An electronic nose for amine detection based on polymer/SWNT-COOH nanocomposite

An electronic nose (e-nose) system based on polymer/carboxylic-functionalized single-walled carbon nanotubes (SWNT-COOH) was developed for sensing various volatile amines. The SWNT-COOH dispersed in the matrix of different polymers; namely, polyvinyl chloride (PVC), cumene terminated polystyrene-co-maleic anhydride (cumene-PSMA), poly(styrenecomaleic acid) partial isobutyl/methyl mixed ester (PSE), and polyvinylpyrrolidon (PVP), were deposited on interdigitated gold electrodes to make the gas sensors. The response of these sensors to volatile amines was studied by both static and dynamic flow measurements. It was found that all sensors exhibited behaviors corresponding to Plateau-Bretano-Stevens law (R2 = 0.81 to 0.99) as the response to volatile amines. Real-world application was demonstrated by applying this e-nose to monitor the odor of sun-dried snakeskin gourami that was pre-processed by salting-preservation. This electronic nose can discriminate sun-dried fish odors with different stored days using a simple pattern recognition based on the principal component analysis (PCA).     

Neuromorphic Processing for Optical Microbead Arrays: Dimensionality Reduction and Contrast Enhancement

This paper presents a neuromorphic approach for sensor-based machine olfaction that combines a portable chemical detection system based on microbead array technology with a biologically inspired model of signal processing in the olfactory bulb. The sensor array contains hundreds of microbeads coated with solvatochromic dyes adsorbed in, or covalently attached on, the matrix of various microspheres. When exposed to odors, each bead sensor responds with corresponding intensity changes, spectral shifts, and time-dependent variations associated with the fluorescent sensors. The bead array responses are subsequently processed using a model of olfactory circuits that capture the following two functions: chemotopic convergence of receptor neurons and center on-off surround lateral interactions. The first circuit performs dimensionality reduction, transforming the high-dimensional microbead array response into an organized spatial pattern (i.e., an odor image). The second circuit enhances the contrast of these spatial patterns, improving the separability of odors. The model is validated on an experimental dataset containing the responses of a large array of microbead sensors to five different analytes. Our results indicate that the model is able to significantly improve the separability between odor patterns, compared to that available from the raw sensor response     

Ceramics for chemical sensing

Many types of sensors have been developed to detect chemical species in the gas phase. These include optical based on color change or fluoresence, surface acoustic wave (SAW) devices, electrochemical, chemoresistive/semiconductive, field effect transistors (FET), metal-insulator-semiconductor (MIS) diode devices, and many other. Among these, resistive type sensors based on ceramic oxides are particularly attractive because of their low cost, wide range of applications and potential for use in electronic nose. This article focuses mainly on the resistive/semiconductive, especially the surface conductive ceramic oxide type gas sensors. The main emphasis is on the basic principles involving gas-solid reactions. Also discussed are selected applications with an emphasis on sensor design issues. Since SnO₂ can be used as a model system for oxide-based sensors, most of the discussions focuses on this system, though other systems are occasionally highlighted illustrating recent developments.     

Optical multibead arrays for simple and complex odor discrimination

A fiber optic bead-based sensor array platform has been employed to discriminate between six different odors and air carrier gas. Six different bead sensor types, with over 250 replicates of each, were monitored before, during, and after odor exposure to produce time-dependent fluorescence response patterns that were unique for each sensor-analyte combination. A total of 2,683 sensors were analyzed with respect to changes in their fluorescence, and signals from identical sensor beads were averaged to improve signal-to-noise ratios. Analyte classification rates of 100% were achieved for three complex (coffee bean) odors and three pure (simple) odors (toluene, acetone, 1,3-dinitrotoluene) measured at their highest relative concentrations. When lower odor concentrations were employed, the system exhibited better than 85% classification rates for analyte discrimination. Sensor response repeatability to these odor stimuli has also been quantified statistically, which is vital in defining the detection limit of the overall system. These results demonstrate, for the first time, the utility of our bead array technology for discriminating between different odor types at various dilution levels.     

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.     

