Gas Sensor Devices Research Articles

Enhanced NO2 gas sensor device based on supramolecularly assembled polyaniline/silver oxide/graphene oxide composites

Herein, we report a successful synthesis of supramolecularly assembled polyaniline/silver oxide/graphene oxide composite (PANI/Ag2O/GO) for enhanced NO2 gas sensing application. The PANI/Ag2O/GO composite was synthesized by facile stirring followed by an ultrasonication process. The prepared material was characterized by different techniques such as x-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy and Raman-scattering spectroscopy. The detailed analysis revealed that the average crystallite sizes of PANI/Ag2O and PANI/Ag2O/GO composites were found to be 37.37 nm and 41.55 nm, respectively. FESEM and TEM analysis showed coral-like rough-surfaced and extensively agglomerated morphology for PANI and ultrathin flexible sheet-like morphology for GO. Ag2O nanoparticles with diameters 20–30 nm were well incorporated in the GO sheets and PANI matrix in the case of PANI/Ag2O/GO composites. The synthesized materials were used to make resistive sensor devices that had a high response to NO2 gas. The fabricated sensors were examined at various temperatures to obtain the optimal sensing temperature. The fabricated NO2 gas sensor device based on PANI/Ag2O/GO composite exhibited a highest sensitivity of 5.85 for 25 ppm at an optimized temperature (100 °C) as compared to the pure PANI (2.5) and PANI/Ag2O composite (3.25). Further, the fabricated sensor device based on PANI/Ag2O/GO composite was also examined at different NO2 gas concentrations.

Controllable Transfer of MOx Nanowires on Microhotplate-Based Gas Sensing Devices

Introduction SnO2 nanowires (NWs) have been synthesized and implemented on CMOS-based gas sensors devices with gold inter-digital electrode structures (IDES). For the implementation process of NWs a polydimethylsiloxane-based stamping transfer was employed with help of a Condor Sigma bond tester. The multi-nanowire gas sensor has demonstrated sensitivity for toluene concentrations as low as 100 ppb. The transfer method applied in this work is one step further from previous manual transfer [1] to the industrial adaptation of the process. Materials and substrates A two-step synthesis was applied to prepare SnO2 nanowires, as described elsewhere [2]. The gas sensing substrate was a commercial chemical sensor platform provided by ams AG, Austria - a 2x2 mm micro-hotplate chip with a Pt-based microheater and Pt-IDES with an electrode width and the distance between the electrode fingers both 5 µm. SnO2 NWs were transferred on the IDES-structure with a PDMS stamp, prepared by 1:10 standard recipe and cut into 3x3mm pieces. Transfer of SnO2 NWs A Condor Sigma bond tester tool was employed to transfer SnO2 NWs. The PDMS-stamp was glued on a specific tool of the bond tester, which is usually employed for testing wire bond connections. The PDMS stamp was pressed with well controlled parameters (force, distance and position) onto the substrate with MOx NWs in order to collect the NWs with the stamp. Afterwards the NWs were transferred onto the gas sensing substrate with maximal force of 0.7N and 10s time of contact between the stamp and the micro-hotplate substrate, which resulted in NW transfer and interconnection to the electrodes. Gas measurement Gas measurements were performed by an automatized setup, with synthetic air (80% N2, 20% O2) as a background gas and a constant flow rate of 1000 sccm. Three different humidity levels of 25, 50 and 75% were investigated in the presence of 5 different toluene concentrations – 0.05, 0.1, 0.5, 1 and 5 ppm. The temperature of the microhotplate was kept constant at 300°C. Response was calculated as follows:R = (Rair - Rgas)/Rair *100%where Rair is a resistance of the sensor in synthetic air before the gas pulse and Rgas is a resistance of the sensor in the presence of a test gas at the end of a gas pulse. Two independent sensors were measured, prepared by the same technique. Results and Conclusions The Condor Sigma bond tester enabled precise manipulation of the PDMS NW transfer stamp not only in all 3 axis but also by using force sensor. Due to the high precision of the machine the transfer process could be performed with high precision and control without damage to the micro-hotplate. This demonstrates that such a NW transfer process is suited for realization of NW-based chemical sensor devices. The response towards toluene is shown in Fig.1. The multi-nanowire based devices are able to sense a gas concentration as low as 100 ppb (where the threshold limit value for Switzerland is 50 ppm [3]). There is also visible dependence towards humidity – but it is not as high as for toluene – i.e. for 5 ppm of toluene the mean response for both sensors is 9%, 8% and 8% for 25, 50 and 75% rH levels, respectively. Presently the work is focused on the NWs transfer on CMOS-based microhotplate array chips. Acknowledgements This work was partly performed within the project “FunkyNano – Optimized Functionalization of Nanosensors for Gas Detection by Screening of Hybrid Nanoparticles” funded by the FFG - Austrian Research Promotion Agency (Project No. 858637). References Sosada-Ludwikowska, R. Wimmer-Teubenbacher, M. Sagmeister, A. Köck, Transfer Printing Technology as a Straightforward Method to Fabricate Chemical Sensors Based on Tin Dioxide Nanowires, Sensors (2019), 19, 3049; doi: 10.3390/s19143049Köck, A. Tischner, T. Maier, M. Kast, C. Edtmaier, C. Gspan, G. Kothleitner, Atmospheric pressure fabrication of SnO2-nanowires for highly sensitive CO and CH4 detection, Sensors and Actuators B: Chemical (2009), 138 no. 1, pp. 160–167 (2009); doi: 10.1016/j.snb.2009.02.055Schweizerische Unfallversicherungsanstalt (SUVA): Grenzwerte – Aktuelle MAK- und BAT-Werte für Toluol. Available online: https://www.suva.ch/de-CH/material/Richtlinien-Gesetzestexte/grenzwerte-am-arbeitsplatz-aktuelle-werte (accessed on 29.11.2019) Figure 1

