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Polymeric mixed ionic–electronic conductors hold great potential for advancing high‐performance organic electronics due to their exceptional electrical conductivity achieved through ion–electron coupling. However, the widespread adoption of these conductors faces obstacles due to their reliance on environmentally harmful solvents during processing and the potential performance degradation under various conditions. In this study, a practical solution to these challenges is proposed by incorporating monomers with ionic liquid‐behaving pendant groups into conjugated polymers. The copolymerization of these charged monomers not only improves solubility in environmentally friendly solvents but also imparts long‐term stability against humidity, high temperatures, and mechanical deformation. The immobilized ion species foster highly selective interactions of these polymers with nitrogen dioxide, paving the way for the development of stretchable sensing devices capable of functioning at exceptionally high temperatures. [ABSTRACT FROM AUTHOR]

This paper describes the process of producing chemosensors based on hybrid nanostructures obtained from Al2O3, as well as ZnO ceramic nanotubes and the following conducting polymers: poly(3-hexylthiophene), polyaniline emeraldine-base (PANI-EB), and poly(3, 4-ethylenedioxythiophene)-polystyrene sulfonate. The process for creating ceramic nanotubes involves three steps: creating polymer fiber nets using poly(methyl methacrylate), depositing ceramic films onto the nanofiber nets using magnetron deposition, and heating the nanotubes to 600 °C to burn off the polymer support completely. The technology for obtaining hybrid nanostructures from ceramic nanotubes and conducting polymers is drop-casting. AFM analysis emphasized a higher roughness, mainly in the case of PANI-EB, for both nanotube types, with a much larger grain size dimension of over 5 μm. The values of the parameter Rku were close or slightly above 3, indicating, in all cases, the formation of layers predominantly characterized by peaks and not by depressions, with a Gaussian distribution. An ink-jet printer was used to generate chemiresistors from ceramic nanotubes and PANI-EB structures, and the metallization was made with commercial copper ink for printed electronics. Calibration curves were experimentally generated for both sensing structures across a wider range of NH3 concentrations in air, reaching up to 5 ppm. A 0.5 ppm detection limit was established. The curve for the ZnO:PANI-EB structure presented high linearity and lower resistance values. The sensor could be used in medical diagnosis for the analysis of breath ammonia and biomarkers for predicting CKD in stages higher than 1. The threshold value of 1 ppm represents a feasible value for the presented sensor, which can be defined as a simple, low-value and robust device for individual use, beneficial at the patient level. [ABSTRACT FROM AUTHOR]

Clean hydrogen is a key aspect of carbon neutrality, necessitating robust methods for monitoring hydrogen concentration in critical infrastructures like pipelines or power plants. While semiconducting metal oxides such as In2O3 can monitor gas concentrations down to the ppm range, they often exhibit cross-sensitivity to other gases like H2O. In this study, we investigated whether cyclic optical illumination of a gas-sensitive In2O3 layer creates identifiable changes in a gas sensor's electronic resistance that can be linked to H2 and H2O concentrations via machine learning. We exposed nanostructured In2O3 with a large surface area of 95 m2 g−1 to H2 concentrations (0–800 ppm) and relative humidity (0–70%) under cyclic activation utilizing blue light. The sensors were tested for 20 classes of gas combinations. A support vector machine achieved classification rates up to 92.0%, with reliable reproducibility (88.2 ± 2.7%) across five individual sensors using 10-fold cross-validation. Our findings suggest that cyclic optical activation can be used as a tool to classify H2 and H2O concentrations. [ABSTRACT FROM AUTHOR]

Sensor arrays based on gold nanoparticle (GNP) films are promising candidates for numerous applications, including medical diagnosis and health monitoring. Their economic fabrication, however, remains challenging. This study presents a facile route to GNP chemiresistors with tunable properties via layer‐by‐layer spin‐coating (LbL‐SC). Key steps involve the alternating deposition of dodecylamine‐stabilized GNPs and mixtures of monothiols (MTs) with 1,9‐nonanedithiol (9DT). The 9DT molecules serve to reinforce the growing film via GNP cross‐linking while the MT ligands are used to tune the interfacial chemistry of the GNP assembly. Hence, by employing differently functionalized MTs the sensors' chemical selectivity can easily be adjusted. Further, by varying the MT‐9DT ratio and adjusting the size of the MT ligands the sensitivity can be tuned along with the conductivity and optical properties of the films. In general, decreasing the 9DT fraction significantly enhances the sensitivity while the response isotherms change from Langmuir‐Henry to Henry type. Finally, the cross‐linked GNP films are robust enough to be patterned via photolithography. Hence, this study demonstrates the fabrication and application of monolithic sensor arrays. Different features of the responses to numerous analytes are used as input data for linear discriminant analyses (LDA), revealing that very similar analytes can be distinguished. [ABSTRACT FROM AUTHOR]

This paper describes the process of producing chemiresistors based on hybrid nanostructures obtained from graphene and conducting polymers. The technology of graphene presumed the following: dispersion and support stabilization based on the chemical vapor deposition technique; transfer of the graphene to the substrate by spin-coating of polymethyl methacrylate; and thermal treatment and electrochemical delamination. For the process at T = 950 °C, a better settlement of the grains was noticed, with the formation of layers predominantly characterized by peaks and not by depressions. The technology for obtaining hybrid nanostructures from graphene and conducting polymers was drop-casting, with solutions of Poly(3-hexylthiophene (P3HT) and Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-bithiophene] (F8T2). In the case of F8T2, compared to P3HT, a 10 times larger dimension of grain size and about 7 times larger distances between the peak clusters were noticed. To generate chemiresistors from graphene–polymer structures, an ink-jet printer was used, and the metallization was made with commercial copper ink for printed electronics, leading to a structure of a resistor with an active surface of about 1 cm2. Experimental calibration curves were plotted for both sensing structures, for a domain of CH4 of up to 1000 ppm concentration in air. A linearity of the curve for the low concentration of CH4 was noticed for the graphene structure with F8T2, presenting a sensitivity of about 6 times higher compared with the graphene structure with P3HT, which makes the sensing structure of graphene with F8T2 more feasible and reliable for the medical application of irritable bowel syndrome evaluation. [ABSTRACT FROM AUTHOR]

