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Chemiresistors based on thin films of the Li-doped CuO–TiO2 heterojunctions were synthesized by a 2-step method: (i) repeated ion beam sputtering of the building elements (on the Si substrates and multisensor platforms), and (ii) thermal annealing in flowing air. The structure and composition of the films were analyzed by several methods: Rutherford Backscattering (RBS), Neutron Depth Profiling (NDP), Secondary Ion Mass Spectrometry (SIMS), and Atomic Force Microscopy (AFM), and their sensitivity to gaseous analytes was evaluated using a specific lab-made device operating in a continuous gas flow mode. The obtained results showed that the Li doping significantly increased the sensitivity of the sensors to oxidizing gases, such as NO2, O3, and Cl2, but not to reducing H2. The sensing response of the CuO–TiO2–Li chemiresistors improved with increasing Li content. For the best sensors with about 15% Li atoms, the detection limits were as follows: NO2&nbsp, → 0.5 ppm, O3→ 10 ppb, and Cl2→ 0.1 ppm. The Li-doped sensors showed excellent sensing performance at a lower operating temperature (200 ∘C), however, even though their response time was only a few minutes, their recovery was slow (up to a few hours) and incomplete.

In this work, we present a complex study of photoregeneration of a zinc phthalocyanine (ZnPc) sensor by illumination from light-emitting diodes (LEDs). It includes an investigation of photoregeneration effectivity for various wavelengths (412–723 nm) of incident light carried out at sensor operating temperatures of 55 °C. It is demonstrated that the efficiency of photoregeneration is increasing with a decrease in the light wavelength. In the region of longer wavelengths (723–630 nm), the regeneration degree (RD) was low and ranged from 12% to 15%. In the region of shorter wavelengths (518–412 nm), the RD rose from 35% for 518 nm to 94% for 412 nm. The efficiency of photoregeneration is also shown to be higher in comparison with the temperature regeneration efficiency. In order to understand the chemism of photoregeneration processes, the electrical measurements are supplemented with Raman and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) studies. The spectroscopic results showed that nitrogen dioxide bonds to the Zn atom in ZnPc in the form of NO2− and NO−, i.e., partial decomposition of NO2 molecules occurs during the interaction with the surface. NAP-XPS spectra proved that light illumination of the ZnPc surface is essential for almost complete desorption of NOx species. At the same time, it is demonstrated that in case of long-time exposure or exposure of a ZnPc chemiresistor with a high concentration of NO2, the oxygen, released due to the NO2 decomposition, slowly but irreversibly oxidizes the layer. This oxidation process is most probably responsible for the sensor deactivation observed in sensor experiments with high NO2 concentrations. Based on these studies, the mechanism of nitrogen dioxide interaction with zinc phthalocyanine both under LED illumination and in dark conditions is proposed, and a special method for the sensor operation called “constant exposure dose” is established.

This work describes the design and fabrication of free-standing carbon nanotube-palladium (CNT-Pd) composite sheets for hydrogen gas sensing. The CNT-Pd composites were made by electroplating palladium onto a solvent-densified and oxygen plasma-treated CNT sheet. The latter was prepared using high purity CNTs drawn from a dense, vertically aligned array grown by chemical vapor deposition on silicon substrates. The CNT-Pd sheets were characterized by energy-dispersive spectroscopy, scanning electron microscopy, and X-ray diffraction. The amount of palladium in the composite was 16.5 wt % as measured via thermogravimetric analysis. Thin strips of the CNT-Pd sheets were assembled as chemiresistor sensors and tested for hydrogen gas detection. The sensors demonstrated a limit of detection of 0.1 mol % and displayed signal reversibility without the need for oxygen removal or heat treatment. A decrease in signal reversibility was observed after multiple exposure cycles; however, redensification with ethanol significantly restored the original reversibility. The sensor showed the Freundlich adsorption isotherm behavior when exposed to hydrogen. The material’s potential application toward a wearable, flexible sensor was demonstrated by integrating the chemiresistor onto a fabric material using hot-press processing and testing the composite for hydrogen sensitivity.

Single-ligand-based electronically conductive porous coordination polymers/metal-organic frameworks (EC-PCPs/MOFs) fail to meet the requirements of numerous electronic applications owing to their limited tunability in terms of both conductivity and topology. In this study, a new 2D π-conjugated EC-MOF containing copper units with mixed trigonal ligands was developed: Cu3 (HHTP)(THQ) (HHTP=2,3,6,7,10,11-hexahydrotriphenylene, THQ=tetrahydroxy-1,4-quinone). The modulated conductivity (σ≈2.53×10-5 S cm-1 with an activation energy of 0.30 eV) and high porosity (ca. 441.2 m2 g-1 ) of the Cu3 (HHTP)(THQ) semiconductive nanowires provided an appropriate resistance baseline and highly accessible areas for the development of an excellent chemiresistive gas sensor.

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.

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.