(Invited) How Nanotechnology Can Help the "Zero Hunger" Goal

The elimination of hunger, food insecurity and malnutrition with the aim of creating the basis for a world whose inhabitants feed themselves sufficiently and in a healthy way, in line with their nutritional needs and their food preferences. These are the goals of FAO (Food and Agriculture Organization) for 2030. All researchers are called to make their important contribution.Today there are 842 million hungry people, of whom 200 million are children. From the other European Food Safety Authority (EFSA) declares that 88 million tons of food are thrown away every year, or 173 Kg per person. Data that FAO reports worldwide, 45% of the food produced is thrown away. How nanotechnology can help and contribute to the reduction of food waste, valuable waste if reusable[1]. Nanotechnologies can support the whole process from farm to fork, ensuring that less food is wasted and therefore better used, without compromising the health of the consumer.In this work several studies will be presented in which, Small Sensor System (S3), based on gas sensors with nanomaterials has been used to identify possible contamination of foodstuff and reduce food waste.All the analyzed samples, were prepared in an identical manner for all the techniques used in parallel so as to minimize possible differences. In general, in the food field, techniques that do not go to destroy the sample are preferred, that are fast, and user-friendly, so as to be able to combine as much as possible the real production and food processing.The S3 (Fig.1), consists of a pneumatic part, an electronic part and a chamber with a maximum 10 MOX gas sensors, and an online data acquisition and processing app. Flow, temperature, humidity sensors, and actuators (valves and pumps) are all embedded inside the S3 device. Techniques have been used in parallel to train the sensors, are chemical (GC-MS with SPME) and microbiological (growth media), depending on the specific objective of the application and the nature of the samples [2]. In the case of the reduction of possible microbiological contaminations, the samples will be prepared with known concentrations of bacteria, yeasts or molds, to reproduce real samples.Once acquired and transmitted, data must be analyzed to extract relevant information. Machine learning algorithms are widely used to achieve this purpose, ranging from unsupervised methods (as PCA) to supervised learning (like neural networks, k-NN, SVM, random forest as few examples).Remarkable results have been obtained that demonstrate how the S3 device is able to discriminate in a surprising manner the samples analyzed. The reported results in the food sector concern different types of high applicative interest:- Identification of possible microbiological contaminants such as that from Campylobacter jejuni (Fig.2)[3], - Geographical identification of raw materials (Fig. 3)[4],- Evolution of the shelf-life of a product during the preparation or storage phases,- Environmental monitoring (Fig. 4), such as water purification, air pollution.The food matrices are all complex and constantly evolving, and a device like S3 turns out to bring a considerable advantage to the online control. An advantage that makes it possible to reduce the loss of products at any time, and at the same time increases control enlarging the number of samples analyzed and reducing costs. The aforementioned results demonstrate how nanotechnology and in particular S3 device can contribute to reach the “zero hunger” goal all over the world.[1] S. Vermani, Farm to Fork: IOT for Food Supply Chain, International Journal of Innovative Technology and Exploring Engineering (IJITEE), 8 (2019) 4915 - 4919. DOI: 10.35940/ijitee.L3551.1081219.[2] A.Abbatangelo, E. Núñez-Carmona, V. Sberveglieri, Application of a novel S3 nanowire gas sensor device in parallel with GC-MS for the identification of Parmigiano Reggiano from US and European competitors, J. Food Eng. 236 (2018) 36–43.doi:10.3390/s18051617.[3] E. Núñez-Carmona, M. Abbatangelo,, V. Sberveglieri, Innovative Sensor Approach to Follow Campylobacter jejuni Development, Biosensors, 9 (1) (2019) Article number bios9010008 DOI: 10.3390/bios9010008[4] M. Abbatangelo, E. Núñez-Carmona, G. Duina, V. Sberveglieri, V., Multidisciplinary approach to characterizing the fingerprint of Italian EVoO, Molecules, 24 (8) (2019), DOI: 10.3390/molecules24081457. Figure 1