ZnO thin films with a thickness of 300 nm were deposited on Si and Al2O3 substrates using an electron beam evaporation technique with the aim of testing them as low cost and low power consumption gas sensors for ozone (O3). Scanning electron microscopy and atomic force microscopy were used to characterize the film surface morphology and quantify the roughness and grain size, recognized as the primary parameters influencing the gas sensitivity due to their direct impact on the effective sensing area. The crystalline structure and elemental composition were studied through Raman spectroscopy and X-ray photoelectron spectroscopy. Gas tests were conducted at room temperature and zero-bias voltage to assess the sensitivity and response as a function of time of the films to O3 pollutant. The results indicate that the films deposited on Al2O3 exhibit promising characteristics, such as high sensitivity and a very short response time (<2 s) to the gas concentration. Additionally, it was observed that the films display pronounced degradation effects after a significant exposure to O3. [ABSTRACT FROM AUTHOR]

Monitoring the quality of drinking water is a crucial responsibility for all water infrastructure networks, as it guarantees access to clean water for the communities they serve. With water infrastructure deteriorating due to age and neglect, drinking water violations are on the rise in the US, underscoring the need for improved monitoring capabilities. Among the different sensor technologies, graphene-based chemiresistors have emerged as a promising technology for water quality monitoring due to advantages such as simple design, sensitivity, and selectivity. This review paper provides an overview of recent advances in the development of graphene-based chemiresistors for water quality monitoring, including principles of chemiresistive sensing, sensor design and functionalization, and performance of devices reported in the literature. The paper also discusses challenges and opportunities in the field and highlights future research directions. The development of graphene-based chemiresistors has the potential to revolutionize water quality monitoring by providing highly sensitive and cost-effective sensors that can be integrated into existing infrastructure for real-time monitoring. [ABSTRACT FROM AUTHOR]

Biorecognition element (BRE)-based carbon nanotube (CNT) chemiresistors have tremendous potential to serve as highly sensitive, selective, and power-efficient volatile organic compound (VOC) sensors. While many research groups have studied BRE-functionalized CNTs in material science and device development, little attention has been paid to optimizing CNT density to improve chemiresistor performance. To probe the effect of CNT density on VOC detection, we present the chemiresistor-based sensing results from two peptide-based CNT devices counting more than 60 different individual measurements. We find that a lower CNT density shows a significantly higher noise level and device-to-device variation while exhibiting mildly better sensitivity. Further investigation with SEM images suggests that moderately high CNT density with a stable connection of the nanotube network is desirable to achieve the best signal-to-noise ratio. Our results show an essential design guideline for tuning the nanotube density to provide sensitive and stable chemiresistors. [ABSTRACT FROM AUTHOR]

DNA sensing is critical in various applications such as the early diagnosis of diseases and the investigation of forensic evidence, food processing, agriculture, environmental protection, etc. As a wide-bandgap semiconductor with excellent chemical, physical, electrical, and biocompatible properties, silicon carbide (SiC) is a promising material for DNA sensors. In recent years, a variety of SiC-based DNA-sensing technologies have been reported, such as nanoparticles and quantum dots, nanowires, nanopillars, and nanowire-based field-effect-transistors, etc. This article aims to provide a review of SiC-based DNA sensing technologies, their functions, and testing results. [ABSTRACT FROM AUTHOR]

Volatile organic compounds (VOCs) are ubiquitous in the surroundings, originating from both industrial and natural sources. VOCs directly impact the quality of both indoor and outdoor air and play a significant role in processes such as fruit ripening and the body's metabolism. VOC monitoring has seen significant growth recently, with an emphasis on developing low-cost, portable sensors capable of both vapor discrimination and concentration measurements. VOC sensing remains challenging, mainly because these compounds are nonreactive, appear in low concentrations and share similar chemical structures that results in poor sensor selectivity. Therefore, individual gas sensors struggle to selectively detect target VOCs in the presence of interferences. Electronic noses overcome these limitations by employing machine learning for pattern recognition from arrays of gas sensors. Here, an electronic nose fabricated with four types of functionalized gold nanoparticles demonstrates rapid detection and quantification of eight types of VOCs at four concentration levels. A robust two-step machine learning pipeline is implemented for classification followed by regression analysis for concentration prediction. Random Forest and support vector machine classifiers show excellent results of 100% accuracy for VOC discrimination, independent of measured concentration levels. Each Random Forest regression analysis exhibits high R2 and low RMSE with an average of 0.999 and 0.002, respectively. These results demonstrate the ability of gold nanoparticle gas sensor arrays for rapid detection and quantification. [ABSTRACT FROM AUTHOR]

We hereby present the fabrication of simple and efficient humidity and ammonia sensors based on a chemiresistor design employing a differential measurement technique. Interdigitated electrodes are connected via PEDOT:PSS or P3HT polymers and a reproducible and scalable fabrication procedure is presented. This procedure utilizes pneumatic nozzle printing instead of drop or spin‐based processes and includes an optimized encapsulation procedure and material for sealing the reference electrodes. The suitability of the NH3 sensor for quantifying ammonia in a range of 2 to 100 ppm within a period of several months is furthermore presented. The developed humidity sensor is operational for several months in a humidity range between 10 and 60% RH. Once this range is exceeded, a sensor decay by irreversible saturation can be observed. Based on the low fabrication costs of the sensors (~0.5 $ when ordering 1000 pcs of the platform + printing work), they provide a very attractive tool for industrial humidity and ammonia sensing applications. [ABSTRACT FROM AUTHOR]