基于Na3Zr2Si2PO12固体电解质的非平衡态SO2传感器

基于Na₃Zr₂Si₂PO₁₂(NASICON)固体电解质, 分别以Na₂SO₄-BaSO₄混合盐和NaRe(SO₄)₂复盐为敏感电极材料制备了片式SO₂非平衡态气体传感器。结果表明, 该类型传感器的输出电动势与SO₂气体浓度的对数呈良好的线性关系, 在低温260℃具有最佳性能, 灵敏度分别达到了160 mV/decade和136 mV/decade。传感器在不同浓度的SO₂气体中的交流阻抗谱测试结果显示, 气体在敏感电极的三相界面处电化学反应的活性随着气体浓度的增大而增强, 结合敏感电极结构, 对该类敏感电极的机理进行了分析。由于NASICON具有良好的低温钠离子导电性, 可以大幅降低传感器的工作温度; 由于Na₂SO₄-BaSO₄混合盐和NaRe(SO₄)₂敏感材料具有更好的化学稳定性, 制备的传感器具有良好的可重复性和稳定性。基于非平衡态设计的传感器, 具有结构简单和成本低的优点。以上特性为该传感器在SO₂气体在环境监测方面的应用提供了可能。     

A Method of Feature Extraction on Recovery Curves for Fast Recognition Application With Metal Oxide Gas Sensor Array

In the fast recognition applications of electronic nose, not only the recognition time is important, another parameter response-recovery time also needs to be considered. The response-recovery time could be defined as the time from the beginning of measuring one sample to the state of being ready for new sample measurement. An electronic nose with nine metal oxide (MOX) gas sensors and a method of feature extraction on sensor recovery curves were presented in this paper. The electronic nose was designed to reduce the recognition time and the response-recovery time synchronously. In the sampling module of the electronic nose, there were two pumps, which could let the sensor quickly recovered. The feature extraction method could rapidly extract features from sensor recovery curves with robust information. Nine volatile organic compounds (VOCs) gas samples were measured with the electronic nose. The correct recognition ratios under 10 and 15 s recognition time are 91.0% and 95.8%, respectively. The mean response-recovery time of these sensors in the measurements was 33.5 s, which was about 42.7% of the response-recovery time in typical traditional gas sample measurements. The results show that the proposed feature extraction method could extract robust information with short recognition time and response-recovery time.     

MWCNT-polymer composites as highly sensitive and selective room temperature gas sensors

Multi-walled carbon nanotubes (MWCNTs)-polymer composite-based hybrid sensors were fabricated and integrated into a resistive sensor design for gas sensing applications. Thin films of MWCNTs were grown onto Si/SiO(2) substrates via xylene pyrolysis using the chemical vapor deposition technique. Polymers like PEDOT:PSS and polyaniline (PANI) mixed with various solvents like DMSO, DMF, 2-propanol and ethylene glycol were used to synthesize the composite films. These sensors exhibited excellent response and selectivity at room temperature when exposed to low concentrations (100 ppm) of analyte gases like NH(3) and NO(2). The effect of various solvents on the sensor response imparting selectivity to CNT-polymer nanocomposites was investigated extensively. Sensitivities as high as 28% were observed for an MWCNT-PEDOT:PSS composite sensor when exposed to 100 ppm of NH(3) and - 29.8% sensitivity for an MWCNT-PANI composite sensor to 100 ppm of NO(2) when DMSO was used as a solvent. Additionally, the sensors exhibited good reversibility.     

A novel integrated acoustic gas and temperature sensor

Acoustic temperature sensors have the advantages of a high-resolution frequency output and ease of integration with other acoustic sensors but require hermetic packaging to prevent sensor contamination. Surface-skimming bulk-wave (SSBW) devices have been found to be much less sensitive to surface contamination than other acoustic devices, and although their temperature response has been studied extensively, they have not been studied specifically as temperature sensors. Surface acoustic wave (SAW) based chemical sensors requiring temperature measurement or control are susceptible to temperature measurement error because the temperature cannot be measured in the same location as the chemical sensor. The objectives of this work were to examine the temperature characteristics and performance of a SSBW temperature sensor when integrated with a SAW condensation and humidity sensor in a novel design. The SSBW temperature sensor had over an order of magnitude less sensitivity to condensation and water uptake in certain polyimide films than an integrated SAW gas sensor indicating that this design is practical for sensing films in the delay path where film thickness is carefully considered.     