Seedless growth of vertically aligned nanostructures, which can induce smoother transport and minimize Ohmic contact between substrate and semiconductor, can be fabricated by in situ growth utilizing modified hydrothermal methods. Such devices can be useful in designing non-invasive ultrasensitive hand-held sensors for diagnostic identification of volatile organic compounds (VOCs) in exhaled air, offering pain-free and easier detection of long-term diseases such as asthma. In the present work, WO3 nanoblocks, with a high surface area and porosity, have been grown directly over transparent conducting oxide to minimize Ohmic resistance, facilitating smoother electron transfer and enhanced current response. Further modification with porous alumina (γ-Al2 O3 ), by electrodeposition, resulted in the selective and ultrasensitive detection of NOX in simulated exhaled air. Crystal phase purity of as-fabricated pristine as well modified samples is validated with X-ray diffraction analysis. Morphological and microstructural analyses reveal the successful deposition of porous alumina over the surface of WO3 . Improved surface area and porosity is presented by porous alumina in the modified WO3 device, suggesting more active sites for the gas molecules to get adsorbed and diffuse through the pores. Oxygen vacancies, which are detrimental in the transport phenomenon in the presented sensors, have been studied using X-ray photoelectron spectroscopic (XPS) analysis. Gas sensing studies have been performed by fabricating chemiresistor devices based on bare WO3 and Al2 O3 -modified WO3 . The higher sensitivity for NOX gas in case of γ-Al2 O3 -modified WO3 based devices, as compared to bare WO3 -based devices, is attributed to the better surface area and charge transport kinetics. The presented device strategy offers crucial understanding in the design and development of non-invasive, hand-held devices for NO gas present in the human breath, with potential application in medical diagnostics.

Noble metals or oxide catalysts have traditionally been loaded or doped to enhance the gas sensing properties of oxide semiconductor chemiresistors. However, the selective detection of various chemicals for a wide range of new applications remains a challenging problem. In this paper, we propose a novel bilayer design with an oxide chemiresistor sensing layer and nanoscale catalytic Au overlayer to provide high controllability for gas sensing characteristics. The Au nanocluster overlayer significantly enhances the methylbenzene response of a SnO2 thick film sensor by reforming gases into more reactive species and suppresses the responses to reactive interference gases through oxidative filtering, leading to excellent selectivity to methylbenzene. Gas sensing characteristics can be tuned by controlling the morphology, amount, and number density of Au nanoclusters through the variation in the Au coating thickness (0.5-3 nm) and thermal annealing conditions (0.5-4 h at 550 °C). Furthermore, the general validity of the proposed Au-coated bilayer sensor design was confirmed through the enhancement of response and selectivity toward methylbenzenes by coating Au nanoclusters onto ZnO and Co3O4 sensors. The sensing mechanism, advantages, and potential applications of bilayer sensors are discussed from the perspective of the separation of sensing and catalytic reactions, as well as the reforming and oxidation of analyte gases in association with the configuration of the sensing layer and Au catalytic overlayer.

Semiconductor-enriched single-walled carbon nanotubes (s-SWCNTs) have potential for application as a chemiresistor for the detection of breath compounds, including tetrahydrocannabinol (THC), the main psychoactive compound found in the marijuana plant. Herein we show that chemiresistor devices fabricated from s-SWCNT ink using dielectrophoresis can be incorporated into a hand-held breathalyzer with sensitivity toward THC generated from a bubbler containing analytical standard in ethanol and a heated sample evaporator that releases compounds from steel wool. The steel wool was used to capture THC from exhaled marijuana smoke. The generation of the THC from the bubbler and heated breath sample chamber was confirmed using ultraviolet-visible absorption spectroscopy and mass spectrometry, respectively. Enhanced selectivity toward THC over more volatile breath components such as CO2, water, ethanol, methanol, and acetone was achieved by delaying the sensor reading to allow for the desorption of these compounds from the chemiresistor surface. Additionally, machine learning algorithms were utilized to improve the selective detection of THC with better accuracy at increasing quantities of THC delivered to the chemiresistor.

All-carbon fiber-based chemiresistor is fabricated by assembling reduced graphene oxide (RGO) fiber and carbon nanotube (CNT) fiber as reversible NO2 sensing layer and flexible heater, respectively. Both graphene oxide (GO) and CNT fibers were synthesized by wet-spinning technique facilitating lyotropic nematic liquid crystal (LC) property. Randomly entangled CNT fiber-based heater, which is embedded in one surface of colorless polyimide (cPI) film with thickness of ˜200 μm, exhibits high bending stability and heating property up to 100 °C. Single reduced graphene oxide (RGO) fiber obtained after heat treatment at 900 °C in H2/N2 ambient was integrated on the CNT fiber-embedded cPI heater, thereby establishing a new type of all-carbon fiber sensing platform. As a result, accelerated NO2 adsorption and desorption kinetics were achieved with RGO fiber at an elevated temperature. In particular, a 9.22-fold enhancement in desorption kinetic (kdes = 8.85 × 10–3 s–1) was observed at 100 °C compared with the desorption kinetic (kdes = 0.96 × 10–3 s–1) at 50 °C, which was attributed to the effective heating by CNT fiber networks. This work pioneered a research on the use of emerging carbonaceous fibers for potential application in wearable chemical detectors.