On-site detection of chemical warfare agents (CWAs) can be performed by various analytical techniques. Devices using well-established techniques such as ion mobility spectrometry, flame photometry, infrared and Raman spectroscopy or mass spectrometry (usually combined with gas chromatography) are quite complex and expensive to purchase and operate. For this reason, other solutions based on analytical techniques well suited to portable devices are still being sought. Analyzers based on simple semiconductor sensors may be a potential alternative to the currently used CWA field detectors. In sensors of this type, the conductivity of the semiconductor layer changes upon interaction with the analyte. Metal oxides (both in the form of polycrystalline powders and various nanostructures), organic semiconductors, carbon nanostructures, silicon and various composites that are a combination of these materials are used as a semiconductor material. The selectivity of a single oxide sensor can be adjusted to specific analytes within certain limits by using the appropriate semiconductor material and sensitizers. This review presents the current state of knowledge and achievements in the field of semiconductor sensors for CWA detection. The article describes the principles of operation of semiconductor sensors, discusses individual solutions used for CWA detection present in the scientific literature and makes a critical comparison of them. The prospects for the development and practical application of this analytical technique in CWA field analysis are also discussed. [ABSTRACT FROM AUTHOR]

The titanium carbide MXenes currently attract an extreme amount of interest from the material science community due to their promising functional properties arising from the two-dimensionality of these layered structures. In particular, the interaction between MXene and gaseous molecules, even at the physisorption level, yields a substantial shift in electrical parameters, which makes it possible to design gas sensors working at RT as a prerequisite to low-powered detection units. Herein, we consider to review such sensors, primarily based on Ti3C2Tx and Ti2CTx crystals as the most studied ones to date, delivering a chemiresistive type of signal. We analyze the ways reported in the literature to modify these 2D nanomaterials for (i) detecting various analyte gases, (ii) improving stability and sensitivity, (iii) reducing response/recovery times, and (iv) advancing a sensitivity to atmospheric humidity. The most powerful approach based on designing hetero-layers of MXenes with other crystals is discussed with regard to employing semiconductor metal oxides and chalcogenides, noble metal nanoparticles, carbon materials (graphene and nanotubes), and polymeric components. The current concepts on the detection mechanisms of MXenes and their hetero-composites are considered, and the background reasons for improving gas-sensing functionality in the hetero-composite when compared with pristine MXenes are classified. We formulate state-of-the-art advances and challenges in the field while proposing some possible solutions, in particular via employing a multisensor array paradigm. [ABSTRACT FROM AUTHOR]

The development of mobile, noninvasive, and portable sensor technologies for diagnostics and emission control is highly demanded. For that purpose, laser carbonization is studied as a tool to produce responsive carbon materials from inexpensive organic precursors for the room-temperature selective detection of volatile organic compounds (VOCs) applicable in future sensor array-based devices. To increase the response of intrinsically low-responsive laser-patterned carbons (LP-C) to analytes in the gas phase, we tested carbonization in the presence of nanoscale ZnO precursors in primary inks. Following the addition of a zinc salt, Zn-(NO3)2, a noticeable 43-fold increase in the sensor response (ΔR/R0 = −21.5% toward 2.5% acetone) was achieved. This effect is explained by a significant increase in the measurable surface area up to ∼700 m2·g–1 due to the carbothermic reduction supported by the in situ formation of ZnO nanoparticles. Varying Zn concentrations or the addition of as-prepared ZnO nanorods lead to different surface properties like the surface area, porosity, and polarity of LP-C. A predominant effect of the surface polarity on the selectivity toward different analytes of the sensors during physisorption, e.g., acetone vs toluene, was identified and tested. The best-performing LP-C sensors were finely characterized by transmission/scanning electron microscopies and X-ray photoelectron/energy-dispersive X-ray/Raman spectroscopies. [ABSTRACT FROM AUTHOR]

Sensor arrays are currently attracting the interest of researchers due to their potential of overcoming the limitations of single sensors regarding selectivity, required by specific applications. Among the materials used to develop sensor arrays, graphene has not been so far extensively exploited, despite its remarkable sensing capability. Here we present the development of a graphene-based sensor array prepared by dropcasting nanostructure and nanocomposite graphene solution on interdigitated substrates, with the aim to investigate the capability of the array to discriminate several gases related to specific applications, including environmental monitoring, food quality tracking, and breathomics. This goal is achieved in two steps: at first the sensing properties of the array have been assessed through ammonia exposures, drawing the calibration curves, estimating the limit of detection, which has been found in the ppb range for all sensors, and investigating stability and sensitivity; then, after performing exposures to acetone, ethanol, 2-propanol, sodium hypochlorite, and water vapour, chemometric tools have been exploited to investigate the discrimination capability of the array, including principal component analysis (PCA), linear discriminant analysis (LDA), and Mahalanobis distance. PCA shows that the array was able to discriminate all the tested gases with an explained variance around 95%, while with an LDA approach the array can be trained to accurately recognize unknown gas contribution, with an accuracy higher than 94%. [ABSTRACT FROM AUTHOR]

The article in Advanced Functional Materials discusses the development of polymeric mixed ion-electron conductors by Ji‐Soo Jang, Mingjiang Zhong, and their colleagues. These conductors, containing charged monomers with ionic liquid-like pendant groups, are soluble in environmentally friendly solvents and exhibit long-term stability against various environmental factors. The research aims to create stretchable sensing devices capable of functioning at high temperatures, with a focus on selective interactions with nitrogen dioxide. [Extracted from the article]