Potential of Impedance Spectroscopy Towards Quantified Analysis of Gas Sensors: A Tutorial

Transduction of the gas sensor devices through Impedance Spectroscopy (IS) provides non-conventional route to derive quantified model of equivalent circuit and to address resonant frequency tuned selectivity aspect. The quantification of the equivalent circuit elements possibly enhances the scientific understandings of sensing performance in computational scheme. Simultaneously, resonant frequency tuned selectivity approach is another possible outcome of the IS, applied to the gas sensors. These methodologies, prove the potentiality of the IS highlighting the theoretical as well as practical aspects of gas sensor devices. The present scope focuses on the step-by-step approach of flexible adoption in measuring techniques of the IS from its liquid phase measurement to gas phase one. The comparative analysis of quantitative equivalent circuit in different gas ambient is fruitful to presume the experimental details as well as the underlying physical phenomena. The input signal frequency also paved the path for the resonant frequency tuned selectivity improvement for the gas sensor devices. Importance of the IS, as transduction technique, is also discussed with physical, theoretical and computational insights. This review also dealt with corresponding constraints of IS in gas sensors by thorough inspection of different bottlenecks reported in literatures. The present review also has a focus to instigate further research to mitigate such constraints and to further improve the potentiality of the IS in gas sensor field.

First-principles study of pristine and Li-doped borophene as a candidate to detect and scavenge SO2 gas

In this research, the potential application of borophene as gas sensor device is explored. The first-principles theory is employed to investigate the sensing performance of pristine and Li-doped borophene for SO2 and five main atmospheric gases (including CH4, CO2, N2, CO and H2). All gases are found to be adsorbed weakly on pristine borophene, which shows weak physical interaction between the pristine borophene and gases. The gas adsorption performance of borophene is improved by the doping of Li atom. The results of adsorption energy suggest that Li-borophene exhibits high selectivity to SO2 molecule. Moreover, analyses of the charge transfer, density of states and work function also confirm the introduction of Li adatom on borophene significantly enhances the selectivity and sensitivity to SO2. In addition, desorption time of gas from pristine and Li doped borophene indicates the Li-borophene has good desorption characteristics for SO2 molecule at high temperatures. This research would be helpful for understanding the influence of Li doping on borophene and presents the potential application of Li-borophene as a SO2 gas sensor or scavenger.

Optical Gas Sensing with Liquid Crystal Droplets and Convolutional Neural Networks.

Liquid crystal (LC)-based materials are promising platforms to develop rapid, miniaturised and low-cost gas sensor devices. In hybrid gel films containing LC droplets, characteristic optical texture variations are observed due to orientational transitions of LC molecules in the presence of distinct volatile organic compounds (VOC). Here, we investigate the use of deep convolutional neural networks (CNN) as pattern recognition systems to analyse optical textures dynamics in LC droplets exposed to a set of different VOCs. LC droplets responses to VOCs were video recorded under polarised optical microscopy (POM). CNNs were then used to extract features from the responses and, in separate tasks, to recognise and quantify the vapours exposed to the films. The impact of droplet diameter on the results was also analysed. With our classification models, we show that a single individual droplet can recognise 11 VOCs with small structural and functional differences (F1-score above 93%). The optical texture variation pattern of a droplet also reflects VOC concentration changes, as suggested by applying a regression model to acetone at 0.9–4.0% (v/v) (mean absolute errors below 0.25% (v/v)). The CNN-based methodology is thus a promising approach for VOC sensing using responses from individual LC-droplets.

