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n -butanol detection is crucial to environmental protection and mankind's health because of its toxicity and irritation to the respiratory system. Mono-metal oxide semiconductors (such as SnO 2) often face disadvantages such as low sensitivity, insufficient selectivity, and a high detection limit. Constructing heterojunctions has been considered an efficient way to enhance the performance of MOS gas sensors. This work presents the synthesis of a series of β -CdSnO 3 -SnO 2 nanorod-like heterostructures with good n -butanol sensitivity using a hydrothermal and post-calcination approach. The optimized CSO/SO-8 shows the highest response value about 90–100 ppm n -butanol at 190 °C, which is 15 times greater than that of SnO 2 , and 6.5 times more than that of β -CdSnO 3. Meanwhile, the CSO/SO-8 also exhibits a short response and recovery time (1–2 s/63 s). More importantly, the detection limit is reduced to ppb level (50 ppb). The improved gas-sensitive properties can be explained by the formation of n - n heterojunctions between β -CdSnO 3 and SnO 2 , which increases the content of chemisorbed oxygen, thus enhancing the performance of the gas-sensitive. Our work not only provides a material with n - n heterostructures but also the strategy for constructing heterojunctions to improve gas-sensitive properties. [Display omitted] • The nanorod-like β -CdSnO 3 -SnO 2 heterojunctions were prepared successfully. • CSO/SO-8 exhibits the optimal gas sensing performance to n -butanol. • CSO/SO-8 enables rapid detection (1–2 s) as well as trace n -butanol detection. • The improved performance can be attributed to the formation of heterojunctions that produce more chemisorbed oxygen. [ABSTRACT FROM AUTHOR]

Water electrolysis is one of the key processes for carbon-free hydrogen (H 2) generation, yet it inherently presents the risk of oxygen (O 2) permeation, a significant concern that must be addressed. Contrary to external atmospheric conditions, the environment within a water electrolysis system is characterized exclusively by high-purity H 2 , little O 2 , and moisture (H 2 O). However, there has been limited research on sensors capable of detecting O 2 crossover in such a pure H 2 atmosphere under high humidity conditions. In this study, we developed an electrolysis O 2 monitoring (E-OM) sensor based on a semiconducting metal oxide (SMO) that can operate at a room-temperature with such high humidity through photoactivation with 365 nm LED light. A porous In 2 O 3 nanofilm, prepared by glancing angle deposition (GLAD), was used as the gas sensing material, and copper nanoparticles (Cu NPs) were decorated on the In 2 O 3 surface through electron beam evaporation to accelerate the O 2 adsorption. Remarkably, the E-OM sensor showed a humidity-independent response to O 2 gas in the H 2 atmosphere. Meanwhile, since the response of the E-OM sensor itself was not fast enough for water electrolysis system application, convolutional neural network (CNN)-based signal processing in the time domain was applied. As a result, our E-OM sensor system could predict O 2 concentrations from 0 to 2 vol% in an H 2 environment with a relative humidity (RH) of 30–90 %, and the detection time was under 5 s. This development could enable safe, rapid, and cost-effective monitoring of O 2 crossover in H 2 production infrastructure. • First-time demonstration on O 2 detection behavior of a semiconductor metal oxide (SMO) gas sensor in pure H 2. • Nanoparticle catalyst study for O 2 dissociation energy barrier in SMO gas sensor. • Humidity-independent response of photoactivated SMO gas sensor for room temperature O 2 detection in H 2 environment. • Highly accurate and fast calibration of O 2 concentration via convolutional neural network(CNN)-based signal processing. [ABSTRACT FROM AUTHOR]

This study unveils an innovative approach to fabricating H 2 S gas sensor prototypes for continuous monitoring, leveraging Pd-decorated CuCrO 2 -based metal oxide semiconductor (MOS) chemiresistors and artificial neural network-assisted impedance-based multivariate analysis. Sensors were exposed to H 2 S in cross-interfering environments containing humidity, NO 2 , NH 3 , CH 4 , H 2 , and CO 2. Impedance-based parameters (Z, phase difference, Z′ and Z") obtained at various frequencies demonstrated that sensors were reproducible and selective for H 2 S detection. A neural network-based multilayer perceptron (MLP) regression model was trained with different impedance-based parameters to estimate the H 2 S concentrations. Continuous operation resulted in larger baseline variation for Z, Z′, and Z" readings; however, the measured phase difference values exhibited less depletion than other parameters. Furthermore, concerns about baseline changes were effectively addressed with a fine-tuned MLP model, which predicted both pure air and H 2 S atmospheres more correctly under cross-interfering and baseline-depleted conditions by employing phase differences at different frequencies as input. Possible reasons for the accurate prediction can be attributed to the confined behaviour of phase difference and discussed with the help of sensor statistical parameters such as mean variation, standard deviation and principal components. [Display omitted] • Impedance-based analysis demonstrated reproducible and selective H 2 S detection. • The neural network model with different impedance based parameters was trained. • The measured ´θ´ values exhibited less depletion than other parameters. • Fluctuating baseline over time was effectively addressed to improve accuracy. [ABSTRACT FROM AUTHOR]

This research investigates a novel supercritical-fluid sulfuration (SCF-S) treatment used to enhance the sensitivity of SnO 2 -based chip-typed gas sensors and enable them to resist the influence of moisture during the alcohol detection. The fluid of supercritical state possesses helpful features, such as high permeability of gas and high solubility of water. The SCF-S process introduces the sulfuration penetrating the whole tin oxide membrane with sulfur bonding and thin film transformation. The surface morphology and the lattice structure have been confirmed by Scanning Electron Microscope (SEM) and X-ray diffraction analysis (XRD), respectively. To further verify the composition content after the novel SCF-S treatment, the qualitative and quantitative analyses are conducted, and both show the sulfur doping results. Moreover, different electronic properties are gained in the experiments where the concentration of alcohol and environment humidity vary. In order to understand the sensing reaction model after SCF treatment, response performance at different temperatures and the reliability of the reaction model are further investigated. From the present results, it is demonstrated that SCF-S process has a good potential in terms of the sensitive and selective detection of alcohol. ● The sulfuration treatment helps to enhance sensitivity of SnO 2 -based gas sensors. ● Sensors treated with sulfuration treatment can operate with low working temperature. ● The treated SnO 2 sensor has high selectivity to alcohol molecule and moisture. ● The treated gas sensor is completely unaffected with humidity. ● Related reaction model is also built to illustrate the SCF sulfuration improvement. [ABSTRACT FROM AUTHOR]

In this study, a self-powered photo-induced humidity sensor was developed using ZnWO 4 /In 2 O 3 heterostructure based Triboelectric Nanogenerator (TENG). The ZnWO 4 /In 2 O 3 heterostructure-based TENG generates 44 V voltage and 1 µA current by finger tapping. The ZnWO 4 /In 2 O 3 heterostructures were synthesized via hydrothermal method. SEM analysis confirmed the In 2 O 3 nanocubes grown on ZnWO 4 nanorods. XRD analysis confirmed the polycrystalline nature of ZnWO 4 /In 2 O 3 heterostructures and the average crystallite size was found to be 32.66 nm. XPS analysis confirmed the presence of Zn2+, In3+, W4+, and O2- oxidation states. Optical properties were analyzed by UV–visible spectroscopy and maximum absorbance was found in the region (315–400 nm) since it is capable of absorbing UV light. In this work, the effect of very low-intensity UV light (2 mW/cm2) on a battery-less moisture detection was investigated at different moisture levels (10–90%RH). The sensor response was around 27.50 at 90%RH of photo-induced self-powered humidity sensor. Also, the fast response and recovery times were observed as 260 ms and 620 ms respectively in the illumination of UV light. The photo-induced self-powered humidity sensor showed highly sensitive and stable for a long time and it can be commercialized for indoor and outdoor humidity detection. [Display omitted] • The ZnWO 4 /In 2 O 3 heterostructure was synthesized via hydrothermal method. • The ZnWO 4 /In 2 O 3 used for the fabrication of Triboelectric Nanogenerator. • The TENG generates 44 V voltage and 1 µA current. • Photo-enhanced self-powered humidity sensor showed 27.50 sensor response at 90%RH. • The response and recovery times were found to be 260 ms and 620 ms respectively. [ABSTRACT FROM AUTHOR]