In the last decades, development of hybrid materials, especially inorganic–organic materials, coordination polymers, conducting polymers, carbon materials, and many more, has produced breakthroughs in diverse applications. Various advance materials have been reported in the literature using metal organic frameworks (MOFs), which compensate for the limitations of sensors. Diverse combinations of HMs not only offer excellent features, but also give a ray of hope for unprecedented advances in materials in different research areas, such as sensing, energy storage, catalysis, non-linear optics, drug-delivery systems, gas storage, etc. Chemiresistor sensors are a core enabling sensor technology and have led to much progress in the field of material science. Here, we have reviewed the recent progress in chemiresistive sensors based on HMs for nitroaromatic compounds, which could be beneficial for researchers that explore this field further. We have put emphasis on sensing mechanisms and the performance of diverse HMs for nitroaromatic sensing applications including pesticides, pollutants, explosives, polycyclic aromatic hydrocarbons (PAHs) and persistent organic pollutants (POPs). In the end, we explored opportunities, challenges, and future perspectives in this emerging field. [ABSTRACT FROM AUTHOR]

Palladium is a well-known hydrogen-absorbing material. When palladium is functionalized with copper-(II) benzene-1,3,5-tricarboxylate (HKUST-1), a hydrogen-adsorbing metal–organic framework, its hydrogen-absorption capacity can be increased. In this work, we show that, by growing the HKUST-1 on palladium nanoparticle-functionalized single-walled carbon nanotubes (Pd NP/SWCNT), we can dynamically monitor the adsorption and desorption of hydrogen from the HKUST-1 and Pd NP composite by using the carbon nanotubes as transducers in chemiresistors. Addition of HKUST-1 to the Pd NP/SWCNT was shown to increase the sensitivity of the nanocomposite material to hydrogen by 300% and limit of detection to hydrogen by 33%. The increase in sensitivity was attributed to the increased hydrogen sorption capacity of the combined HKUST-1/Pd NP. A factor of 8 improvement in sensitivity was further achieved by using semiconductor-enriched SWCNT instead of mixed metallic/semiconducting nanotubes and a corresponding improvement in the theoretical limit of detection down to 70 ppb. [ABSTRACT FROM AUTHOR]

The remarkable potential of breath analysis in medical care and diagnosis, and the consequent development of electronic noses, is currently attracting the interest of the research community. This is mainly due to the possibility of applying the technique for early diagnosis, screening campaigns, or tracking the effectiveness of treatment. Carbon nanotubes (CNTs) are known to be good candidates for gas sensing, and they have been recently considered for the development of electronic noses. The present work has the aim of reviewing the available literature on the development of CNTs-based electronic noses for breath analysis applications, detailing the functionalization procedure used to prepare the sensors, the breath sampling techniques, the statistical analysis methods, the diseases under investigation, and the population studied. The review is divided in two main sections: one focusing on the e-noses completely based on CNTs and one reporting on the e-noses that feature sensors based on CNTs, along with sensors based on other materials. Finally, a classification is presented among studies that report on the e-nose capability to discriminate biomarkers, simulated breath, and animal or human breath. [ABSTRACT FROM AUTHOR]

Ammonia sensors with high sensitivity, reproducible response, and low cost are of paramount importance for medicine, i.e., being a biomarker to diagnose lung and renal conditions, and agriculture, given that fertilizer application and livestock manure account for more than 80% of NH3 emissions. Thus, in this work, we report the fabrication of ultra-sensitive ammonia sensors by a rapid, efficient, and solvent-free laser-based procedure, i.e., laser-induced forward transfer (LIFT). LIFT has been used to transfer carbon nanowalls (CNWs) onto flexible polyimide substrates pre-patterned with metallic electrodes. The feasibility of LIFT is validated by the excellent performance of the laser-printed CNW-based sensors in detecting different concentrations of NH3 in the air, at room temperature. The sensors prepared by LIFT show reversible responses to ammonia when exposed to 20 ppm, whilst at higher NH3 concentrations, the responses are quasi-dosimetric. Furthermore, the laser-printed CNW-based sensors have a detection limit as low as 89 ppb and a response time below 10 min for a 20 ppm exposure. In addition, the laser-printed CNW-based sensors are very robust and can withstand more than 200 bending cycles without loss of performance. This work paves the way for the application and integration of laser-based techniques in device fabrication, overcoming the challenges associated with solvent-assisted chemical functionalization. [ABSTRACT FROM AUTHOR]

Air contamination is one of the foremost concerns of environmentalists worldwide, which has elevated global public health concerns for monitoring air contaminants and implementing appropriate safety policies. These facts have generated nascent global demand for exploring sustainable and translational strategies required to engineer affordable, intelligent, and miniaturized sensors because commercially available sensors lack lower detection limits, room temperature operation, and poor selectivity. The state‐of‐the‐art sensors are concerned with architecting advanced nanomaterials to achieve desired sensing performance. Recent studies demonstrate that neither pristine metal carbides/nitrides (MXenes) nor polymers (P) can address these practical challenges. However, synergistic combinations of various precursors as hybrid‐nanocomposites (MXP‐HNCs) have emerged as superior sensing materials to develop next‐generation intelligent environmental, industrial, and biomedical sensors. The expected outcomes could be manipulative due to optimizing physicochemical and morphological attributes like tunable interlayer‐distance, optimum porosity, enlarged effective surface area, rich surface functionalities, mechanical flexibility, and tunable conductivity. This review intends to detail a comprehensive summary of the advancements in state‐of‐the‐art MXP‐HNCs chemiresistors. Moreover, the underlying sensing phenomenon, chemiresistor architecture, and their monitoring performance are highlighted. Besides, an overview of challenges, potential solutions, and prospects of MXP‐HNCs as next‐generation intelligent field‐deployable sensors with the integration of IoT and AI are outlined. [ABSTRACT FROM AUTHOR]

Sensitivity and selectivity are the two major parameters that should be optimized in chemiresistive devices with boosted performances towards Volatile Organic Compounds (VOCs). Notwithstanding a plethora of metal oxides/VOCs combinations that have been investigated so far, a close inspection based on theoretical models to provide guidelines to enhance sensors features has been scarcely explored. In this work, we measured experimentally the sensor response of a WO3 chemiresistor towards gaseous acetone and toluene, observing a two orders of magnitude higher signal for the former. In order to gain insight on the observed selectivity, Density Functional Theory was then adopted to elucidate how acetone and toluene molecules adsorption may perturb the electronic structure of WO3 due to electrostatic interactions with the surface and hybridization with its electronic structure. The results of acetone adsorption suggest the activation of the carbonyl group for reactions, while an overall lower charge redistribution on the surface and the molecule was observed for toluene. This, combined with acetone's higher binding energy, justifies the difference in the final responses. Notably, the presence of surface oxygen vacancies, characterizing the nanostructure of the oxide, influences the sensing performances. [ABSTRACT FROM AUTHOR]