A novel method for reducing the dimensionality in a sensor array

Specific types of gas sensors are normally produced by adding different dopants to a common substrate. The advancement of technology has made the fabrication of many dopants and consequently various sensors possible. As a result, in each family of gas sensors, one can find tens of different sensors which are only slightly different in the spectrum of response to various volatile compounds. The wide variety of available gas sensors creates a selection problem for any specific application. Sensor selection/reduction becomes even more important when cost and technology limitations are issues of concern. Accordingly, a methodology by which one can tailor a sensor array to a specific need is highly desirable. In this paper, a novel method is introduced to address this task using data from an electronic nose that uses polymer gas sensors. This method has been delineated based on the geometry of eigenvectors in Karhunen-Loeve expansion. The methodology is general and therefore suitable for many other feature selection problems     

Preparation of modified MWCNTs-doped PANI nanorods by oxygen plasma and their ammonia-sensing properties

Multi-wall carbon nanotubes (MWCNTs)-doped polyaniline (PANI) nanopowders were prepared by chemical oxidation polymerization. Then, the MWCNTs-doped PANI nanopowders were modified by a radio frequency (RF) oxygen plasma source. The morphology and structure of modified MWCNTs-doped PANI nanorods were analyzed by SEM and FI-IR. Gas sensors were fabricated based on plasma modified MWCNTs-doped PANI nanorods to detect ammonia at room temperature. The response amplitude of the gas sensor based on modified MWCNTs-doped PANI nanorods was much higher than those of MWCNTs-doped PANI nanopowders and pure PANI nanopowders sensors, respectively, in ammonia concentration range of 10–150 ppm. Cross responses of modified MWCNTs-doped PANI nanorods sensor to ammonia, ethanol, formaldehyde, and toluene were tested. The sensor showed good selectivity and stability. The sensing mechanism of modified MWCNTs-doped PANI nanorods gas sensor was analyzed.     

Polyaniline Nanofiber Based Surface Acoustic Wave Gas Sensors—Effect of Nanofiber Diameter on H2 Response

A template-free rapidly mixed reaction was employed to synthesize polyaniline nanofibers using chemical oxidative polymerization of aniline. Hydrochloric acid (HCl) and camphor sulfonic acid (CSA) were used in the synthesis to obtain 30- and 50-nm average diameter polyaniline nanofibers. The nanofibers were deposited onto layered ZnO/64deg YX LiNbO₃ surface-acoustic-wave transducers. The sensors were tested toward hydrogen (H₂) gas while operating at room temperature. The dopant for the polyaniline nanofiber synthesis was found to have a significant effect on the device sensitivity. The sensor response was found to be larger for the 50-nm diameter CSA-doped nanofiber based sensors, while the response and recovery times were faster for the 30-nm diameter HCl-doped nanofibers     

Effect of solution pH and adsorbent concentration on the sensing parameters of TGN‐based electrochemical sensor

The response of trilayer graphene nanoribbon (TGN)‐based ion‐sensitive field‐effect transistor (ISFET) to different pH solutions and adsorption effect on the sensing parameters are analytically studied in this research. The authors propose a TGN‐based sensor to electrochemically detect pH. To this end, absorption effect on the sensing area in the form of carrier concentration, carrier velocity, and conductance variations are investigated. Also, the caused electrical response on TGN as a detection element is analytically proposed, in which significant current decrease of the sensor is observed after exposure to high pH values. In order to verify the accuracy of the model, it is compared with recent reports on pH sensors. The TGN‐based pH sensor exposes higher current compared to that of carbon nanotube (CNT) counterpart for analogous ambient conditions. While, the comparative results demonstrate that the conductance of proposed model is lower than that of monolayer graphene‐counterpart for equivalent pH values. The results confirm that the conductance of the sensor is decreased and V_g‐min is obviously right‐shifted by increasing value of pH. The authors demonstrate that although there is not the experimental evidence reported in the part of literature for TGN sensor, but the model can assist in comprehending experiments involving nanoscale pH sensors.Inspec keywords: adsorption, graphene, ion sensitive field effect transistors, nanoribbons, electrochemical sensors, pH measurement, nanosensors, absorptionOther keywords: adsorbent concentration, TGN‐based electrochemical sensor, trilayer graphene nanoribbon‐based ion‐sensitive field‐effect transistor, adsorption effect, carbon nanotube counterpart, monolayer graphene‐counterpart, nanoscale pH sensors, pH solution effect, TGN‐based pH sensor, ISFET, CNT, C     