• Presentation of the precise electric potential profiles inside metal oxides for both of the depletion and accumulation cases. • Investigation of the grain shape, grain size and doping level on the electric potential profile. • Investigation of the validity of the approximated solutions of the Poisson-Boltzmann equation. • Extraction of characteristic curves for estimating the surface oxygen density based on the surface electric potential. • Extraction of characteristic curves for estimating the near surface electron concentration based on the surface oxygen density. Chemiresistive metal oxides are widely used in the fabrication of gas sensors. The theory of gas sensitivity of metal oxide sensors has been developed based on the gas-solid interactions on the metal oxide surface. These interactions led to a charge transfer that is well described by the Poisson-Boltzmann equation. The receptor function of metal oxides can be modeled based on this differential equation. Here, the precise results of numerical solution of the Poisson equation coupled with Fermi-Dirac statistics and its approximated version that is the Poisson-Boltzmann equation is calculated to find the exact spatial variation of the electrical potential inside the metal oxide grain. The results are utilized to derive characteristic curves that relate surface oxygen ion density to the surface electrical potential and near surface electron density. The effects of grain size, grain shape and doping level on the results are also studied. It is shown that approximated solutions are only valid in limited range of conditions and the numerical solution should be used when a valid receptor function is sought. [ABSTRACT FROM AUTHOR]

Highlights • One single SnO 2 nanowire bridging 2 electrodes is used as resistive sensor. • The nanosensor undergoes a thermal gradient, becoming a virtual array. • 7 Different gases are tested, giving each a different thermal fingerprint. • The nanosensor shows classification (94.3%) and quantitative estimate (error <25%). Abstract Nowadays the analysis of ambient atmosphere in order to monitor the presence of dangerous gases and volatile compounds is more and more important. For this reason, a network of tiny sensors capable to discriminate the presence of pollutants and distinguish them is crucial. We present here a resistive sensor based on a single tin oxide nanowire (60 nm in diameter and 3.5 μm long) that can detect the presence of different gases and estimate their concentration in the range of 1–50 ppm. The SnO 2 nanowire (NW) is grown by chemical vapor deposition and then used to bridge to metal electrodes. Under a temperature gradient, 5 signals can be extracted, forming the thermal fingerprint of each specific gas that can be present in the measuring chamber. Applying machine learning algorithms to these thermal fingerprints, the system can recognize which gas is present in the chamber (with an 94.3% accuracy) and estimate the concentration of the gas (with an average error of 24.5%). The limit of detection has been found to be under 1 part per million for all the gases tested. [ABSTRACT FROM AUTHOR]

Highlights • CO sensors using an anion-conducting polymer electrolyte and oxide-based electrodes were developed in this study. • The Pt or Au loading onto the SnO 2 electrode and the Au loading onto the In 2 O 3 electrode improved their CO responses. • Only the Pt loading onto the SnO 2 electrode of the sensor drastically improved the CO selectivity against H 2. • The heat treatment of the Pt-loaded SnO 2 in hydrogen at 250 °C enhanced both the CO and H 2 responses of the sensor. Abstract Potentiometric CO-sensing properties of electrochemical gas sensors using an anion-conducting polymer as an electrolyte and a metal oxide (MO; Bi 2 O 3 , CeO 2 , In 2 O 3 , SnO 2 , or V 2 O 5) loaded with or without 2.0 wt% noble metal (N; Ag, Au, Ir, Ru, Rh, Pd, or Pt) as an electrode material (EC(N/MO) or (EC(MO) sensor; N/MO: N-loaded MO) were investigated in wet synthetic air (57%RH) at 30 °C. Among all the EC(MO) sensors, the EC(CeO 2) sensor showed the largest CO response and excellent CO selectivity against H 2 and the EC(Bi 2 O 3) sensor showed relatively large CO and H 2 responses (no CO selectivity against H 2). Other EC(MO) sensors hardly showed both CO and H 2 responses. However, the Au loading just onto In 2 O 3 and SnO 2 was effective in improving the magnitude of CO and H 2 responses of the EC(In 2 O 3) and EC(SnO 2) sensors, respectively, which resulted in relatively poor CO selectivity against H 2. On the other hand, the Pt loading only onto SnO 2 extremely enhanced only the magnitude of CO response of the EC(SnO 2) sensor, and thus the EC(Pt/SnO 2) sensor showed the most excellent CO selectivity against H 2 , among all the sensors in this study. The heat treatment of N/MO powders in H 2 at 250 °C reduced only the H 2 response of the EC(Au/SnO 2) sensor, leading to an improvement of the CO selectivity of only the EC(Au/SnO 2) sensor against H 2. On the other hand, the heat treatment drastically enhanced both CO and H 2 responses of the EC(Pt/SnO 2) sensor, resulting in a decrease in its CO selectivity against H 2. The CO and H 2 responses of other EC(N/SnO 2) sensors before and after the heat treatment in H 2 at 250 °C were also examined, and the effectiveness of the noble metal loading and the heat treatment on the CO-sensing properties was discussed in this study. In addition, gas-sensing mechanism was also proposed on the basis of chemical surface states of representative N/MO. [ABSTRACT FROM AUTHOR]

Abstract Metal oxide resistive-type nano-scale gas sensors have been investigated for their low cost, high sensitivity, and environmentally friendly fabrication. In these sensors, electrical resistance measurements are used to detect the presence of gas. In n-type metal oxides, resistance is increased by coverage of adsorbed oxygen and lowered by removal of adsorbed oxygen through reactions with reducing gasses. The sensitivity and selectivity of these sensors have been improved by incorporation of heterostructures. Heterostructures may improve sensor performance through facilitating catalytic activity, increasing adsorption, and creating a charge carrier depletion layer that produces a larger modulation in resistance. Synergistic effects in these gas sensors describe the improved sensor signal due to these combined effects which act to amplify the reception and transduction of the sensor signal. Receptive mechanisms may be improved by increasing adsorption and reactivity. Transduction mechanisms may be improved by restriction of the major charge conduction channels which helps to maximize resistance modulation. In this review, the synergistic effect achieved by combining these two mechanisms are examined. Fundamental properties of the metal oxide surface are used to provide insight for the large body of experimental evidence available for metal oxide resistive-type gas sensors. This review aims to connect experimental evidence to conceptual mechanistic descriptions by examining adsorption processes, charge transfer, reaction mechanisms, morphology, and ambient gas interactions. [ABSTRACT FROM AUTHOR]