International audience; Chemiresistors are highly important for monitoring and detection of harmful gases produced from industrial processes and vehicle emissions. We report herein an experimental investigation of the gas-sensing properties of Zn0.95Co0.05O thin film deposited by spray pyrolysis technique. The X-ray photoelectron spectroscopy indicated the presence of Co2+ ions into de ZnO lattice. The gas-sensing measurements revealed the sensitivity of Zn0.95Co0.05O film towards ozone gas in a wide range of concentrations from 20 to 1040 ppb, exhibiting good repeatability and total reversibility after consecutive exposures. Selectivity towards ozone is observed in comparison to NO2, NH3, and CO gases even at low levels. This manuscript reports the effectiveness of spray-pyrolysis method for obtaining nanostructured Zn1-xCoxO thin films for practical applications as an ozone gas sensor.

A growing demand for indoor atmosphere monitoring relies critically on the ability to reliably and quantitatively detect carbon dioxide. Widespread adoption of CO2 sensors requires vastly improved materials and approaches because selective sensing of CO2 under ambient conditions, where relative humidity (RH) and other atmosphere contaminants provide a complex scenario, is particularly challenging. This report describes an ambient CO2 chemiresistor platform based on nanoporous, electrically conducting two-dimensional metal-organic frameworks (2D MOFs). The CO2 chemiresistive sensitivity of 2D MOFs is attained through the incorporation of imino-semiquinonate moieties, i.e., well-defined N-heteroatom functionalization. The best performance is obtained with Cu3(hexaiminobenzene)2, Cu3HIB2, which shows selective and robust ambient CO2 sensing properties at practically relevant levels (400-2500 ppm). The observed ambient CO2 sensitivity is nearly RH-independent in the range 10-80% RH. Cu3HIB2 shows higher sensitivity over a broader RH range than any other known chemiresistor. Characterization of the CO2-MOF interaction through a combination of in situ optical spectroscopy and density functional theory calculations evidence autogenously generated hydrated adsorption sites and a charge trapping mechanism as responsible for the intriguing CO2 sensing properties of Cu3HIB2.

The high-performance p-type metal-oxide-semiconductor (MOS)-based gas sensor is an important subject of research in the field of gas-sensing technology. In this work, we demonstrated a p-type MOS-based thin-film transistor (TFT) nitrogen dioxide (NO2) gas sensor that used tin oxide (SnOX) for both the channel and sensing layers. The crystalline status, surface morphology, and atomic-bonding configuration of the thin-film were examined using X-ray diffraction, field emission-scanning electron microscopy, and X-ray photoelectron spectroscopy. The results indicated that the deposited thin-film was mainly composed of polycrystalline SnO with a tetragonal structure. The fabricated p-type SnOX TFT showed a maximum response value of 19.4-10 ppm NO2 at room temperature (RT, 25 °C) when operated in the subthreshold region, which was significantly higher than that of 2.8–10 ppm NO2 obtained from a p-type SnOX thin-film chemiresistor at RT. In addition, the SnOX TFT gas sensor showed significantly higher sensitivity to NO2 gas than to other target gases such as NH3, H2S, CO2, and CO at RT. To the best of our knowledge, this is the first study to a p-type MOS-based field-effect transistor-type gas sensor. Our experimental results demonstrate that the p-type SnOX TFT is a promising gas sensor that can operate at RT with high sensitivity and selectivity to NO2 gas.

The analyte sensitivity of a chemical sensor is commonly tuned by the type of chemical recognition element. Here, it is shown that the solvent also plays a critical role in the sensing performance of thiol-capped gold nanoparticle film chemiresistors. Both the resistance change and speed of response of the chemiresistor can be optimised by adjusting the composition of methanol and water used for detection of the pesticide permethrin. It is shown that the resistance change is governed by the partitioning of the analyte between the thiol coating of the gold nanoparticles and the solvent. The partition coefficient, log P, has a strong influence on the response magnitude and is directly correlated to the response time. By varying the solvent, the log P of extremely high partitioning analytes such as permethrin in water can be lowered, and a much larger resistance change can be obtained in a shorter time period (5 min). A theoretical model is developed that correlates the experimental findings to changes in analyte partition coefficient with solvent type and composition. The results have significant implications for the design of chemical sensors and solvent conditions that can offer an enhanced signal within a practical timeframe.

A semiconducting metal oxide (SMO) chemiresistor (ZnFe2 O4 ) is used for discriminating two isomeric volatile organic compounds (VOCs), namely 1- and 2-propanol. The transient current of the SMO chemiresistor is correlated with the aerobic oxidation of organic vapors on its surface. The changes in transient current of the ZnFe2 O4 chemiresistor are measured at different temperatures (260-320 °C) for detecting equal concentrations (200 ppm) of the two structural isomers of propanol. The transient current of ZnFe2 O4 reflects a faster oxidation of 2-propanol than 1-propanol on the surface. First-principles calculations and kinetic studies on the interaction of 1- and 2-propanol over ZnFe2 O4 provide further insight in support of the experimental evidence. The calculations predict more spontaneous adsorption of 2-propanol on the (111) surface of ZnFe2 O4 than 1-propanol. Kinetic parameters for the oxidation of isomeric vapors are estimated by modelling the transient current of ZnFe2 O4 using the Langmuir-Hinshelwood reaction mechanism. The faster oxidation of 2-propanol and comparatively lower activation energy for the respective process over ZnFe2 O4 is justified in accordance to the chemical structures of vapors. The findings have strong implications in exploring a new technique for discriminating isomeric VOCs, which is significant for environmental monitoring and medical applications.

