bio)sensors, including: the main target analytes and their relationships with clinical conditions; most usual electrochemical techniques of transduction used according to the nature of the target analytes; issues connected to the collection of representative sweat samples; aspects regarding the associated, miniaturized electronic instrumentation used for signal processing and communication; and signal processing by machine learning. Full article

Chiral enantiomer recognition has important research significance in the field of analytical chemistry research. At present, most prepared chiral sensors are used for recognizing amino acids, while they are rarely used in the identification of drug intermediates. This work found that combining CS and reduced graphene oxide can enhance conductivity, increasing the recognition effect by connecting CS with BSA. Based on the above preparation, a new type of chiral sensor (3D–rGO–CS–BSA) was synthesized for the identification of drug intermediates, including the 1–Boc–3–hydroxypyrrolidine enantiomer. An obvious difference was achieved (I_R/I_S = 2.82) in the oxidation peak currents between the two enantiomers. The detection limits of the R–enantiomer and S–enantiomer were 4.85 nM and 11.76 nM, respectively. The proposed electrochemical sensing platform also has better potential for detecting the percentage content of mixed chiral enantiomer drugs. Full article

Ni- and Fe-based metal-organic frameworks (NiFe-MOFs) have abundant valence states and have the potential to be used as bifunctional electrode materials. However, unannealed NiFe-MOFs are still not widely used in electrode materials, including electrochemical sensing, supercapacitors, and overall water splitting. In addition, the direct growth of active material on a conductive carrier has been developed as a binder-free strategy for electrode preparation. This strategy avoids the use of insulating binders and additional electrode treatments, simplifies the preparation process of the NiFe-MOFs, and improves the conductivity and mechanical stability of the electrode. Therefore, in this study, we employed a simple solvothermal method combined with an in situ growth technique to directly grow NiFe-MOF-X (X = 4, 8, 12) nanomaterials of different sizes and morphologies on nickel foam at low reaction temperatures and different reaction times. The NiFe-MOF-8 electrode exhibited high capacitive properties, with an area-specific capacitance of 5964 mF cm⁻² at 2 mA cm⁻² and excellent durability. On the other hand, NiFe-MOF-12 exhibited strong catalytic activity in electrocatalytic tests performed in a 1 M KOH aqueous solution, demonstrating hydrogen evolution reaction (η₁₀ = 150 mV) and oxygen evolution reaction (η₅₀ = 362 mV) activities. The electrochemical sensing tests demonstrated a good response to BPA. Overall, our results suggest that the direct growth of NiFe-MOFs on nickel foam using a simple solvothermal method combined with an in situ growth technique is a promising strategy. Full article

Fatty liver diseases are a spectrum of liver disorders consisting of the benign fatty liver, which could eventually lead to cirrhosis or even hepatocellular cancer (HCC) without timely treatment. Therefore, early diagnosis is crucial for fatty liver diseases. Liver biopsy is regarded as the gold standard in the diagnosis of fatty liver diseases. However, it is not recommended for routine use due to its invasiveness and complicated operation. Thus, it is urgent to diagnose fatty liver diseases with non-invasive and precise methods. In this regard, fluorescence imaging technology has attracted intensive attention and become a robust non-invasive method for fatty liver visualization, and a series of fluorescent probes are being intensively designed to track the biomarkers in fatty liver. In this brief review, the small molecular fluorescent probes employed in fatty liver are summarized, mainly focusing on the last four years. Moreover, current opportunities and challenges in the development of fluorescent probes for fatty liver will be highlighted. Full article

The detection of human serum albumin (HSA) is of significant clinical importance in disease diagnoses. In this work, polymer-based synthetic receptors are designed by incorporating Ag-ZnS microspheres in molecularly imprinted poly(methacrylic acid-co-ethylene glycol dimethacrylate) (MIPs) for the gravimetric detection of HSA. Among different compositions of Ag-ZnS@MIPs, MIPs having methacrylic acid and ethylene glycol dimethacrylate volume ratio of 3:2 exhibit enhanced HSA sensitivity in the concentration range of 5–200 ng/mL. A remarkably low threshold limit of detection (LOD = 0.364 ng/mL) is achieved with quartz crystal microbalance (QCM) based gravimetric sensors. Furthermore, the Ag-ZnS@MIPs/QCM sensors show high selectivity for HSA compared to other proteins, e.g., bovine serum albumin (BSA), glycoprotein, ribonuclease, and lysozyme. Hence, the gravimetric quantification of HSA realizes a highly sensitive, selective, and label-free detection mechanism with a limit of quantification down to 1.1 ng/mL. Full article