Highlights • Gas sensor based on individual nanowire. • Electron Beam Lithography on top of non flat Surface. • Micro-sized suspended platforms with low consumption. Abstract A methodology based on the use of Electron Beam Lithography for contacting individual nanowires on top of non-flat micromembranes and microhotplates has been implemented, and the practical details have been exhaustively described. The different fabrication steps have been adapted to the substrate's topology, requiring specific holders and conditions. The methodology is demonstrated on individual SnO 2 nanowires, which, after fabrication, have been characterized as functional resistive gas nanosensors towards NH 3 and benchmarked against similar devices fabricated using more conventional Dual Beam Focused Ion Beam techniques, demonstrating the superior properties of the here presented methodology, which can be further extended to other non-conventional suspended substrates and nanomaterials. [ABSTRACT FROM AUTHOR]

Highlights • Nickel oxide polycrystalline nanowires are grown via hydrothermal way & calcination. • The nanosensor undergoes a thermal gradient, becoming a virtual array. • 7 hazardous gases are tested, giving each a different thermal fingerprint. • The nanosensor shows categorization (100%) and quantitative prediction (error <15%). Abstract Monitoring of hazardous gases is nowadays very important, since the urbanized environment is more subject to this kind of pollutants. Therefore, a capillary network of small gas sensors capable to check the quality of the environment is necessary. Metal oxide gas nanosensors are small economic devices that can be easily integrated in any context, however they unfortunately lack of selectivity. We present an approach using hydrothermally grown nickel oxide nanowires working at different temperatures and creating a virtual sensors array, thus exploiting the thermal fingerprints (sensor response as a function of temperature) of the gases. Using only one nanostructured material (nickel oxide) and different machine learning techniques, the system can easily discriminate any of 7 harmful gases (C 2 H 5 OH, H 2 , CO, LPG, CO 2 , NH 3 and H 2 S, all of them reducing gases) with an accuracy of 100%. Furthermore, the nanosensor also evaluates the gas concentration with an average error lower than 15%. Our results show that, exploiting thermal fingerprints from a temperature gradient, single metal oxide resistive nanosensors can efficiently discriminate specific hazardous gases. [ABSTRACT FROM AUTHOR]

High sensitivity of metal oxide gas sensors has increased the tendency to use them in various applications. The sensitivity of metal oxide sensors exposed to target gases is determined based on their electrical resistance. To determine the resistance, the resistance model of each grain is usually used and it is generalized to the whole sensor. Here, a new and simple model is used to extract the electrical resistance of a spherical metal oxide grain. Also, the effect of various factors on this resistance, such as grain size, surface potential and donor density is studied. Next, by examining the chemical reactions of the surface, the transient response of a spherical metal oxide grain to oxygen and hydrogen gases is extracted. The results of the gas-dependent resistivity model show that changing the structural and reaction parameters changes the electrical resistance of the metal oxide grain and thus changes the sensitivity of these sensors. • Presenting a gas-dependent resistivity model to obtain the electrical resistance of a spherical metal oxide grain using charge transfer kinetics • Investigating the grain size, surface potential and donor density on the electrical resistance of a spherical metal oxide grain • Extracting the transient response curve of a spherical metal oxide grain to gas pulses • Investigating the effect of changing structural and reaction parameters on the electrical resistance of metal oxide grains [ABSTRACT FROM AUTHOR]

The prevalence of nitrogen dioxide (NO 2) in the atmosphere is causing a steady increase in air pollution. As such, there is an urgent need to create a sensitive and energy-efficient NO 2 gas sensor to ensure environmental and health safety. In this study, we explore the effectiveness of nitrogen-doped zinc oxide nanowires in detecting harmful NO 2 gas. Through the hydrothermal method, we synthesized zinc oxide (NW-0) and nitrogen-doped zinc oxide nanowire sensors (NW-1, NW-2 & NW-4). The introduction of nitrogen induced changes in both the crystallite size and nanowire density in ZnO. Additionally, an increase in oxygen vacancy concentration was observed with nitrogen doping. At room temperature, the NW-2 sensor exhibited exceptional NO 2 sensing performance (response of 110) compared to NW-0 sensor (response of 1.9) for 10 ppm of gas. The NW-2 sensor also demonstrated a rapid response/recovery time of 50 s/5 s for a 10 ppm NO 2 concentration. Moreover, the sensor showed stability over an extended period, minimal response variation with changes in relative humidity, and selectivity towards NO 2 over other gases. Complementing the experimental findings, density functional theory (DFT) calculations were employed to estimate the adsorption energies of NO 2 on the sensor surface. • N-ZnO NW sensor has been fabricated using hydrothermal synthesis. • Nitrogen doped NWs shows superior sensing characteristics. • N-ZnO NW shows good selectivity towards NO 2. [ABSTRACT FROM AUTHOR]

Short-chain fatty acids (SCFAs) are key molecules, produced by gut bacteria in the intestine, that are absorbed into the bloodstream and strongly influence human health. SCFA disruption and imbalances have been linked to many diseases; however, SCFAs are seldom used diagnostically as their detection requires extensive sample preparation and expensive equipment. In this work, an electrochemical sensor was developed to enable real-time (requiring less than 2 min per measurement), quantitative measurement of SCFAs from complex samples in liquid phase without the need for extraction, evaporation, or destruction. An impedance-based sensor for in vitro detection of acetic acid, propionic acid, and butyric acid (accounting for more than 85% of SCFAs in the intestine) was fabricated by the deposition of ZnO and polyvinyl alcohol (PVA) on the surface of a microfabricated interdigitated gold electrode. The sensor was first exposed to a broad, physiologically relevant range of concentrations of SCFAs in isolation (0.5–20 mg/ml in electroanalytical cell), and, unlike previously published SCFA sensors that could detect only in gas form with the aid of evaporation, it was able to detect them directly in the liquid phase at room temperature. Subsequently, Electrochemical Impedance Spectroscopy (EIS) analysis was utilized in a complex medium, accompanied by Fe (CN) 6 3 − / 4 − . This was applied to a mixture of SCFAs, prepared in varying ratios and concentrations spanning from 0.5 to 10 mg/ml. The analysis successfully demonstrated the sensor's capacity to accurately measure SCFAs in a mixture of experimental relevance. The recorded faradaic responses were then used to train a fit-to-data model to screen human bacterial isolates and detect which species secrete SCFAs in vitro. This research paves the way for swift, non-destructive assessment of SCFA levels in complex biological samples. The end product is a miniaturized sensor that is highly stable and sensitive, making it ideal for real-time monitoring applications. [Display omitted] • A real-time detector of short chain fatty acids was developed using a synthesized ZnO-PVA-based microfluidic chemiresistor. • The hand-held sensor can detect SCFAs directly in the liquid phase at room temperature without evaporation or extraction. • The simultaneous detection of analytes in a mixture and in biological samples is shown to be as accurate as GC-MS. • The creation of this sensor opens the doors to real-time detection of bacterially produced molecules in the clinical setting. [ABSTRACT FROM AUTHOR]