The development of freestanding fiber-type chemiresistors, having high integration ability with various portable electronics including smart clothing systems, is highly demanding for the next-generation wearable sensing platforms. However, critical challenges stemming from the irreversible chemical sensing kinetics and weak reliability of the freestanding fiber-type chemiresistor hinder their practical use. In this work, for the first time, we report on the potential suitability of the freestanding and ultraporous reduced graphene oxide fiber functionalized with WO3 nanorods (porous WO3 NRs-RGO composite fiber) as a sensitive nitrogen dioxide (NO2) detector. By employing a tunicate cellulose nanofiber (TCNF), which is a unique animal-type cellulose, the numerous mesopores are formed on a wet-spun TCNF-GO composite fiber, unlike a bare GO fiber with dense surface structure. More interestingly, due to the superior wettability of TCNF, the aqueous tungsten precursor is uniformly adsorbed on an ultraporous TCNF-GO fiber, and subsequent heat treatment results in the thermal reduction of a TCNF-GO fiber and hierarchical growth of WO3 NRs perpendicular to the porous RGO fiber (porous WO3 NRs-RGO fiber). The freestanding porous WO3 NRs-RGO fiber shows a notable response to 1 ppm NO2. Furthermore, we successfully demonstrate reversible NO2 sensing characteristics of the porous WO3 NRs-RGO fiber, which is integrated on a wrist-type wearable sensing device.

Ambipolar tris(phthalocyaninato) rare earth semiconductors, Eu2[Pc(OC8H17)8]2[Pc(OCH2CF3)8] (1) and Eu2{Pc[(OC2H4)3OCH3]8}2[Pc(OCH2CF3)8] (2), are synthesized and fabricated into well-organized films via a Langmuir-Shafer (LS) method. Driven by intermolecular π–π stacking, as well as hydrophilic–hydrophobic interactions, 1 and 2 aggregated in different modes, H for 1 and J for 2, in LS films. Consequently, higher conductivity and carrier mobilities for electrons and holes in the LS films of 1 are achieved compared to those of 2, with gate-induced sheet electron and hole densities (ne and nh) of 2.48 × 1012 and 8.31 × 109 cm−2 for 1, and 1.77 × 1012 and 8.01 × 1010 cm−2 for 2, respectively. Comparative studies on the gas sensing behavior of the LS films of 1 and 2 revealed that, towards oxidative NO2, both 1 and 2 show n-type sensing behavior, with a higher sensitivity of 2 (54.6% ppm−1) than 1 (12.5% ppm−1) in the 0.10–0.85 ppm range, which can be attributed to the lower ne of 2. Unexpectedly, towards reductive NH3 (3.0–7.0 ppm) and H2S (5.0–9.0 ppm), only ambipolar 1 exhibits current responses, determined by the lower nh of 1. More importantly, the current of 1 responds reversely towards NH3 and H2S (decreases vs. increases) even though they are both reductive gases. This is because weakly reductive NH3 reduces nh while strongly reductive H2S increases ne. By combining the distinct response patterns of ambipolar 1 and 2, these three toxic gases can be distinguished unambiguously from each other, showing the potential for these two molecules in efficient chemiresistor sensor arrays.

The ability to measure and monitor the concentration of specific chemical and/or gaseous species (i.e., “analytes”) is the main requirement in many fields, including industrial processes, medical applications, and workplace safety management. As a consequence, several kinds of sensors have been developed in the modern era according to some practical guidelines that regard the characteristics of the active (sensing) materials on which the sensor devices are based. These characteristics include the cost-effectiveness of the materials’ manufacturing, the sensitivity to analytes, the material stability, and the possibility of exploiting them for low-cost and portable devices. Consequently, many gas sensors employ well-defined transduction methods, the most popular being the oxidation (or reduction) of the analyte in an electrochemical reactor, optical techniques, and chemiresistive responses to gas adsorption. In recent years, many of the efforts devoted to improving these methods have been directed towards the use of certain classes of specific materials. In particular, ionic liquids have been employed as electrolytes of exceptional properties for the preparation of amperometric gas sensors, while metal–organic frameworks (MOFs) are used as highly porous and reactive materials which can be employed, in pure form or as a component of MOF-based functional composites, as active materials of chemiresistive or optical sensors. Here, we report on the most recent developments relative to the use of these classes of materials in chemical sensing. We discuss the main features of these materials and the reasons why they are considered interesting in the field of chemical sensors. Subsequently, we review some of the technological and scientific results published in the span of the last six years that we consider among the most interesting and useful ones for expanding the awareness on future trends in chemical sensing. Finally, we discuss the prospects for the use of these materials and the factors involved in their possible use for new generations of sensor devices.