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Here in this research, Activated Carbon (AC)-γFe2O3 composite structures were prepared by pyrolysis of bamboo at 700–1000 °C temperatures. Characterization of the as-obtained samples by field-emission scanning electron microscopy, Raman spectroscopy, and X-ray diffraction analysis confirmed the formation of AC-γFe2O3 composite. The alcohol sensing performance of the composite structure at room temperature was investigated for different ethanol concentrations, which showed self-activated faster response/recovery, improved range of detection, and sensitivity. Ethanol sensing by the drop-casted AC-γFe2O3 films on a glass substrate was tested for various ethanol concentrations ranging from 50 to 500 ppm in water. The chemiresistive p-n-type heterojunctions formed between AC and γFe2O3 were attributed to the room temperature alcohol sensing performance of the material. AC-γFe2O3 prepared at 700 °C showed better sensitivity of 49% for 500 ppm ethanol concentration. The quickest response time was found to be 26 s for 100 ppm ethanol concentration with a recovery time of 15 s. [ABSTRACT FROM AUTHOR]

Poly(aniline co – pyrrole) (PAP) nanospheres (diameter ~50 nm) have been synthesized by using cost-effective electrochemical oxidation of monomers, i.e., aniline, and pyrrole (1:1 ratio), and evaluated as sulfur dioxide chemiresistor. PAP chemiresistor exhibited an improved sensing response (6.1%) toward a low concentration of sulfur dioxide (5 parts per million) compared to its precursor-based chemiresistors at room temperature. It is attributed to the remarkable characteristics of PAP, such as higher surface-to-volume ratio, larger effective surface area, higher branching, and lower conductivity value. Furthermore, the cause of low conductivity was evaluated using low-temperature direct current charge transport studies. The dependence of activation energy on temperature discarded the probability of band conduction mechanism in the synthesized PAP nanospheres. PAP nanospheres were found to possess 3-Dimensional VRH conduction of charge carriers. The low conductivity of PAP nanospheres is further explained by evaluating Mott's parameters, i.e., characteristic temperature, hopping radius, hopping energy, and density of states. [ABSTRACT FROM AUTHOR]

Tungsten oxide (WO3) was deposited through the successive ionic layer adsorption reaction (SILAR) method at low temperature. The polycrystalline monoclinic structure of deposited WO3 is confirmed by its x-ray diffraction (XRD) pattern. Sensing studies revealed that the nanostructured WO3 has a good response towards formaldehyde (HCHO) at 350°C. A response time of 28 s was observed for 5 ppm HCHO. Density functional theory (DFT) studies were performed to understand the detection mechanism based on the lattice plane growth orientation. The electronic properties of the WO3 were analyzed using the density of states (DOS) and Mulliken population analysis for adsorption of oxygen and HCHO on the WO3 surface. A significant change in the Fermi energy was observed during oxygen and HCHO adsorption on the surface of WO3. The computational results were compared with the proposed HCHO detection mechanism. The results obtained through the present work highlight the possibilities of developing a sensor to detect HCHO concentrations at the ppm level. [ABSTRACT FROM AUTHOR]

The development of sensors enabling the identification of volatile organic compounds (VOCs) and their mixtures is an ongoing challenge driven by many potential applications, such as breath analysis in medical diagnosis, food quality control, and environmental monitoring. In this context, hybrid materials of organically stabilized or cross-linked metal nanoparticles show great potential for the chemiresistive detection of VOCs. Here, we show that the unique nanostructure of these materials induces analyte-dependent response kinetics, which can be tuned and employed for analyte recognition. First, the response kinetics of different α,ω-alkanedithiol (ADT) cross-linked GNP film chemiresistors were measured by exposing them to vapors of toluene, 1-butanol, 1-propanol, 2-propanol, ethanol, water, 4-methylpentan-2-one, and different homologous alkanes. The t80 response times and the integrated area below the response transients were used as characteristic features of the response kinetics. The response times depended strongly on the molecular volume of the analyte as opposed to the maximum sensor response, which was governed by the analytes' polarity and vapor pressure. We attribute the variation in response time to molecular transport of the analyte molecules entering the sensor film. Second, using different-sized cross-linkers, different film thicknesses, or by preexposing the sensors to deep ultraviolet radiation, it is possible to tune the kinetics of the sensors' responses to different analytes. Third, by employing linear discriminant analysis on data sets containing the maximum responses and kinetic response features of a GNP/ADT sensor array with adjusted response kinetics, it was possible to differentiate between very similar analytes over a broad range of concentrations (100–4000 ppm). Hence, the methods described herein will be very useful for various applications of nanoparticle-based chemiresistor arrays. [ABSTRACT FROM AUTHOR]

A considerable multitude of chemiresistive type of gas sensors has been estimated to sense minute amounts of hazardous or toxic gases using inexpensive setups. Although the ultimate value of the response of gas sensors is well-established, analyzing other characteristics has been mostly neglected. In the present paper, we demonstrate the modeling of time-dependent responses by slightly modified Gompertz functions whose derived parameters can be correlated with the detection mechanism, thus, expanding analysis methods towards a better understanding of gas sensing. [ABSTRACT FROM AUTHOR]