Due to the excellent oxidation effect on phenolic compounds, the present copper-based laccase-mimics are mostly used for the degradation and detection of phenolic pollutants in environment. As the oxidation substrates of laccase are only limited to compounds containing specific phenolic hydroxyl groups, which makes the application of laccase mimetics to non-phenolic pollutants faces great challenges and bottlenecks. Herein, a non-copper-based laccase mimics like cubic Ag 2 O nanoparticles is prepared. The maximum reaction velocity and affinity of cubic Ag 2 O nanoparticles to substrates are significantly better than that of natural laccase and most artificial laccase mimetics. Cubic Ag 2 O nanoparticles also exhibits high stability under various conditions and has good tolerance to organic solvents. Moreover, it is found that the laccase-mimic activity of cubic Ag 2 O nanoparticles can be effectively inhibited by kanamycin, which enable us to construct a colorimetric sensor for rapid detection of kanamycin in environment. The sensor has excellent sensitivity with the limit of detection (LOD) of 1.3 nM for kanamycin, and has high selectivity for other competitive substances. Besides, a standard curve for rapidly detecting kanamycin by smartphones is established, which expect to provide a novel idea for on-site monitoring of other hazards in environment. [Display omitted] • Cubic Ag2O NPs exhibits high laccase catalytic stability under extreme conditions. • Laccase-mimic activity is used in a sensor for non-phenolic hazards analysis. • The inhibitory mechanism of kanamycin on laccase-mimic activity was revealed. • Smartphone-assisted sensor enables the result to be recognized by naked eye. • The colorimetric sensor has excellent sensitivity with a LOD as low as 1.3 nM. [ABSTRACT FROM AUTHOR]

In this review the state of the art and future prospects of 2-dimensional (2D) metal oxide nanoflakes used as active sensing elements for the detection of solutes, gases and radiation are discussed. 2D material geometries are particularly interesting for sensing applications because they provide large specific surface areas and are suitable for crystal facet engineering. In addition, unique material properties of atomically thin nanosheets due to quantum size effects provide engineering possibilities beyond the realm of their bulk counterparts. A variety of possibilities in materials, synthesis routes, (hierarchical) sensor architectures and application areas is sketched. The discussion is focused on high-performing sensors and innovative concepts. The scope is limited to nanoflakes with a thickness of up to 50 nm. Special attention is given to sensing based on material properties that are unique to atomically thin nanosheets. [ABSTRACT FROM AUTHOR]

An unconventional method is developed to fabricate flexible and transparent sensors for real-time, wireless sensing of alcohol vapors using hybrid nanostructures of indium oxide and Pt nanoparticles (as an active channel) with random networks of metal nanowires (as electrodes and antennas). The hybrid structures of indium oxide and Pt nanoparticles present high response and selectivity for ethanol vapor sensing with detecting the blood alcohol concentration range corresponding to the license suspension or revocation in the Road Traffic Act of many countries (blood alcohol concentration 200 ppm). The integration of a Bluetooth system or an inductive antenna enables wireless operations of the alcohol sensor using smartphones for applications as wearable and hands-free devices with flexible, transparent film geometries. Furthermore, these sensor systems exhibit outstanding thermal reliabilities for their stable operations over wide temperature ranges between −40 °C and 125 °C, which can extend their practical use for automobile electronics. Such devices can be transferable onto diverse nonplanar surfaces including steering wheels and curved glasses of phones, which suggests substantial promise for their applications in next-generation automobile or wearable electronics. [ABSTRACT FROM AUTHOR]

Nowadays the detection of unwanted volatile compounds in air is increasingly important in a wide range of fields. Metal oxide nanosensors are extremely small and cheap devices that could be integrated in any application, but their single resistance response make them non-selective. For this reason, sensor arrays are used where pollutant recognition is needed. Unfortunately, these electronic noses, consisting of different active materials, are complex and expensive. Here, we present a simple visual nanosensor that can detect and recognize selectively several volatile compounds at a relatively low temperature (200–300 °C). The dynamic resistance of a conductometric NiO nanosensor is simply transformed in a visual output that allows to recognize different gases and their concentration with a quick look. This way, one single nanostructured metal oxide can act as a sensitive and selective electronic nose, using the powerful post-processing given by the human eyes and brain. The sensor has proven to discriminate 8 different gases diluted in air (1 oxidizing and 7 reducing) and their respective concentration. [ABSTRACT FROM AUTHOR]

Noble metal-functionalized, reduced graphene oxide (rGO)-loaded metal oxides are a new class of ternary composites that combine the advantages of each component, resulting in exceptional materials. But, there are few reports on their use as gas sensors. This paper reports the gas sensing behavior of Au or Pd-functionalized rGO-loaded ZnO nanofibers (NFs) synthesized by using a combination of facile, cost-effective sol-gel and electrospinning methods. An examination of the gas sensing properties revealed that Au-functionalized NFs have a very high response to CO gas. In particular, the gas response (R a /R g ) to 1 ppm of CO was as high as 23.5, whereas Pd-functionalized NFs showed a high response to C 6 H 6 gas (11.8 to 1 ppm C 6 H 6 ). The presence of rGO/ZnO heterointerfaces, the catalytic effect of Au and Pd nanoparticles (NPs), and the high surface area of NFs were the main factors that contributed to the strong response of the Au or Pd-functionalized rGO-loaded ZnO NFs sensors. These results show that the combination of noble metals, such as Au or Pd NPs, with rGO and ZnO can impart new gas sensing functionality that is potentially useful for CO or C 6 H 6 sensing applications, respectively. [ABSTRACT FROM AUTHOR]

Nano-helical array of p -NiO/n-SnO 2 p - n junctions with well-defined and highly-gas-accessible hetero-interfaces is presented for designable and highly selective gas sensors. The gas sensor having the nanoscale p - n junction sensing layer with a top-and-bottom electrodes configuration was fabricated by conventional photolithography and oblique-angle deposition. The unique device structure enables a predominant modulation in barrier height at the hetero-interfaces, consistent with simulation results, resulting in unusual-yet-promising sensing behaviors upon H 2 and NO 2 exposure. Based on our experimental and simulation results, and the ability to fabricate a variety combination of metal oxides heterostructures with a reproducible and controllable way, we believe that it becomes possible to design and realize highly-selective sensor array electronic-noses optimized towards target gases on demand. [ABSTRACT FROM AUTHOR]

In the present study, a simple hydrothermal approach has been successfully applied for a large scale synthesis of anatase titanium dioxide nanoparticles (TiO 2 NPs) using titanium glycolate precursors and is utilized for the fabrication of low-cost high performance acetone (CH 3 COCH 3 ) gas sensors after corroborating the crystallinity, phase-purity, and surface morphology investigations. Several randomly distributed TiO 2 aggregates, composed of NPs, are noticed from morphology analysis. Chemiresistive properties of as-fabricated TiO 2 sensors attempted towards host of oxidizing and reducing gases, reveal a superior selectivity to CH 3 COCH 3 with a maximum response of 15.24 (1000 ppm) @270 °C compared to other target gases. One of the key features of as-fabricated TiO 2 sensor is the lowest detection limit of 500 ppb to CH 3 COCH 3 with rapid response and recovery times, signifying commercial potential of the developed sensor materials. The effect of operating temperature along with various concentrations of CH 3 COCH 3 on the gas sensing properties of TiO 2 sensor has thoroughly been investigated and reported. Finally, the interaction mechanism between the CH 3 COCH 3 molecules and the TiO 2 NPs sensor was elaborated in depth for a thorough understanding sensor performance experimentally and supposedly. [ABSTRACT FROM AUTHOR]

In the present work, flame-spray-made CeO x -doped SnO 2 nanoparticles with 0.1–1 wt% Ce contents were systematically studied for ethanol detection. Structural characterizations by electron microscopy, Nitrogen adsorption and X-ray analysis indicated that SnO 2 nanoparticles were highly crystalline with tetragonal structure and CeO x crystallites with mixed Ce 3+ and Ce 4+ oxidation states should form a solid solution with SnO 2 matrix. The sensing films were tested towards 3–200 ppm ethanol at operating temperatures ranging from 200 to 400 °C in dry air. Gas-sensing results demonstrated that the SnO 2 sensing film with the optimal Ce content of 0.5 wt% exhibited a very high response of ∼2654–200 ppm ethanol with a short response time of 1.1 s at the optimal operating temperature of 350 °C. Moreover, the optimal sensor displayed high ethanol selectivity against C 3 H 6 O, CH 4 , H 2 , NO 2 , H 2 S and H 2 O. Therefore, the flame-made CeO x -doped SnO 2 sensor is a promising candidate as a sensitive and selective ethanol detector for drunk-driving and biomedical applications. [ABSTRACT FROM AUTHOR]