In this study, we report on the rapid preparation of WO₃ nanoplates decorated with noble metals and evaluate their gas-sensing performance using a high-throughput screening technique. The incorporation of Pd significantly enhanced the gas-sensing properties, and, among all of the samples tested, the WO₃ nanoplate containing 0.3 mol% Pd exhibited the highest response to 100 ppm xylene at 250 °C (R_a/R_g = 131.2), which was almost 56 times greater than that of the pure WO₃ sample. Additionally, this sample demonstrated rapid response and recovery times (τ_response = 3.9 s and τ_recovery = 189.2 s, respectively). The nanoplate samples were also classified and screened using cluster analysis, and the selected samples were optimized for use in a sensor array. By applying principal component analysis and Fisher discriminant analysis, four typical gases were identified and a potential sensitization mechanism was elucidated. Full article

This study proposes a low-cost, portable paper-fluidic vertical flow assay bacterium counter with high accuracy. We designed sensors with low fabrication costs based on e-beam evaporation and three-dimensional printing based on the impedance measurement principle. Interdigitated (IDT) electrodes were coated on the filter membrane by e-beam evaporation with a shadow mask. We could print wafer-scale frames with low melting temperature three-dimensional-printing materials for confining liquid bacterial samples within the IDT sensing region. This novel fabrication technique significantly reduced the chip’s cost to less than 1% of that of silicon-based chips. Two equivalent circuit models were proposed for different concentration ranges to analyze the principle of paper-based impedance bacterial sensors. We proposed an improved model based on the Randles model for low concentrations by considering the leaky double-layer capacitor effect and spherical diffusion from the nano-structural electrodes of the gold-coated filter membrane. The phenomenon in which charge transfer resistance, R_ct, declines at high concentration ranges was found and explained by the pearl chain effect. The pearl effect could cause a false-negative at high concentrations. We modeled the pearl chain effect as an R and C, connected parallel to the low-concentration model. When users properly applied both models for analyses, this sensor could quantitatively measure cell concentrations from 400 to 400 M per milliliter with superior linearity. Full article

Chemoenzymatic assay systems are widely used to detect toxicants in various samples, including food and environment specimens. These methods are based on the ability of various types of toxicant to specifically inhibit/activate the functions of individual enzymes or enzyme systems. The present study examines the possibility of using the proteolytic enzyme trypsin as a specific marker to detect protease inhibitors in different samples. The study shows that trypsin activity is not affected by various heavy metals, pesticides, or quinones at levels considerably greater than their maximum allowable concentrations (MACs) in water bodies. At the same time, the IC₅₀ value for the food preservative potassium sorbate (E202) is 15 mg/L, which is substantially lower than its acceptable daily intake (ADI). The quenching of trypsin fluorescence in the presence of potassium sorbate suggests that inhibition could occur due to the binding of the preservative to the enzyme in the region adjacent to the active center. The trypsin was immobilized in starch gel to ensure its stability in the enzyme inhibition based assay. Single-use reagents were prepared as dry starch disks that could be stored over long periods. Their sensitivity to copper (II) chloride, potassium sorbate, and chromium (III) chloride was similar to the sensitivity of the free trypsin. Full article

In recent years, there has been a growing need for the development of low-power gas sensors. This paper proposes pulse heating and a corresponding measurement strategy using a Pulse Width Modulation (PWM) signal to realize the ultra-low power consumption for metal oxide semiconductor (MOS) gas sensors. A Micro-Hot-Plate (MHP) substrate was chosen to investigate the temperature and power characteristics of the MHP under different applied heating methods. The temperature of this given substrate could respond to the applied voltage within 0.1 s, proving the prac ticability of a pulse heating strategy. In addition, Pd-doped SnO₂ was synthesized as the sensing material in the implementation of an ultra-low power gas sensor. The sensing performance and power consumption under different conditions were compared in the detection of reducing gases such as ethanol (C₂H₅OH) and formaldehyde (HCHO). Additionally, the results revealed that the sensor could work under PWM excitation while reducing the operating power to less than 1mW. The features shown in the measurements provide the feasibility for MOS gas sensors’ application in wearable and portable devices. Full article