Low-temperature direct current charge transport mechanism of charge carriers in polyaniline–tungsten oxide (PAN-WO3) nanocomposite has been investigated. Charge transport in pristine PAN was found to govern by Mott’s three-dimensional variable range hopping (3-D VRH) model. However, the inclusion of WO3 in nanocomposite shifts the dimension of hopping from 3-D to 1-D. The room-temperature conductivity of PAN-WO3 nanocomposite (5.55 × 10–3 S/cm) was also found to be enhanced compared to pristine PAN (1.27 × 10–5 S/cm). The reasons for crossover in hopping dimensionality and enhanced conductivity of PAN-WO3 nanocomposite have been explained in terms of Mott’s parameters, i.e., small hopping radius, lower hopping energy, high inter-chain distance, and prominent intra-chain transport. Furthermore, PAN-WO3 exhibited enhanced reversible sensing behaviour (27%) towards 10 parts per million of hydrazine at room temperature compared to that of pristine PAN (12%). Enhanced sensing characteristics of PAN-WO3 nanocomposite can be attributed to its higher conductivity and prominent intra-chain unidirectional charge transport. Present communication opens a new window for energy-saving, eco-friendly, cost-effective, recoverable and reproducible, easily processable and efficient PAN-WO3 nanocomposite-based hydrazine detecting device. [ABSTRACT FROM AUTHOR]

In this work, we report the detection and discrimination of five commercially available NESTT K-9 explosive compounds, including 2,4,6-trinitrotoluene (TNT), cyclotrimethylenetrinitramine (RDX), pentaerythritol tetranitrate (PETN), potassium nitrate, and potassium chlorate. An array of 48 chemiresistors were fabricated and assembled with four types of gold nanoparticle sensing materials. The functional groups include tetradecylamine (TDA), octadecylamine (ODA), 3-mercaptopropionic acid (MPA), and 4-aminothiophenol (ATP). Machine learning methods were applied to analyze sensor data. The discrimination accuracy of the sensor array was studied over time at various vapor concentrations (${p}/{p}_{0}$), using several classifiers, including linear discriminant analysis (LDA), support vector machine (SVM), k-nearest neighbors (KNN), and bagged trees (BT), with five-fold cross-validation. More than 90% accuracy was achieved using datasets with more than 2500 sensor measurements. Sensor arrays and classification algorithms exhibit good stability over a range of vapor concentrations. The results demonstrate the utility of machine learning for detection and classification of volatiles, including explosive and related compounds. [ABSTRACT FROM AUTHOR]

On the perspective to develop CuO–TiO2 MOS, multilayered Cu and Ti thin layers were alternatively deposited on silicon wafers using 25 keV Ar + ion beam sputtering and, subsequently, oxidized by thermal annealing in air at 400 °C for 24 h. The deposited films have variable ratios of the Cu and Ti % at. One of the main goal is to obtain such multilayers avoiding the presence of Cu–Ti–O compounds. The samples were characterized in terms of composition (by RBS and SIMS analyses) and morphology (by AFM and SEM investigations). In particular, SIMS maps allows to observe the spatial distribution and thickness of each phase of the Cu/Ti multilayers, and further to observe Cu diffusion and mixing with Ti, as well as phase separation of CuO and TiO2 in the samples. The reasons of this effect represent an open issue that has to investigated, in order to improve the MOS fabrication. [ABSTRACT FROM AUTHOR]

Miniaturization of gas sensors carries added benefits of better portability, lower costs, and allowing for multianalyte detection by integrating various sensors in one device. In this study, laser writing technology is proposed to fabricate a micro gas sensor based on 11-mercaptoundecanoic acid functionalized gold nanoparticles, which are assembled and deposited over the spacing between two electrodes. During the response process to volatile organic compounds (VOCs), conductivity change is dominated by interparticle properties, such as dielectric constant, interparticle spacing. The microsensor showed the maximum response to ortho-xylene among seven target analytes. The microsensor exhibited larger response ($\Delta \text{R}/\text{R}_{a}= 0.412$) than the drop-cast film ($\Delta \text{R}/\text{R}_{a} = 0.044$) at 5000 ppm for ortho-xylene. The response time of the microsensor was 57 s, shorter than the drop-cast film of 127 s. Improved sensitivity and shorter response time were also observed for other VOCs (toluene, ethanol, benzene, methanol, acetone, chloroform). The better sensing performance can be attributed to the larger surface-volume ratio brought by the rough surface and the miniaturization of device. Therefore, the shorter response time and higher response were obtained by laser writing technology. [ABSTRACT FROM AUTHOR]

Ethanol is widely used in preparing alcoholic beverages, thermometers, perfumes, flavorings, colorings, and medicines. Ethanol sensors are mainly used in biofuel plants and breweries to quantify ethanol concentrations through infrared spectral studies as well as in forensics and direct alcohol fuel cells. This overview discusses the various materials and methods used in the determination of ethanol concentrations in sensing applications using resistance-based measurements. A comprehensive view of the different nanomaterials that have been studied is provided, with an outlook on the shortcomings as well as merits of the material architectures. Statistical methods of interpreting sensing performance are analyzed as well, through which possibilities for improving the adaptability of materials towards their end use is projected. [ABSTRACT FROM AUTHOR]

A paper based chemiresistor has been fabricated to selectively sense ethanol in human breath. The chemiresistor was composed of a sensing mixture of multiwall-carbon-nano-tubes (MWCNTs), poly (diallyl-dimethyl-ammonium chloride) (PDDA), alcohol dehydrogenase (ADH), and coenzyme (NADH). The aluminum electrode was deposited on the paper surface, followed by drop-casting of the aforementioned sensing mixture. The resistance of the sensors was measured by exposing the same in gas-vapor mixture as well as the sample solution. The surface-modified MWCNTs specifically broke down ethanol present in the gas-vapor mixture or in a solution to generate a quantitative electronic response proportional to the ethanol concentration. Subsequently, the interference of other volatile organic materials was also tested to prove the selectivity and sensitivity of the sensor towards ethanol in the presence of different volatile organic compounds (VOCs). The variation of the resistance during the interaction between sensor and ethanol was also characterized by measuring the surface potential of the channel material under ethanol exposure using Kelvin probe force microscopy (KPFM). The sensor was integrated with a voltage divider circuit, a display, and a microcontroller unit to make a proof-of-concept prototype for the point-of-care (POC) detection of ethanol in human breath. [ABSTRACT FROM AUTHOR]