In this study, we have reported the fabrication of p-NiO/n-SnO 2 heterojunction based formaldehyde (HCHO) sensor by decorating the surface of pre-deposited SnO 2 thin film with laser ablated NiO nanoparticles (NPs). In atmospheric air, NiO NPs were produced by a moving laser beam on the surface of pure (99.99%) Ni pellet to decorate sputtered deposited SnO 2 thin film directly to form NiO/SnO 2 p-n heterojunction. After fabrication, gas sensing properties of NiO/SnO 2 sensor were investigated systemically towards HCHO and it exhibited higher response (R a /R g) of about 31.04 towards 50 ppm HCHO at 210 °C with good selectivity compared with pristine SnO 2 sensor. Moreover, adsorption (K a) and desorption rate constant (K d), response time (τ res), recovery time (τ rec) and surface coverage (θ) of NiO/SnO 2 sensor were extracted from experimental data using Langmuir gas adsorption-desorption model via curve fitting method and the models demonstrated the irreversible type of gas sensing behaviour towards HCHO. The experimental results revealed that the laser ablation method has a great potential to use as alternative chemical free surface decoration route to fabricate metal oxide heterojunction based sensors for the detection of formaldehyde or other toxic gases. [Display omitted] • The NiO nanoparticles were synthesized in air by laser ablation technique. • The p-NiO/n-SnO 2 heterojunction has been formed by direct deposition of NiO nanoparticles on SnO 2 thin-film. • The NiO/SnO 2 sensor shows fast and high response to formaldehyde at 210 °C with good selectivity. • Langmuir adsorption-desorption model and curve fitting were used to extract gas sensing parameters. [ABSTRACT FROM AUTHOR]

We investigated a discriminant method for volatile organic compounds (VOCs) in exhaled breath under contaminant air as well as purified air conditions using an array of multiple semiconductive gas sensors and the linear discriminant analysis (LDA) space. We analyzed the sensor signals of resistance changes employing our developed discriminant method in the LDA space. In particular, the sensor resistances at intervals at which a resistance change occurred owing to target gas introduction were used as test data. The linear discriminant (LD) scores from the test data in the two-dimensional (2-D) LDA space gradually shifted in the direction of the correct LD scores of the training data. Therefore, the target gases were discriminated using the angle of locus of the test data in the 2-D LDA space. Our results proved the viability of the proposed method to discriminate target gases under contaminant air conditions. • Multiple semiconductive gas sensors using linear discriminant analysis (LDA). • Using LD scores of sensor signals for discrimination. • The rapid method to discriminate the target gas. • Discrimination from the angle of the locus of test data on LDA scores. [ABSTRACT FROM AUTHOR]

The triethylamine (TEA) sensors with high response and selective detection have been widely reported. However, sluggish kinetics of recovery process, high operating temperature, unsatisfied long-term stability and high detection limit still restrict their further application. In this work, NiO nanofibers with different contents of Sn2+ are prepared via electrospinning method. Gas sensing investigation indicates that the sensor based on 6 at% Sn2+ doped NiO nanofibers with an elevated baseline resistance exhibits the highest gas response and excellent recovery characteristic to TEA at 200 °C. The sensor shows a low detection limit at ppb level and good long-term stability within three months. In addition, the sensor only has a small variation on baseline resistance, gas response and response/recovery speed under different relative humidity. The promising gas sensing performance can be mainly attributed to the increase of the oxygen vacancies defects caused by the difference of Sn2+ and Ni2+, resistance changes and the hollow structure of as-prepared nanofibers. • Sn2+ doped NiO nanofibers are prepared by a facile electrospinning method. • Isovalent Sn2+ doping can obviously boost the oxygen vacancy of NiO nanofiber. • The sensor of 6 at% Sn2+ shows an enhanced response and humidity tolerance to TEA. [ABSTRACT FROM AUTHOR]

Monitoring emission levels of combustion related gases such as H 2 , CO and NO 2 is beneficial both to optimize performance of combustion systems as well as to reduce the impacts of the target gases on the environment. Although considerable work has been done with regards to high temperature sensing in plasmonics based sensor research, the sensing mechanisms have not been fully developed. Another area that needs further investigative studies is in the optimization of material parameters that give the optimal sensing performance to tested gases. In this work nanorods encapsulated with a metal oxide of yttria-stabilized zirconia (YSZ) patterned through electron beam lithography are shown to be sensitive to the tested gases, and mechanisms pertinent to the sensing reactions have been proposed. The optimal thicknesses of fabricated films have been identified, which will enable predictive design of such nanocomposite films for applications requiring compatibility to high temperatures and harsh environments. [ABSTRACT FROM AUTHOR]

An attempt has been made in the present work to develop a hybrid nanocomposite sensor by exploiting the novel properties of Tin oxide (SnO 2 ), MWCNTs (Carbon nanotubes) and reduced graphene oxide (RGO) for the efficient detection of SO 2 gas. MWCNT-SnO 2 and RGO-SnO 2 hybrid nanocomposite sensors have been prepared by incorporating MWCNTs and RGO into SnO 2 nanoparticle colloidal solution respectively by chemical route. Low sensing response of 1.2 is obtained for bare SnO 2 sensor at 220 °C towards 500 ppm SO 2 gas. However, enhanced sensing response of ∼22 has been observed for the RGO-SnO 2 sensor compared to ∼5 obtained for MWCNT-SnO 2 sensor at a lower operating temperature of 60 °C towards the same concentration of SO 2 gas. Modulation of space charge region at the interface of n-SnO 2 and p-RGO on interaction with the target SO 2 gas is responsible for the enhanced response. [ABSTRACT FROM AUTHOR]

Present work focuses on the growth of SnO 2 thin films with high electrical resistivity using RF sputtering technique for fabrication of SO 2 gas sensors. NiO catalyst has been incorporated over the surface of SnO 2 thin film in the form of continuous layer and dotted nanoclusters to improve the response characteristics. Enhanced sensing response of about 56 has been observed for the NiO dotted cluster/SnO 2 sensor at a relatively lower operating temperature of 180 °C towards 500 ppm SO 2 gas. The contribution of both (i) modulation of depletion region at the interface of n type SnO 2 film and p type NiO catalyst on interaction with the target SO 2 gas and (ii) the spill over mechanism are considered to be the main reasons for the obtained enhanced response. [ABSTRACT FROM AUTHOR]

Metal oxide (MOX) has been used as resistive gas sensors for a long time. The gas sensitive response under constant temperature measurement is a broad-spectrum response, resulting in poor selectivity. To solve this problem and enhance the selectivity of a single sensing film, gas sensor based on Pt-modified zinc oxide was prepared by homogeneous precipitation and screen printing method. Afterwards, the temperature & light modulation method is first proposed herein. This method allows us to obtain different Resistance-Temperature (R-T) curves and characteristic parameters, photoresponse and photorelaxation. Combined with pattern recognition method, five tested gases, including ethanol, methanol, acetone, formic acid and ethyl ether, can be successfully classified. Compared with temperature modulation, which is usually used as a method to enhance selectivity, temperature & light modulation method can increase the classification rate from 95.55% to 100% for sensing these five tested gases. The results indicate that the temperature & light modulation method is an effective method for improving the selectivity of the gas sensor. [ABSTRACT FROM AUTHOR]