The fabrication technology of surface nanocomposites based on hexagonally ordered gold nanoparticle (AuNP) layers (quasi-arrays) and their possible application as surface-enhanced Raman spectroscopy (SERS) substrates are presented in this paper. The nanoparticle layers are prepared using a nanotextured template formed by porous anodic alumina (PAA) and combined with gold thin-film deposition and subsequent solid-state dewetting. Three types of hexagonal arrangements were prepared with different D/D₀ values (where D is the interparticle gap, and D₀ is the diameter of the ellipsoidal particles) on a large surface area (~cm² range), namely, 0.65 ± 0.12, 0.33 ± 0.10 and 0.21 ± 0.09. The transfer of the particle arrangements to transparent substrates was optimized through three generations, and the advantages and disadvantages of each transfer technology are discussed in detail. Such densely packed nanoparticle arrangements with high hot-spot density and tunable interparticle gaps are very beneficial for SERS applications, as demonstrated with two practical examples. The substrate-based enhancement factor of the nanocomposites was determined experimentally using a DNA monolayer and was found to be between 4 × 10⁴ and 2 × 10⁶ for the different particle arrangements. We also determined the sensing characteristics of a small dye molecule, rhodamine 6G (R6G). By optimizing the experimental conditions (e.g., optimizing the laser power and the refractive index of the measurement medium with an ethylene-glycol/water mixture), concentrations as low as 10⁻¹⁶ M could be detected at 633 nm excitation. Full article

In the era of liquid biopsies, the reliable and cost-effective detection and screening of cancer biomarkers has become of fundamental importance, thus paving the way for the advancement of research in the field of point-of-care testing and the development of new methodologies and technologies. Indeed, the latter ones can help designing advanced diagnostic tools that can offer portability, ease of use with affordable production and operating costs. In this respect, impedance-based biosensing platforms might represent an attractive alternative. In this work, we describe a proof-of-concept study aimed at designing portable impedimetric biosensors for the monitoring of human urokinase-type plasminogen activator (h-uPA) cancer biomarker by employing small synthetic receptors. Aberrant levels of h-uPA were correlated with different types of cancers. Herein, we report the use of two bicyclic peptides (P₂ and P₃) which have been engineered to bind h-uPA with high affinity and exquisite specificity. The synthetic receptors were immobilized via biotin-streptavidin chemistry on the surface of commercial screen-printed electrodes. The impedimetric changes in the electrode/solution interface upon incubation of spiked h-uPA samples in the presence of a redox probe were followed via electrochemical impedance spectroscopy. The P₃-based impedimetric assay showed the best outcomes in terms of dynamic range and linearity (0.01–1 μg mL⁻¹) and sensitivity (LOD = 9 ng mL⁻¹). To fully assess the performances of P₃ over P₂, and to compare the label-free architecture vs. labelled architecture, a voltammetric assay was also developed. Full article

Amperometric sensors evaluate current changes that occur as a result of redox reactions under constant applied potential. These changes in current intensity are stoichiometrically related to the concentration of analytes. Owing to their unique features, such as fast reaction velocity, high specificity, abundant existence in nature, and feasibility to be immobilized, enzymes are widely used by researchers to improve the performance of amperometric sensors. Unfortunately, natural enzymes have intrinsic disadvantages due to their protein structures. To overcome these proteinic drawbacks, scientists have developed nanozymes, which are nanomaterials with enzymatic properties. As the result of significant advances in materiology and analytical science, great progress has been achieved in the development of nanozyme-based amperometric sensors with outstanding performance. To highlight achievements made in recent years, we first summarize the development directions of nanozyme-based amperometric sensors. Then, H₂O₂ sensors, glucose sensors, sensors combining natural enzymes with nanozymes, and sensors targeting untraditional specific targets will be introduced in detail. Finally, the current challenges regarding the nanozymes utilized in amperometric sensors are discussed and future research directions in this area are suggested. Full article