The study demonstrates that resistivity of an alumina wafer is highly sensitive to trace concentrations of acetone vapors at room temperature. Though, a thermal pretreatment is necessary to precede the room-temperature sensing of acetone vapors, whilst the sensitiveness increases with the pretreatment temperature. This advocates the alumina being suitable for an adequate acetone sensor in the ppm range. A plausible mechanism of the response is discussed. [ABSTRACT FROM AUTHOR]

Two configurations of single-walled carbon nanotube-based chemi-capacitive gas sensors were fabricated, i.e., horizontal and vertical. Further, their sensing properties for hexane and ammonia (NH3) vapor were characterized and compared with chemi-resistive-type sensing properties. Upon exposure to hexane and NH3 vapor, both capacitance and resistance varied as the analyte concentration increased. The sensing sensitivity measured along the horizontal direction increased with the device resistance. However, the capacitive sensing response along the vertical direction was independent of the number and fraction of semi-conductive single-walled carbon nanotubes. Furthermore, the vertical chemi-capacitive sensing response was dependent on the dipole moment of analytes. [ABSTRACT FROM AUTHOR]

Nowadays, sensitive biosensors with high selectivity, lower costs and short response time are required for detection of DNA. The most preferred materials in DNA sensor designing are nanomaterials such as carbon and Au nanoparticles, because of their very high surface area and biocompatibility which lead to performance and sensitivity improvements in DNA sensors. Carbon nanomaterials such as carbon nanotubes (CNTs) can be considered as a suitable DNA sensor platform due to their high surface‐to‐volume ratio, favourable electronic properties and fast electron transfer rate. Therefore, in this study, the CNTs which are synthesised by pulsed AC arc discharge method on a high‐density polyethylene substrate are used as conducting channels in a chemiresistor for the electrochemical detection of double stranded DNA. Moreover, the response of the proposed sensor is investigated experimentally and analytically in different temperatures, which confirm good agreement between the presented model and experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Water testing for microbial contamination is essential to ensure safe drinking water. In the present work, an organic nanocomposite based sensor is designed and fabricated to detect the presence of E. coli in water. The detection is carried out by measuring the change in two parameters of the organic nanocomposite film – resistance and impedance. The lower detection limit of E. coli cell counts up to 10 CFU mL−1 and 107 CFU mL−1 in 100 mL test solution is observed by impedance spectroscopy and resistance change respectively. A conjugated molecule with a specific pendant amine group was used as the receptor moiety that can interact and exhibit affinity to the functional groups like carboxyl groups present on the outer membrane of the cell wall of the E. coli. The binding of E. coli cells to conjugated molecule was characterized by chemical, physical and structural properties. The electrostatic interaction between positively charged amine groups on conjugated molecule and negatively charged E. coli is utilized for sensing. This interaction is also observed to be acting as p‐dopant to conjugated molecule which synergistically induces a change in electrical resistance in the composite. This work shows that the conjugated molecules designed with suitable moieties could be used for sensing microorganisms, when most sensors for detection of E. coli cells, use antibodies as detecting element. [ABSTRACT FROM AUTHOR]

Abstract: Chemiresistive and chemicapacitive circuit elements are fashioned from polymer thin films that are bio‐nanocomposites of polyaniline‐chloride (PAn‐Cl) nanofibers within a chitosan (CHI) matrix deposited on microfabricated electrodes (IAME‐co‐IME; 2 µm lines and 1 µm spacing). UV–vis spectroscopy of 0–100 wt% PAn‐Cl/CHI confirms no electronic coupling between PAn and CHI. When aqueous dispersions of the bio‐nanocomposite are slow dried or cast, frozen, and lyophilized, they produce dense (chemiresistive) or highly porous foam (chemicapacitive) membranes. Studied in air, deionized water, and in physiological buffers (PBS and HEPES), four‐band probe measurements of the ionic to polaronic conduction show a conductivity that is composition dependent with a percolation threshold of ≈30 wt%. Cyclic voltammetry reveals all compositions to be electroactive. Electrical impedance spectroscopy (EIS) shows that resistive and capacitive properties can be controlled by composition and morphology with 50–50 wt% being favored. EIS modeling confirms a modified Randles circuit of low chi‐square values (<0.05) that appropriately models the chemiresistive and chemicapacitive properties. DC offset voltages can externally control the predominantly chemiresistive and chemicapacitive properties of the devices for biosensor and biocircuit applications. [ABSTRACT FROM AUTHOR]

ZnO thin films were prepared on glass substrates at low (5 × 10 mbar) and high (3 × 10 mbar) sputter pressure using dc reactive magnetron sputtering. The structural, morphological, compositional and optical properties of the thin films were investigated. XRD patterns of both films confirmed the polycrystalline nature of the films with hexagonal Wurtzite structure. SEM study indicates that the surface of the film formed at high sputter pressure was more uniform, compact and porous in nature. From the EDAX analysis, no other characteristic peaks of other impurities were observed and the formation of single phase of ZnO was confirmed. From the study of photoluminescence, three peaks were observed, one strong near band-edge emission at 390 nm followed by weak and broad visible emissions around 420-480 nm. Room temperature ammonia sensing characteristics of ZnO nanothin films formed at higher sputter pressure were studied for different ammonia vapour concentration levels. The response of the Ammonia sensor at room temperature (30 °C) operation was observed to be of high sensitivity with quick response and recovery times. [ABSTRACT FROM AUTHOR]