We report here on the results of a study into the response of a tungsten oxide based low power MEMS gas sensor to ppb of nitrogen dioxide at low levels of ambient oxygen. It was found that the resistive gas sensors not only had a high sensitivity to NO 2 (3.4%/ppb vs. 0.2%/ppb obtained for commercial MOX) but can still operate reliably at lower oxygen levels (down to 0.5%) - albeit with slightly longer response and recovery times. The optimal operating temperature was determined to be ca. 350 °C and so easily within the range of a MEMS based SOI CMOS substrate. The response was sensitive to significant changes in ambient humidity, but was found to have low cross-sensitivity to CO, hydrogen, methane, and acetone even at much higher ppm levels. We believe that these tungsten oxide gas sensors could be exploited in harsh applications, i.e. with a low oxygen (lean) environment often associated in the exhaust gases from combustion systems. [ABSTRACT FROM AUTHOR]

A microwave system dedicated to the detection of nitrogen dioxide in the harsh environment of the Norway highways is proposed. An optimized transmission line type of sensor coated with a tungsten trioxide thin film that changes its electrical properties under NO 2 gas exposure is developed. The sensors' response (S) is given in °/GHz and it is calculated based on wideband measurements. The advantage of wideband measurements in comparison to a single value is based on multiple measurements taken at different frequencies, which greatly suppresses noise and enables measuring low target-gas concentrations within environments of high interfering compounds. Herein, the developed system works in 1.5 GHz – 4.5 GHz, and NO 2 varies in the 0–20 ppm range. The optimal thickness of the gas-sensing layer is estimated to be around 410 nm taking the advantage of the magnetron sputtering technology with the glancing angle deposition technique. The advantage of the developed sensor is the possibility to work at ambient temperature without the need to heat up the sensors. The disadvantage of the developed sensors is longer response and recovery times; however, this issue will be a subject of research in the future. [ABSTRACT FROM AUTHOR]

A microwave system dedicated to the detection of nitrogen dioxide in the harsh environment of the Norway highways is proposed. An optimized transmission line type of sensor coated with a tungsten trioxide thin film that changes its electrical properties under NO 2 gas exposure is developed. The sensors' response (S) is given in °/GHz and it is calculated based on wideband measurements. The advantage of wideband measurements in comparison to a single value is based on multiple measurements taken at different frequencies, which greatly suppresses noise and enables measuring low target-gas concentrations within environments of high interfering compounds. Herein, the developed system works in 1.5 GHz – 4.5 GHz, and NO 2 varies in the 0–20 ppm range. The optimal thickness of the gas-sensing layer is estimated to be around 410 nm taking the advantage of the magnetron sputtering technology with the glancing angle deposition technique. The advantage of the developed sensor is the possibility to work at ambient temperature without the need to heat up the sensors. The disadvantage of the developed sensors is longer response and recovery times; however, this issue will be a subject of research in the future. [ABSTRACT FROM AUTHOR]

Nanosecond and picosecond pulsed laser deposition (PLD) was used to prepare metal oxide nanostructures with different morphologies as gas sensing materials on top of oxidized silicon substrates and commercial SGX Sensortech SA MEMS microheater platforms. The layers were formed of different types of nanostructures including nanoparticles, agglomerates, and nanotrees with fractal-like growth. Clear dependencies between the deposition parameters, structural morphology, and gas sensing performance were found. Also, some differences in the morphologies of the layers were seen when picosecond PLD was used instead of nanosecond PLD. Many of the sensing materials were found to be highly sensitive to different types of gaseous species. We investigated inorganic gases in the ppm range (2–400 ppm) including NO, CO, and NH 3 , and the selectivity and sensitivity were shown to be dependent, not only on layer morphology, but also on the measurement temperature. Moreover, an investigation with volatile organic compound gases in the ppb range demonstrated that WO 3 layers are highly sensitive and selective towards naphthalene at least down to 2.5 ppb. [ABSTRACT FROM AUTHOR]

The manner how nano building blocks assemble into hierarchical architectures exerts a tremendous influence on gas-sensing performance of the metal oxides. Here, we focus on tuning the 2D SnO 2 nanosheets into 3D hierarchical nanoflowers by manipulating the presence of NaOH, and investigate their gas-sensing functionalities. We find that the blooming SnO 2 nanoflowers assembled by ultrathin nanosheets (∼50 nm) are shrunk into semi-blooming state, and that the semi-blooming nanoflowers based sensor shows enhanced gas-sensing performance towards the ethanol, which is attributed mainly to the confined effect due to numerous nano or micro reaction rooms by keeping oxygen and ethanol molecules to complete gas-sensing reactions. While the semi-blooming nanoflowers turn into ordered mesoporous via thermally removing the periodically arranged polyvinyl pyrrolidone micelles, their gas-sensing performance is found to be improved dramatically, indicating that sufficient amount of gas diffusion is crucial to gas-sensing properties rather than the fast gas diffusion speed. As a final verification, we fabricate the sensors using the mesoporous semi-blooming SnO 2 nanoflowers and successfully monitor the existence of beer by a simple integrated device, making it a promising candidate in detecting drunk driving. [ABSTRACT FROM AUTHOR]

Sulfur dioxide (SO 2 ) is one of the most toxic and hazardous gas having adverse effect on the health and environment. In the present report, an attempt is made to fabricate SO 2 gas sensor using SnO 2 thin film integrated with various metal oxide catalyst (PdO, CuO, NiO, MgO, V 2 O 5 ) in the form of 10 nm thin dotted clusters of 600 μm diameter. The structural, optical and surface morphological properties of the SnO 2 thin film have been studied. The bare SnO 2 thin film sensor exhibits the sensing response of about 1.3 at 220 °C whereas, NiO/SnO 2 sensor shows the enhanced sensing response (∼56) towards 500 ppm SO 2 gas at a relatively low operating temperature (180 °C). A correlation has been established between the observed sensing response and the properties of the sensing film. Selectivity of the NiO/SnO 2 sensor was tested towards different interfering gases and found to be highly selective for SO 2 gas. [ABSTRACT FROM AUTHOR]

Quasi-one-dimensional (1D) heterostructures act as promising gas-sensing materials due to their high surface-to-volume ratio, large depletion area, and synergistic behaviors of the two components that contribute to dramatic change in resistance. To date, the noble metals and p-type metal oxides, a component of the heterostructure systems, have been extensively applied as good catalysts owing to their excellent selectivity and sensitivity to analyte gases. In particular, much effort has been devoted to the fabrication and processing of nanoscale quasi-1D metal-oxide-based heterostructural materials for a variety of practical applications especially as gas sensors. In this review, we first introduce the two-step preparation process of various quasi-1D nanostructures and then discuss how the primary parameters, e.g. the amount of activated adsorption sites, the gas diffusion ability, and the coverage the secondary materials, can have an impact on the gas-sensing performances. We finally review gas-sensing mechanism in terms of the species of heterojunctions, such as the Schottky-junction, p – n or n – p junction, and n – n or p – p junction. [ABSTRACT FROM AUTHOR]