Tactile Chemomechanical Transduction Based on an Elastic Microstructured Array to Enhance the Sensitivity of Portable Biosensors

Tactile sensors capable of perceiving biophysical signals such as force, pressure, or strain have attracted extensive interest for versatile applications in electronic skin, noninvasive healthcare, and biomimetic prostheses. Despite these great achievements, they are still incapable of detecting bio/chemical signals that provide even more meaningful and precise health information due to the lack of efficient transduction principles. Herein, a tactile chemomechanical transduction strategy that enables the tactile sensor to perceive bio/chemical signals is proposed. In this methodology, pyramidal tactile sensors are linked with biomarker‐induced gas‐producing reactions, which transduce biomarker signals to electrical signals in real time. The method is advantageous as it enhances electrical signals by more than tenfold based on a triple‐step signal amplification strategy, as compared to traditional electrical biosensors. It also constitutes a portable and general platform capable of quantifying a wide spectrum of targets including carcinoembryonic antigen, interferon‐γ, and adenosine. Such tactile chemomechanical transduction would greatly broaden the application of tactile sensors toward bio/chemical signals perception which can be used in ultrasensitive portable biosensors and chemical‐responsive chemomechanical systems.     

The human-based multisensor fusion method for artificial nose andtongue sensor data

Presently, an increased interest is apparent for the development of integrated human-like smell and taste sensing capabilities, e.g. for chemical, paper pulp, food, and medicine applications. This paper will present an original sensor fusion method based on human expert opinions about smell and taste and measurement data from artificial nose and taste sensors. The “electronic nose” consists of an array of gas sensors with different selectivity patterns, signal handling, and a sensor signal pattern recognition and decision strategy. The “electronic tongue”, which was developed for the taste analysis of liquids is based on pulse voltammetry. Measurement data from the artificial smell and taste sensors are used to produce sensor-specific opinions about these two human-like sensing modalities. This is achieved by a team of artificial neural networks and conventional signal handling which approximates a Bayesian decision strategy for classifying the sensor information. Further, a fusion algorithm based on the maximum likelihood principle provides a combination of the smell and, respectively, taste opinions, into an overall integrated opinion similar to human beings. The proposed integrated smell- and taste-sensing method is then illustrated by an application of real world measurements in the food industry     

Highly sensitive and rapidly responding room-temperature NO<Subscript>2</Subscript> gas sensors based on WO<Subscript>3</Subscript> nanorods/sulfonated graphene nanocomposites

Metal oxide/graphene nanocomposites are emerging as promising materials for developing room-temperature gas sensors. However, the unsatisfactory performances owing to the relatively low sensitivity, slow response, and recovery kinetics limit their applications. Herein, a highly sensitive and rapidly responding room-temperature NO₂ gas sensor based on WO₃ nanorods/sulfonated reduced graphene oxide (S-rGO) was prepared via a simple and cost-effective hydrothermal method. The optimal sensor response of the WO₃/S-rGO sensor toward 20 ppm NO₂ is 149% in 6 s, which is 4.7 times higher and 100 times faster than that of the corresponding WO₃/rGO sensors. In addition, the sensor exhibits excellent reproducibility, selectivity, and extremely fast recovery kinetics. The mechanism of the WO₃/S-rGO nanocomposite gas sensor is investigated in detail. In addition to the high transport capability of S-rGO as well as its excellent NO₂ adsorption ability, the superior sensing performance of the S-rGO/WO₃ sensor can be attributed to the favorable charge transfer occurring at the S-rGO/WO₃ interfaces. We believe that the strategy of compositing a metal oxide with functionalized graphene provides a new insight for the future development of room-temperature gas sensors.