Continuous monitoring of hydrogen sulfide (H2S) in human breath for early stage diagnosis of halitosis is of great significance for prevention of dental diseases. However, fabrication of a highly selective and sensitive H2S gas sensor material still remains a challenge, and direct analysis of real breath samples has not been properly attempted, to the best of our knowledge. To address the issue, herein, we introduce facile cofunctionalization of WO3 nanofibers with alkaline metal (Na) and noble metal (Pt) catalysts via the simple addition of sodium chloride (NaCl) and Pt nanoparticles (NPs), followed by electrospinning process. The Na-doping and Pt NPs decoration in WO3 grains induces the partial evolution of the Na2W4O13 phase, causing the buildup of Pt/Na2W4O13/WO3 multi-interface heterojunctions that selectively interacts with sulfur-containing species. As a result, we achieved the highest-ranked sensing performances, that is, response (Rair/Rgas) = 780 @ 1 ppm and selectivity (RH2S/REtOH) = 277 against 1 ppm ethanol, among the chemiresistor-based H2S sensors, owing to the synergistic chemical and electronic sensitization effects of the Pt NP/Na compound cocatalysts. The as-prepared sensing layer was proven to be practically effective for direct, and quantitative halitosis analysis based on the correlation (accuracy = 86.3%) between the H2S concentration measured using the direct breath signals obtained by our test device (80 cases) and gas chromatography. This study offers possibilities for direct, highly reliable and rapid detection of H2S in real human breath without the need of any collection or filtering equipment.

Made available in DSpace on 2022-04-28T19:41:05Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-06-25 Quality control in the production and processing of raw meat is currently one of the biggest concerns for food industry and would benefit from portable and wireless sensors capable of detecting the onset of spoilage. Raw meat is a natural source of biogenic and volatile amines as byproducts of decarboxylation reactions, and the levels of these compounds can be utilized as quality control parameters. We report herein a hybrid chemiresistor sensor based on inorganic nanofibers of SiO2:ZnO (an n-type material) and single-walled carbon nanotubes functionalized with 3,5-dinitrophenyls (a p-type material) with dosimetric sensitivity ∼40 times higher for amines than for other volatile organic compounds, which also provides excellent selectivity. The hybrid nanomaterial-based chemiresistor sensory material was used to convert radio-frequency identification tags into chemically actuated resonant devices, which constitute wireless sensors that can be potentially employed in packaging to report on the quality of meat. Specifically, the as-developed wireless tags report on cumulative amine exposure inside the meat package, showing a decrease in radio-frequency signals to the point wherein the sensor ceased to be smartphone-readable. These hybrid material-modified wireless tags offer a path to scalable, affordable, portable, and wireless chemical sensor technology for food quality monitoring without the need to open the packaging. Department of Chemistry Institute for Soldier Nanotechnologies Massachusetts Institute of Technology Cambridge, 77 Massachusetts Avenue Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge, 77 Massachusetts Avenue Nanotechnology National Laboratory for Agriculture (LNNA) Embrapa Instrumentação, São Paulo Institute of Geosciences and Exact Sciences São Paulo State University (UNESP), São Paulo Institute of Geosciences and Exact Sciences São Paulo State University (UNESP), São Paulo

A high-performance chemiresistive gas sensor is described for the detection of hydrogen sulfide (H2S), an acutely toxic and corrosive gas. The chemiresistor operates at room temperature with low power requirements potentially suitable for wearable sensors or for rapid in-field detection of H2S in settings such as pipelines and wastewater treatment plants. Specifically, we report chemiresistors based on single-walled carbon nanotubes (SWCNTs) containing highly oxidizing platinum-polyoxometalate (Pt-POM) selectors. We show that by tuning the vanadium content and thereby the oxidation reactivity of the constituent POMs, an efficient chemiresistive sensor is obtained that is proposed to operate by modulating CNT doping during aerobic H2S oxidation. The sensor shows exceptional sensitivity to trace H2S in air with a ppb-level detection limit, multimonth stability under ambient conditions, and high selectivity for H2S over a wide range of interferants, including thiols, thioethers, and thiophene. Finally, we demonstrate that the robust sensing material can be used to fabricate flexible devices by covalently immobilizing the SWCNT-P4VP network onto a polyimide substrate, further extending the potentially broad utility of the chemiresistors. The strategy presented herein highlights the applicability of concepts in molecular aerobic oxidation catalysis to the development of low-cost analyte detection technologies., ACS Central Science, 7 (9), ISSN:2374-7951

This paper proposes a new carbon black composite thin film and its depositing method. The thin film functions as a chemiresistor which changes its resistance in response to gas vapor exposure. A composite of carbon black (CB) powder and a gelatin (GE) polymer is used as the resistor material. A microelectrode structure is formed on a glass substrate by using microfabrication. A mixture solution of CB and GE is deposited on the microelectrode by a needle dispenser to form the chemiresistor with a thickness of approximately 400 nm. The vapor response of the chemiresistor is tested for water and ethanol at room temperature. The chemiresistor exhibits extreme large response (200∼300% in resistance increment) upon exposure to these vapors.

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.