Advanced sensors are needed for development of next-generation fossil fuel power generation technologies and for enhancing efficiencies of existing power generation systems. Optical waveguide-based sensing technologies have become increasingly important for harsh environment energy applications. In this manuscript, we present sensing results for fiber-optic evanescent wave hydrogen sensors employing La-doped SrTiO 3 layers as the active sensing element. These sensors show a rapid, reproducible sensing response to hydrogen fuel gas streams at elevated temperatures (600–800 °C). The presence of hydrogen results in a reversible and reproducible decrease in near-infrared transmission through the sensor. Sensors were also tested directly in the anode assembly of an operating solid oxide fuel cell (SOFC) with the sensor response correlating with both H 2 concentration and SOFC cell potential. [ABSTRACT FROM AUTHOR]

Heterojunction based metal oxide semiconductors have a great potential to detect toxic gases and volatile organic compounds. Here, we report a synthesis technique to obtain ZnO nanoparticles (NPs) via laser ablation method in ambient atmosphere using a nanosecond pulse laser. The laser ablated ZnO NPs were used for the decoration of sputtered deposited SnO 2 thin film to realize n-type ZnO/n-type SnO 2 heterojunction based formaldehyde (HCHO) gas sensor. Fabricated ZnO/SnO 2 sensor shows 3 times higher response (response = 20) to 50 ppm HCHO, faster response-recovery dynamics (τ res = 4 s and τ rec = 30 s) and better selectivity compared to pristine SnO 2 sensor at lower operating temperature (250 °C). The ZnO/SnO 2 heterojunction based sensor also can detect < 0.25 ppm HCHO, which allows for lower detectable limit of sub-ppm HCHO. Therefore, the ZnO/SnO 2 sensor shows enhanced sensing performance due to oxygen vacancies and n-n heterojunction structures provides more effective electrons for oxygen adsorption which speeds up the electronic transport at the interface of two materials. Synthesis of metal oxide composite NPs using pulse laser ablation method could be a promising alternative rapid route for the fabrication of highly sensitive gas sensors to detect most common indoor air pollutant. • Fabrication of highly selective and ultrasensitive n-SnO 2 /n-ZnO heterojunction-based formaldehyde gas sensor. • ZnO nanoparticles were synthesized using rapid and cost-effective laser ablation techniques in air. • The sensor exhibits high gas sensing response at 200 °C with ultra-low detection limit of 0.25 ppm. • SnO 2 /ZnO sensor shows better sensitivity and selectivity of as compared to pristine SnO 2. [ABSTRACT FROM AUTHOR]

Porous film of semiconducting metal oxides provides large surface area for efficient gas-sold interactions. Nanocasting using solid templates such as closely packed colloidal crystals represents a straightforward strategy for creating ordered porous materials, yet the preparation of templates typically involves a complex, time-consuming process of self-assembly. In this study, on-chip preparation of SnO 2 nanoarrays with porous net-like structure has been achieved by employing in-situ grown NiO nanoarrays as the template. In contrast to the poor stability of the response of NiO nanoarrays to H 2 S, the as-prepared SnO 2 nanoarrays exhibit excellent selectivity, stability and capability of responding linearly to H 2 S in a broad concentration range (0.4–80 ppm). Due to the free access of gas molecules to the porous net-like microstructure, SnO 2 nanoarrays are found to possess much higher and faster H 2 S response as compared to the dense film counterpart. A portable sensor device based on SnO 2 nanoarrays has also been fabricated, which demonstrates the practical H 2 S detection with high accuracy. [Display omitted] • Chemical bath deposition of NiO nanosheet arrays. • NiO-templated preparation of net-like SnO2 nanoarrays. • Linear response to H2S in a broad concentration range (0.4–80 ppm). • A portable sensor device fabricated for practical H2S analysis. [ABSTRACT FROM AUTHOR]

Fluorine-doped ZnO (FZO) thin films were fabricated on glass substrates using a chemical spray pyrolysis method for NO 2 gas detection. The physical properties of the films were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet–visible (UV–Vis) spectrometry as a function of fluorine concentration (2 and 5 at%). The films exhibited a hexagonal wurtzite structure, and the inclusion of fluorine did not significantly influence the morphology of the films. Optical measurements showed that the band gap energies of the films were affected by the fluorine concentration. Furthermore, the NO 2 gas-sensing properties of pristine and FZO thin films were investigated in the concentration range of 100–1000 ppb. Fluorine doping in ZnO was found to improve the NO 2 -sensing capability at the least. All gas sensor measurements showed an optimal sensor response at a temperature of 200 °C. In addition, the 2% FZO was found to be more suitable candidate for detecting NO 2 gas. [ABSTRACT FROM AUTHOR]

Metal oxide type (MOx) sensors are one of several technologies being employed in low-cost air quality monitors. Their size and cost make them ideally suited for portable and remote monitoring applications. Quantifying the sensor response, however, remains a significant challenge due to sensitivity to ambient temperature and humidity, and interference with non-target pollutant species. Temporal characteristics such as signal hysteresis and sensor drift over time also affect MOx sensor response. We present a quantification model rooted in principles of semiconductor science and chemistry, yet informed by and simplified using empirical observations. This model predicts concentration from resistance while providing a correction for temperature effects, usually the most significant confounder in ambient air quality monitoring with MOx sensors, and drift due to change in the sensor heating element. The model parameter values are first determined using lab calibrations, then refined using a field collocation with reference instruments. The work thus provides both a quantification model and a means to effectively calibrate sensors for practical applications. [ABSTRACT FROM AUTHOR]

Metal oxide-based resistive-type gas sensors are solid-state devices which are widely used in a number of applications from health and safety to energy efficiency and emission control. Nanomaterials such as nanowires, nanorods, and nanoparticles have dominated the research focus in this field due to their large number of surface sites facilitating surface reactions. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can have drastic effects on gas sensor performance, especially the selectivity. Recently, these effects have been amplified by designing heterojunctions on the nano-scale. These designs have evolved from mixed commercial powders and bi-layer films to finely-tuned core–shell and hierarchical brush-like nanocomposites. This review details the various morphological classes currently available for nanostructured metal-oxide based heterojunctions and then presents the dominant electronic and chemical mechanisms that influence the performance of these materials as resistive-type gas sensors. Mechanisms explored include p–n and n–n potential barrier manipulation, n–p–n response type inversions, spill-over effects, synergistic catalytic behavior, and microstructure enhancement. Tables are presented summarizing these works specifically for SnO 2 , ZnO, TiO 2 , In 2 O 3 , Fe 2 O 3 , MoO 3 , Co 3 O 4 , and CdO-based nanocomposites. Recent developments are highlighted and likely future trends are explored. [ABSTRACT FROM AUTHOR]

The objective of this paper is to apply a closed-loop control based on dielectric excitation to MOX gas sensors in order to improve their response time. The control implements a feedback loop in which temperature modulations keep constant the sensor reactance, measured at constant temperature. The required fast temperature switching has been implemented on MEMS microhotplates. The mean temperature generated by the control is the new output signal. This technique is applied to an in-house sensor made of WO 3 nanowires decorated with gold nanoparticles to detect NH 3 and to a commercial MEMS MOX sensor (CCS801). [ABSTRACT FROM AUTHOR]