Silver particles have been widely used in SERS detection as an enhancement substrate. The large-scale synthesis of Ag particles with controllable size and shape is still a challenge. We demonstrate a high-throughput method for the preparation of monodisperse submicron silver particles using S-shaped microfluidic chips. Submicron silver particles were prepared by a simplified reduction method. By adjusting the concentration of the reducing agent ascorbic acid and the stabilizer PVP, the particle size and morphology could be controlled, obtaining a size distribution of 1–1.2 μm for flower-like silver particles and a size distribution of 0.5–0.7 μm for quasi-spherical silver particles. This microfluidic system can be used to fabricate submicron silver particles on a large scale, continuously and stably, with a production efficiency of around 1.73 mg/min. The synthesized submicron silver particles could realize ultra-sensitive SERS detection, and the lowest concentration of rhodamine 6G (R6G) that could be detected was 10⁻⁹ M. Full article

Organic phototransistors (OPTs) as optical chemical sensors have progressed excitingly in recent years, mainly due to the development of new materials, new device structures, and device interfacial engineering. Exploiting the maximum potential of low-cost and high-throughput fabrication of organic electronics and optoelectronics requires devices that can be manufactured in a fully printed way that also have a low operation voltage. In this work, we demonstrate a fully printed fabrication process that enables the realization of a high-yield (~90%) and low-voltage OPT array. By solution printing of a high-quality organic crystalline thin film on the pre-printed electrodes, we create a van der Waals contact between the metal and organic semiconductor, resulting in a small subthreshold swing of 445 mV dec⁻¹ with a signal amplification efficiency over 5.58 S A⁻¹. Our OPTs thus exhibit both a low operation voltage of −1 V and a high photosensitivity over 5.7 × 10⁵, making these devices suitable for a range of applications requiring low power consumption. We further demonstrate the capability of the low-voltage OPT array for imaging and show high imaging contrasts. These results indicate that our fabrication process may provide an entry into integrated and low-power organic optoelectronic circuits fabricated by scalable and cost-effective methods for real-world applications. Full article

The development of sensitive and affordable testing devices for infectious diseases is essential to preserve public health, especially in pandemic scenarios. In this work, we have developed an attractive analytical method to monitor products of genetic amplification, particularly the loop-mediated isothermal amplification reaction (RT-LAMP). The method is based on electrochemical impedance measurements and the distribution of relaxation times model, to provide the so-called time-constant-domain spectroscopy (TCDS). The proposed method is tested for the SARS-CoV-2 genome, since it has been of worldwide interest due to the COVID-19 pandemic. Particularly, once the method is calibrated, its performance is demonstrated using real wastewater samples. Moreover, we propose a simple classification algorithm based on TCDS data to discriminate among positive and negative samples. Results show how a TCDS-based method provides an alternative mechanism for label-free and automated assays, exhibiting robustness and specificity for genetic detection. Full article

Incineration leachate is a hazardous liquid waste that requires careful management due to its high levels of organic and inorganic pollutants, and it can have serious environmental and health implications if not properly treated and monitored. This study applied a novel electronic nose to monitor the microbial communities and chemical characteristics of incineration leachate. The e-nose data were aggregated using principal component analysis (PCA) and T-distributed stochastic neighbor embedding (TSNE). Random forest (RF) and gradient-boosted decision tree (GBDT) algorithms were employed to establish relationships between the e-nose signals and the chemical characteristics (such as pH, chemical oxygen demand, and ammonia nitrogen) and microbial communities (including Proteobacteria, Firmicutes, and Bacteroidetes) of the incineration leachate. The PCA-GBDT models performed well in recognizing leachate samples, achieving 100% accuracy for the training set and 98.92% accuracy for the testing data without overfitting. The GBDT models based on the original data performed exceptionally well in predicting changes in chemical parameters, with R² values exceeding 0.99 for the training set and 0.86 for the testing set. The PCA-GBDT models also demonstrated superior performance in predicting microbial community composition, achieving R² values above 0.99 and MSE values below 0.0003 for the training set and R² values exceeding 0.86 and MSE values below 0.015 for the testing set. This research provides an efficient monitoring method for the effective enforcement and implementation of monitoring programs by utilizing e-noses combined with data mining to provide more valuable insights compared with traditional instrumental measurements. Full article