In this article, polythiophene (PTh) and a sequence of PTh/(1, 5, 10 wt.%) ZnO composites were prepared by in situ chemical oxidative polymerization method. The successful formation of PTh, ZnO, and interaction between PTh and ZnO were confirmed by various techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry, UV–vis spectroscopy, fluorescence studies, followed by DC electrical conductivity. The XRD spectra showed crystallinity modification for PTh with ZnO wt.%, demonstrating the crystal structure of the sulfur modification. The SEM micrographs showed the existence of randomly linked ZnO nanoparticles, confirming the interaction of ZnO nanoparticles with the polymer matrix. A good agreement was observed in comparison to spectral studies. From the Tauc plot, it was found that the pure PTh bandgap was 2.0 eV and eventually decreased on decreasing the (wt.%) of doping. PTh/10(wt.%) ZnO showed enhanced conductivity (i.e., 0.00982 S cm−1) compared to pure PTh (0.000472 Scm−1). At room temperature (30 °C), the sensing performance was evaluated in terms of percent sensing and response/recovery time. It was noticed that the prepared composites were suitable for acetone sensing. Pure PTh showed 48.40% sensitivity, and sensitivity response for PTh/1(wt.%) ZnO, PTh/5(wt.%) ZnO, and PTh/10(wt.%) ZnO was 55.35%, 58.08%, and 75.11%, respectively. Sensitivity of the PTh/10(wt.%) ZnO composites–based sensors increased more than that of PTh. PTh/10(wt.%) ZnO showed a 122 s response time compared to another fabricated sensor. In reversibility test for PTh/10(wt.%) ZnO, an oscillating trend in sensitivity for four cycles was observed. The sensor’s operating stability was checked over a 16-day period and a fluctuating trend was observed in percentage sensitivity, reversibility, and response/recovery time.

This paper investigates the response of chemiresistor gas sensor at room temperature with ZnS/PbS core-shell quantum dots as a gas-sensitive layer to detect hydrogen sulfide as a toxic gas, which is a product of decaying organic matter, refineries, and several industrial processes such as extracting crude oil. The dark current of the sensor is simulated and calculated by COMSOL Multiphysics software. The sensor's response to different concentrations of hydrogen sulfide gas and how it changes over time is measured experimentally. Finally, over 2 minutes, the sensor's response to 300 ppm of hydrogen sulfide gas was 2062, an excellent response in gas sensors against target gas.

Acetone is a metabolic byproduct found in the exhaled breath and can be measured to monitor the metabolic degree of ketosis. In this state, the body uses free fatty acids as its main source of fuel because there is limited access to glucose. Monitoring ketosis is important for type I diabetes patients to prevent ketoacidosis, a potentially fatal condition, and individuals adjusting to a low-carbohydrate diet. Here, we demonstrate that a chemiresistor fabricated from oxidized single-walled carbon nanotubes functionalized with titanium dioxide (SWCNT@TiO2) can be used to detect acetone in dried breath samples. Initially, due to the high cross sensitivity of the acetone sensor to water vapor, the acetone sensor was unable to detect acetone in humid gas samples. To resolve this cross-sensitivity issue, a dehumidifier was designed and fabricated to dehydrate the breath samples. Sensor response to the acetone in dried breath samples from three volunteers was shown to be linearly correlated with the two other ketone bodies, acetoacetic acid in urine and β-hydroxybutyric acid in the blood. The breath sampling and analysis methodology had a calculated acetone detection limit of 1.6 ppm and capable of detecting up to at least 100 ppm of acetone, which is the dynamic range of breath acetone for someone with ketosis. Finally, the application of the sensor as a breath acetone detector was studied by incorporating the sensor into a handheld prototype breathalyzer.

The response time is one of the main functional parameter for gas sensors, including metal oxide chemiresistors. This parameter is widely investigated for devices working in isothermal mode but it is much less investigated for gas sensors working in temperature modulation mode. In this work, considering ethanol as target gas, we compare the response times of a metal oxide chemiresistor working according to these two modes. In order to compare them, we worked with nearly the same average temperature in both cases, supplying a constant voltage to the heater in the isothermal mode and a squared voltage wave in temperature modulation. Our results show that, depending on the average working temperature, one mode or the other may be faster. At high average working temperature, the response time recorded with the isothermal mode is shorter than the thermal-period of the temperature modulation mode. Lowering the average working temperature, the response time increases for both modes, but the increase is more marked for the isothermal mode, which become slower than the temperature modulation one.

International audience; The monitoring of K+ in saliva, blood, urine, or sweat represents a future powerful alternative diagnostic tool to prevent various diseases. However, several K+ sensors are unable to meet the requirements for the development of point‐of‐care (POC) sensors. To tackle this grand‐challenge, the fabrication of chemiresistors (CRs) based on 3D networks of Au nanoparticles covalently bridged by ad‐hoc supramolecular receptors for K+, namely dithiomethylene dibenzo‐18‐crown‐6 ether is reported here. A multi‐technique characterization allows optimizing a new protocol for fabricating high‐performing CRs for real‐time monitoring of K+ in complex aqueous environments. The sensor shows exceptional figures of merit: i) linear sensitivity in the 10^–3 to 10^–6 m concentration range; ii) high selectivity to K+ in presence of interfering cations (Na+, Ca2+, and Mg2+); iii) high shelf‐life stability (>45 days); iv) reversibility of K+ binding and release; v) successful device integration into microfluidic systems for real‐time monitoring; vi) fast response and recovery times (

In this work, a TiO2-coated GaN nanowire-based back-gate field-effect transistor (FET) device was designed and implemented to address the well-known cross-sensitive nature of metal oxides. Even though a two-terminal TiO2/GaN chemiresistor is highly sensitive to NO2, it suffers from lack of selectivity toward NO2 and SO2. Here, a Si back gate with C-AlGaN as the gate dielectric was demonstrated as a tunable parameter, which enhances discrimination of these cross-sensitive gases at room temperature (20 °C). Compared to no bias, a back-gate bias resulted in a significant 60% increase in NO2 response, whereas the increase was an insignificant 10% in SO2 response. The differential change in gas response was explained with the help of a band diagram, derived from the energetics of molecular models based on density functional theory (DFT). The device geometries in this work are not optimized and are intended only for proving the concept.