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The effect of carbon nanotube dispersion on CO gas sensing characteristics of polyaniline gas sensor

Polyaniline is one of the most promising conducting polymers for gas sensing applications due to its relatively high stability and n or p type doping capability. However, the conventionally doped polyaniline still exhibits relatively high resistivity, which causes difficulty in gas sensing measurement. In this work, the effect of carbon nanotube (CNT) dispersion on CO gas sensing characteristics of polyaniline gas sensor is studied. The carbon nanotube was synthesized by Chemical Vapor Deposition (CVD) using acetylene and argon gases at 600 degrees C. The Maleic acid doped Emeradine based polyaniline was synthesized by chemical polymerization of aniline. CNT was then added and dispersed in the solution by ultrasonication and deposited on to interdigitated AI electrode by solvent casting. The sensors were tested for CO sensing at room temperature with CO concentrations in the range of 100-1000 ppm. It was found that the gas sensing characteristics of polyaniline based gas sensor were considerably improved with the inclusion of CNT in polyaniline. The sensitivity was increased and response/recovery times were reduced by more than the factor of 2. The results, therefore, suggest that the inclusion of CNT in MA-doped polyaniline is a promising method for achieving a conductive polymer gas sensor with good sensitivity, fast response, low-concentration detection and room-operating-temperature capability.     

Monolithic miniaturized quartz microbalance array and its application to chemical sensor systems for liquids

We report on the design, fabrication, and application of novel monolithic miniaturized quartz microbalance (QMB) arrays. Up until now, almost all reported resonator arrays (often designated as "electronic noses" or "electronic tongues", respectively, dependent on their application) are assembled from single QMBs. We fabricate arrays with up to 36 QMBs on a single AT-cut quartz blank. Mass sensitive devices based on AT-cut quartz resonators are suitable as (bio)chemical sensors. A frequency shift caused by mass accumulation on the sensor surface increases theoretically with f/sup 2/, hence the detection limits for the application as chemical sensors should be decreased with increasing frequency. Since the quality factor Q of a quartz crystal decreases with f, the frequency stability is reduced, thus limiting mass sensitivity. The mass sensitivity of resonators with different resonant frequencies was examined by means of electrochemical copper deposition on their surface. Subsequently, the manufactured resonators were coated with different layers (polystyrene, amyl-calix[8]arene, /spl beta/-cyclodextrine). In order to examine the applicability of such coatings as sensitive layers, their sensitivities to toluene in water were investigated. Moreover, arrays with up to four different resonant frequencies on one chip were fabricated for comparing the resonator behavior of the same coating at different frequencies. In another test setup, different layers were sprayed onto an array of microbalances having all the same resonant frequency. This allowed for comparing the different coating behavior under equivalent test conditions. Arrays were tested for viscosity measurement to find an optimum resonant frequency.     

A highly sensitive chemical gas detecting transistor based on highly crystalline CVD-grown MoSe<Subscript>2</Subscript> films

Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities,large surface-to-volume ratios,and rapid electrical responses to their surrounding environments.Here,we report the first implementation of a highly sensitive chemical-vapor-deposition-grown multilayer MoSe2 field-effect transistor (FET) in a NO2 gas sensor.This sensor exhibited ultra-high sensitivity (S =ca.1,907 for NO2 at 300 ppm),real-time response,and rapid on-off switching.The high sensitivity of our MoSe2 gas sensor is attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed in the MoSe2,which leads to a significant increase in hole current in the off-state regime.Device modeling and quantum transport simulations revealed that the variation of gap states with NO2 concentration is the key mechanism in a MoSe2 FET-based NO2 gas sensor.This comprehensive study,which addresses material growth,device fabrication,characterization,and device simulations,not only indicates the utility of MoSe2 FETs for high-performance chemical sensors,but also establishes a fundamental understanding of how surface chemistry influences carrier transport in layered semiconductor devices.