The synthetically tunable properties and intrinsic porosity of conductive metal-organic frameworks (MOFs) make them promising materials for transducing selective interactions with gaseous analytes in an electrically addressable platform. Consequently, conductive MOFs are valuable functional materials with high potential utility in chemical detection. The implementation of these materials, however, is limited by the available methods for device incorporation due to their poor solubility and moderate electrical conductivity. This manuscript describes a straightforward method for the integration of moderately conductive MOFs into chemiresistive sensors by mechanical abrasion. To improve electrical contacts, blends of MOFs with graphite were generated using a solvent-free ball-milling procedure. While most bulk powders of pure conductive MOFs were difficult to integrate into devices directly via mechanical abrasion, the compressed solid-state MOF/graphite blends were easily abraded onto the surface of paper substrates equipped with gold electrodes to generate functional sensors. This method was used to prepare an array of chemiresistors, from four conductive MOFs, capable of detecting and differentiating NH3, H2S and NO at parts-per-million concentrations. [ABSTRACT FROM AUTHOR]

Polyaniline nanofibres ( NFs) with controlled diameters were synthesized using a template-free method, with ammonium peroxidisulfate ( APS) or ferric chloride ( FeCl3) as oxidants. Porosity studies reveal that NFs prepared with FeCl3 possess higher effective surface area and larger pore volume compared to NFs prepared with APS. The FeCl3-assisted NFs show around twofold enhanced sensing response ( ca 4.5%) towards 5 ppm of SO2 at room temperature compared to APS-assisted NFs ( ca 2%). The enhancement can be attributed to the lower diameter, higher effective surface area and larger porosity of FeCl3-assisted NFs. To further explain this enhanced sensing response, the conduction mechanism was studied. NFs possessing a smaller diameter ( ca 10 nm) are found to follow the one-dimensional variable range hopping ( VRH) model, whereas NFs with larger diameter ( ca 100 nm) follow the conventional three-dimensional VRH model. This can be due to the restriction of charge carrier transport into only one direction due to quantum size effects. Furthermore, the calculated Mott parameters suggest that the NFs prepared using FeCl3 provide a better pathway for charge transport of charge carriers as compared to NFs prepared using APS in terms of shorter hopping distance, lower activation energy and lower hopping energy, and weaker localization of charge carriers. © 2016 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

Graphene-based gas/vapor sensors have attracted much attention in recent years due to their variety of structures, unique sensing performances, room-temperature working conditions, and tremendous application prospects, etc. Herein, we summarize recent advantages in graphene preparation, sensor construction, and sensing properties of various graphene-based gas/vapor sensors, such as NH, NO, H, CO, SO, HS, as well as vapor of volatile organic compounds. The detection mechanisms pertaining to various gases are also discussed. In conclusion part, some existing problems which may hinder the sensor applications are presented. Several possible methods to solve these problems are proposed, for example, conceived solutions, hybrid nanostructures, multiple sensor arrays, and new recognition algorithm. [ABSTRACT FROM AUTHOR]

We report the novel application of cetryltrimethylammonium bromide ( CTAB) assisted polyaniline ( PAN) nanostructures to fabricate cost effective chemiresistive sensors for monitoring a low parts per million level of sulfur dioxide ( SO2) at room temperature. PAN nanoparticles ( NPs, diameter ca 200 nm), nanofibres ( NFs, diameter ca 100 nm) and nanoneedles ( NNs, diameter ca 50 nm) were synthesized via template-directed chemical oxidative polymerization of aniline by varying concentration of the solitary cationic surfactant CTAB (from 5 mmol L−1 to 15 mmol L−1). It was observed that the PAN NN based sensing device showed an excellent sensing response ( ca 4.2%) towards a low concentration of SO2 (10 ppm) compared to the device based on PAN NFs ( ca 3%) and PAN NPs ( ca 1.2%) at room temperature. The PAN NN based sensing device was also found to be efficient in terms of stability, selectivity, reproducibility, recovery and response time, and detection range. The mechanism of SO2 sensing has been explored for the first time by using in situ Fourier transform infrared spectroscopy. © 2015 Society of Chemical Industry [ABSTRACT FROM AUTHOR]

A chemiresistor vapor sensor based on electron transport through an ensemble of ligand-stabilized gold nanoclusters is made exceptionally sensitive and selective by terminal carboxylic acid functionalization of the alkanethiol ligand. The directionality of the response (conductance increase or decrease) is strongly dependent on the nanoscale dimensions of the gold core and ligand shell thickness. Films of gold nanoclusters composed of a 2-nm metal core with a 0.5-nm S(CH2)5COOH shell are compared with those based on an 8-nm core and a 1.5-nm S(CH2)15 COOH shell. Sensitivity toward amine vapors covered spanned 4-5 orders of magnitude with a detection limit lower than 0.1 ppm and respective selectivity factors of 100× and 20× over interferent vapors. Sensor isotherm measurements on a series of trialkylamine vapors displayed a sensitivity dependence that correlates with vapor pressure. Comparative responses to deuterated and nondeuterated triethylamine vapor exposures were identical and ruled out a sensor transduction mechanism that couples analyte vibrational modes with electron tunneling between gold nanocluster cores. [ABSTRACT FROM PUBLISHER]

Novel chemiresistive gas sensors based on graphene/AlO quantum dots composites were fabricated and examined for carbon dioxide sensing. Composite samples with different wt% of graphene (20-80 wt%) mixing in constant 1 g AlO were prepared and characterized by X-ray diffraction, transmission electron microscopy along with selected area electron diffraction, ultraviolet-visible spectroscopy, fluorescence spectroscopy and thermo gravimetric-differential thermal analysis. The experimental results showed that the graphene/AlO-based chemiresistor exhibited much higher sensing response and it enhanced linearly with addition of graphene. The gas sensing mechanism was discussed on the basis of defect chemistry through fluorescence measurements. 80 wt% graphene/AlO composite exhibited good sensing response (10.84) at room temperature, low operating temperature (398 K), fast response time (14 s) and recovery time (22 s) along with good stability. [ABSTRACT FROM AUTHOR]