2D transition metal carbides and nitrides (MXenes) open up novel opportunities in gas sensing with high sensitivity at room temperature. Herein, 2D Mo2 CTx flakes with high aspect ratio are successfully synthesized. The chemiresistive effect in a sub-µm MXene multilayer for different organic vapors and humidity at 101 -104 ppm in dry air is studied. Reasonably, the low-noise resistance signal allows the detection of H2 O down to 10 ppm. Moreover, humidity suppresses the response of Mo2 CTx to organic analytes due to the blocking of adsorption active sites. By measuring the impedance of MXene layers as a function of ac frequency in the 10-2 -106 Hz range, it is shown that operation principle of the sensor is dominated by resistance change rather than capacitance variations. The sensor transfer function allows to conclude that the Mo2 CTx chemiresistance is mainly originating from electron transport through interflake potential barriers with heights up to 0.2 eV. Density functional theory calculations, elucidating the Mo2 C surface interaction with organic analytes and H2 O, explain the experimental data as an energy shift of the density of states under the analyte's adsorption which induces increasing electrical resistance.

Sensors, capable of detecting trace amounts of gas molecules or volatile organic compounds (VOCs), are in great demand for environmental monitoring, food safety, health diagnostics, and national defense. In the era of the Internet of Things (IoT) and big data, the requirements on gas sensors, in addition to sensitivity and selectivity, have been increasingly placed on sensor simplicity, room temperature operation, ease for integration, and flexibility. The key to meet these requirements is the development of high-performance gas sensing materials. Two-dimensional (2D) atomic crystals, emerged after graphene, have demonstrated a number of attractive properties that are beneficial to gas sensing, such as the versatile and tunable electronic/optoelectronic properties of metal chalcogenides (MCs), the rich surface chemistry and good conductivity of MXenes, and the anisotropic structural and electronic properties of black phosphorus (BP). While most gas sensors based on 2D atomic crystals have been incorporated in the setup of a chemiresistor, field-effect transistor (FET), quartz crystal microbalance (QCM), or optical fiber, their working principles that involve gas adsorption, charge transfer, surface reaction, mass loading, and/or change of the refractive index vary from material to material. Understanding the gas-solid interaction and the subsequent signal transduction pathways is essential not only for improving the performance of existing sensing materials but also for searching new and advanced ones. In this review, we aim to provide an overview of the recent development of gas sensors based on various 2D atomic crystals from both the experimental and theoretical investigations. We will particularly focus on the sensing mechanisms and working principles of the related sensors, as well as approaches to enhance their sensing performances. Finally, we summarize the whole article and provide future perspectives for the development of gas sensors with 2D materials.

The acetone sensing mechanism by WO 3 was investigated through a combined experimental and theoret- ical approach. The ?-monoclinic WO 3 powder was synthesized by a template-mediated sol-gel method and characterized on structural, surface, morphological and optical points of view. A thin film of WO 3 was deposited on interdigitated Au electrodes by hot-spray method and tested at 300 oC (while apply- ing a bias of 1.0 V) for acetone gas sensing, both in presence and absence of oxygen in the gas carrier. Interestingly, the absence of oxygen had no significant effect on the sensor response intensity but it dra- matically increased the recovery times (from 120 s to 2700 s). In order to explain these experimental results, by means of ab initio density functional theory calculations, we modeled a defective ?-WO 3 surface structure and simulated the adsorption of acetone and oxygen molecules on top of it. We un- precedentedly evidenced that, in presence of surface oxygen vacancies, both acetone adsorption and its oxidation reaction can occur. However, their contribution to the sensor response strictly depends on the inert/oxidative atmosphere present in the sensing chamber, which in turn strongly affects the surface oxygen population. Our findings can either be the guidelines for future studies aimed at delineating the possible reaction products or pave the way for the engineering of tailored nanomaterials having specific surface features and enhanced sensing properties.