Au-nanoparticle incorporated metal oxide based sensing layers show significant promise for high temperature optical sensing applications at temperatures approaching 800 °C or even higher depending upon the base oxide material. Several Au-nanoparticle incorporated oxide systems were synthesized and investigated here, namely TiO2, ZrO2, and Yttria-Stabilized Zirconia (YSZ). Gas (CO, H2, and O2) and temperature sensing responses were observed at wavelengths near the localized surface plasmon resonance (LSPR) absorption peak of the Au nanoparticles and addition of 1% O2 content to a N2 baseline gas stream resulted in significantly enhanced recovery kinetics for H2 sensing. TiO2 films with a relatively small bandgap as compared to ZrO2 and YSZ enabled band-edge monitoring yielding a strong temperature sensing response with minimal cross-correlation to changes in gas composition. Testing of the films in high H2-level gas streams demonstrated that monotonic responses to H2 up to 98% H2 by volume in 2% O2 balance N2 gas streams could be achieved by interrogation at wavelengths shorter than the transmittance minimum associated with the Au LSPR absorption peak. These results collectively demonstrate the importance of careful wavelength selection or broadband wavelength interrogation to minimize cross-correlation between composition and temperature and to optimize the gas sensing response in high temperature gas streams. Although the tested films were stable in the presence of simple gas mixtures (N2, H2, CO, O2) used for gas and temperature sensing experiments, a preliminary study of film stability in a contaminated (H2S-containing) high temperature fuel gas stream relevant for solid oxide fuel cell applications was also carried out and yielded two important conclusions deserving further investigation: (1) enhanced microstructural stability of Au nanoparticle incorporated TiO2 due to grain boundary pinning and (2) significant mass loss of Au with an associated reduction in LSPR absorption. [ABSTRACT FROM AUTHOR]

Abstract: An Electronic nose (E-nose) made up of an array of metal oxide thin film based SAW sensors is proposed as a detection system for the binary mixture of DMMP (simulant of nerve agent) and methanol at room temperature. The SAW devices are one port surface acoustic wave (SAW) resonators having center frequency of 433.9MHz. The E-nose sensor array has SAW sensors coated with four different chemical sensitive coatings of ZnO, TeO2, SnO2 and TiO2 in order to recognize the individual components in a binary mixture of DMMP (in ppb levels) and methanol (in sub ppm levels). Principal Component Analysis (PCA) as a data pre-processing technique and Artificial Neural Network (ANN) as a pattern classification technique have been applied for obtaining a clear discrimination and correct classification. [Copyright &y& Elsevier]

We report a novel chemical sensing array based on metal oxide nanoparticles as a portable and inexpensive paper-based colorimetric method for polyphenol detection and field characterization of antioxidant containing samples. Multiple metal oxide nanoparticles with various polyphenol binding properties were used as active sensing materials to develop the sensor array and establish a database of polyphenol standards that include epigallocatechin gallate, gallic acid, resveratrol, and Trolox among others. Unique charge-transfer complexes are formed between each polyphenol and each metal oxide on the surface of individual sensors in the array, creating distinct optically detectable signals which have been quantified and logged into a reference database for polyphenol identification. The field-portable Pantone/X-Rite© CapSure® color reader was used to create this database and to facilitate rapid colorimetric analysis. The use of multiple metal-oxide sensors allows for cross-validation of results and increases accuracy of analysis. The database has enabled successful identification and quantification of antioxidant constituents within real botanical extractions including green tea. Formation of charge-transfer complexes is also correlated with antioxidant activity exhibiting electron transfer capabilities of each polyphenol. The antioxidant activity of each sample was calculated and validated against the oxygen radical absorbance capacity (ORAC) assay showing good comparability. The results indicate that this method can be successfully used for a more comprehensive analysis of antioxidant containing samples as compared to conventional methods. This technology can greatly simplify investigations into plant phenolics and make possible the on-site determination of antioxidant composition and activity in remote locations. [ABSTRACT FROM AUTHOR]

NO 2 is an oxidizing gas, which is considered to be the most dangerous and most toxic gas that should be controlled. Because of the advantages of high sensitivity, selectivity, stability and fast response/recovery speed, the chemical resistance sensor is used as a NO 2 gas sensor. This article reviews the NO 2 gas sensor based on metal oxides. Firstly, the hazards of NO 2 and the major disadvantages of chemiresistive sensors based on metal oxides were discussed. Then, different sensitive materials of chemical resistance sensors were discussed, including metal oxides (MO), conductive polymers and carbon-based materials. Finally, the NO 2 gas sensors based on metal oxide (including ZnO, NiO, CuO, SnO 2 , WO 3) were discussed from the aspects of morphology, preparation method, modification, doping and composite. [ABSTRACT FROM AUTHOR]

While electronic noses have been around for over 30 years, little effort has been devoted to the development of transferable calibration models, which are models that can be applied to multiple equivalent devices without adjustment. The majority of published results limit itself to data sets gathered with a single device. This lack of insight in transferable models hampers large scale implementations of eNose-based applications as individual calibration of a multitude of devices for a specific application area is generally unrealistic due to the requirement of actual samples to be measured. For simple gases this may be do-able, but in the case of more complex samples such as biological patient material it is logistically impossible. In this paper we show the influence of the deviation of the sensor temperature on the measurement reproducibility and by inference on the transferability of calibration models. We introduce the total inertia (φ 2) as a measure for the heterogeneity within the measured data. The total inertia is an objective measure known from linear algebra, where it is used to calculate the correspondence between matrices. We use 5 micro-hotplate metal-oxide sensors from the same wafer, with an inter-sensor heater temperature difference of approximately 15°C in combination with 2 substances, n-butyl-acetate and hexane. This research demonstrates the increase in heterogeneity of the measured response values in relation to a temperature shift. A shift of 15°C at the sensor surface causes an increase of heterogeneity that is 10–15 times higher than the increase in heterogeneity caused by inter-sensor responses to the substances when operated at exactly the same temperature. Some mixtures of substances will be separable by pattern recognition under virtually any condition, and strict temperature control will neither improve nor deteriorate results. However the significant contribution of temperature deviation toward data heterogeneity renders it plausible that optimized temperature control, and by inference lower data heterogeneity, is a prerequisite for transferability of a calibration model. This holds true when applied to metal-oxide sensors and for mixtures containing substances showing a fair degree of similarity. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, SAW sensors coated with various metal oxides as sensitive layer have been exploited for distinct detection of ammonia. Thin films of different metal oxides (ZnO, SnO2, TeO2 and TiO2) of same thickness (40nm) were deposited under optimized parameters using rf sputtering technique for selective detection of liquor ammonia. As deposited ZnO thin film was highly c-axis oriented, whereas SnO2 thin film was polycrystalline. However the coatings of TiO2 and TeO2 thin films were amorphous. SAW sensor coated with ZnO film (ZnO/SAW) was found to be highly sensitive towards liquor ammonia as compared to other metal oxide coated SAW sensors. Effect of the thickness of ZnO sensing layer (20–80nm) has been studied on the sensing response characteristics. Mass loading and elastic loading are identified as the major contributions towards the response of SAW sensor for ammonia and their contributions were evaluated by fitting with the theory. [Copyright &y& Elsevier]

The emerging hydrogen economy has created a demand for the development of improved hydrogen sensors operating at room temperature. In this work, we present hydrogen detectors based on metal decorated WO3 nanowires that were able to detect 1000ppm of H2, even at room temperature (30°C), with relatively short recovery time and high sensitivity. The nanowires were synthesized by a hydrothermal process and decorated with PdO and PtO x nanoparticles by decomposition of Pd(acac)2 and Pt(acac)2 precursors. The gas responses were tested for H2, NO, H2S and CO analyte gases in an air buffer at 150, 200 and 250°C (H2 also at 30, 70 and 130°C). [ABSTRACT FROM AUTHOR]