Sensors capable of detecting and classifying volatile organic compounds (VOC) have been gaining more attention by the advent of internet-of-things (IoT) enabled devices and integration of various sensing elements into hand-held and portable devices. The recent applications of VOC sensors call for detectors that operate with low power consumption as well as fabrication techniques that allow integration of sensing elements with devices. Hence, in this dissertation, the application of chemiresistive sensing elements based on polymer nanocomposites, which operate at near room temperature, for low-power detection of VOCs was investigated. As a versatile fabrication method, drop-on-demand inkjet printing (DoD IJP) was used to transfer the sensing material onto the sensor platform. A group of commercially available polymers with representative solubility parameters were selected to detect a wide range of chemically diverse analytes. A highly conductive high-structure carbon black (CB) was used as the conductive filler. The inkjet inks containing different polymers and CB were then formulated via solution mixing. Due to various constraints on the physical properties of inkjet inks, a systematic process for ink formulation had to be established, especially when formulating inks that contained multiple components. For this aim, a systematic approach consisting of successive characterization steps, including assessment of rheology, evaporation profile, and CB particle size distribution was proposed. At the end of this step, inkjet inks containing different polymer composites were formulated, allowing stable droplet jetting and deposition of uniform sensory films. The effect of ink formulation on the sensing performance of the printed sensors was investigated. It was demonstrated that by changing CB concentration in the composite, the sensor sensitivity varied. Lower CB loading resulted in better sensitivity (55% higher sensitivity to pentane for 3.3 vol% compared to 8.3 vol% CB loading) but inferior baseline noise (45% increase in baseline noise for 3.3 vol% compared to 8.3 vol% CB loading). By optimizing the CB concentration, high sensitivity with an improved limit of detection (LoD) was obtained as demonstrated by comparing the LoD of composites containing 3.3, 5.5, and 8.3 vol% CB to pentane, which was approximately 80, 20, and 90~ppm, respectively. The concentration dependence of the sensor response upon exposure to chemically diverse analytes, namely water, ethanol, acetone, pentane, and heptane, was studied. It was shown that the sensor response to each analyte was in line with the analyte-polymer affinity expected from their respective solubility parameters. In the studied concentration range, all sensors showed a linear response to the concentration change of those analytes to which they were sensitive, allowing to extract the sensor sensitivity and expected LoD. The sensor LoD values appeared to be in the range of few to tens of ppm, depending on the analyte and the printing parameters. By performing the principal component analysis (PCA), it was demonstrated that using three principal components would allow the classification of the tested analytes. Overall, the methods, materials, and results of this work demonstrated the potential of DoD IJP for the fabrication of versatile and sensitive low-temperature VOC detectors, paving the way to fully printed low-cost and low-power electronic nose devices.

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

The present work shows the synthesis of a graphene-based nanocomposite with titania nanoparticles through a simple one-step, microwave assisted method. The GR/TiO2 nanocomposite was used for the fabrication of a chemiresistor device and characterized towards oxidizing and reducing gases under controlled environment. The results show how the addiction of titania nanoparticles to graphene nanosheets enables the sensing of ethanol at room temperature. A sensitivity curve to ethanol was recorded in the range 15–50 ppm. The interaction with water, the main interferent in ethanol sensing, was also investigated and the findings disclose a weaker interaction respect to ethanol, thus suggesting that steric factors can play a role in the sensing mechanism.

A chemiresistive sensor is described for the detection of methane (CH(4)), a potent greenhouse gas that also poses an explosion hazard in air. The chemiresistor allows for the low-power, low-cost, and distributed sensing of CH(4) at room temperature in air with environmental implications for gas leak detection in homes, production facilities, and pipelines. Specifically, the chemiresistors are based on single-walled carbon nanotubes (SWCNTs) noncovalently functionalized with poly(4-vinylpyridine) (P4VP) that enables the incorporation of a platinum-polyoxometalate (Pt-POM) CH(4) oxidation precatalyst into the sensor by P4VP coordination. The resulting SWCNT-P4VP-Pt-POM composite showed ppm-level sensitivity to CH(4) and good stability to air as well as time, wherein the generation of a high-valent platinum intermediate during CH(4) oxidation is proposed as the origin of the observed chemiresistive response. The chemiresistor was found to exhibit selectivity for CH(4) over heavier hydrocarbons such as n-hexane, benzene, toluene, and o-xylene, as well as gases, including carbon dioxide and hydrogen. The utility of the sensor in detecting CH(4) using a simple handheld multimeter was also demonstrated.

The additives change the conductance of the metal oxide chemiresistor gas sensor. A model based on the Freundlich adsorption isotherm, giving a power law relation between the electrical conductance of the gas sensor and the concentration of the target gas, describes the steady state response of the metal oxide chemiresistor gas sensor in the best manner. In this paper, the effect of platinum (Pt) and palladium (Pd) on various tin dioxide (SnO 2 ) nanostructures are analyzed and the model parameters are evaluated for the conduction model based on adsorption. The model parameters are fitted using the data extracted from different papers that report the effect of additives to tin dioxide (SnO 2 ) in sensing gases like hydrogen (H 2 ), carbon monoxide (CO), nitric oxide (NO 2 ), or ethanol (C 2 H 5 OH). The model parameters are evaluated for the sensing material with and without the additives. The result shows that not only the exponent but also the pre-exponential coefficient, of the power law relation, is changed when doping is done to the sensing material. The major amount of conductance change is observed to be due to the power law exponent when SnO2 with Pt is used for detecting gases like CO, NO 2 , and H 2 , etc., while the pre-exponential factor controls conductance change when SnO 2 with Pd is used in detecting H 2 . Even though this study is done on SnO 2 with palladium or platinum, it can be shown that the steady state response of most of the metal oxide chemiresistors with additives can be modeled by the method proposed.