Fabrication of H2S sensitive gas sensors formed of SnO2–Fe2O3 composite nanoparticles

Nanocomposites of metal oxide are considered promising candidates for developing novel gas sensors of exceptional performance. Composite SnO2–Fe2O3 nanoparticles are produced in this work by a modified sole-gel method, and tested for their gas response towards H2S gas in air. The nanoparticles are deposited on a printed substrate with interdigitated electrodes to produce gas sensor devices. Electrical characteristics of the produced devices are tested by means of impedance spectroscopy and revealed an activation energy of 0.13±0.01eV. The sensors exhibit novel characteristics where they are functional at room temperature for low H2S concentration of 2.5 ppm. The practical production process, low power demand, enhanced sensitivity, and low recovery and response times of the fabricated sensors specify their potential for practical implementation for applications of portable gas sensors. Furthermore, the utilization of magnetic nanoparticles enables the recycling of nanocomposites for further applications.

A sensitive thick film as a candidate for ethanol gas sensor from Jarosite-based minerals

We report this study that aims to know thick film sensitivity to ethanol gas. The thick film was fabricated from local mineral, is Jarosite as a source of iron oxide, and mixed with zinc oxide and Mangan oxide. The semiconductor mixture powder was synthesized by the sol-gel method, whereas the film was fabricated by a screen printing method. The film sensitivity was known with a gas sensing performance analyzer. An ethanol gas was adsorbed well by a thick film with different electrical resistance values. Due to the difference in the electrical resistance value, the thick film exhibited good sensitivity performance. The highest sensitivity was achieved approximately 91.7% of 4 gas concentration level of ethanol. Besides, the morphology of the thick film was examined by SEM that showed a pyramid-like form with a little impurity around them. The pyramid-like form looks like a spinel structure with a complex spinel-type where this structure is very well utilized for gas sensor device applications. Furthermore, these characteristics of the thick film indicated an opportunity that it can be a candidate for an ethanol gas sensor device.

Molecular bridge-mediated ultralow-power gas sensing

We report the electrical detection of captured gases through measurement of the quantum tunneling characteristics of gas-mediated molecular junctions formed across nanogaps. The gas-sensing nanogap device consists of a pair of vertically stacked gold electrodes separated by an insulating 6 nm spacer (~1.5 nm of sputtered α-Si and ~4.5 nm ALD SiO2), which is notched ~10 nm into the stack between the gold electrodes. The exposed gold surface is functionalized with a self-assembled monolayer (SAM) of conjugated thiol linker molecules. When the device is exposed to a target gas (1,5-diaminopentane), the SAM layer electrostatically captures the target gas molecules, forming a molecular bridge across the nanogap. The gas capture lowers the barrier potential for electron tunneling across the notched edge region, from ~5 eV to ~0.9 eV and establishes additional conducting paths for charge transport between the gold electrodes, leading to a substantial decrease in junction resistance. We demonstrated an output resistance change of >108 times upon exposure to 80 ppm diamine target gas as well as ultralow standby power consumption of <15 pW, confirming electron tunneling through molecular bridges for ultralow-power gas sensing.

Fabrication of ZnO nanorods based gas sensor pattern by photolithography and lift off techniques

In this paper, mainly focused on the fabrication of ZnO nanorod based gas sensor. This sensor was aimed to detect a small amount of volatile organic compounds (VOCs) with low power, low cost and simple manufacturing method. Low power consumption is achievable by completion of metal oxide sensors. So, high sensitivity of sensor is requested to sense the target gas. It is important to find out optimal sensing structures of materials. The components of sensor were platinum electrode and heater and ZnO sensing film. SiO2 was used as thermal and electricity insulator. Herein, the ZnO nanorods was synthesized by wet chemical method, namely chemical bath deposition(CBD) and hydrothermal method (HD), were tried to check well alignment of ZnO nanorods on the sensing area. Morphology of sensing material and sensor pattern was analyzed by Scanning Electron Microscope (SEM) and EDS, phase analysis was carried out using X-ray Diffractometer (XRD). The morphology images of ZnO confirmed the formation of nanorod like structure with average diameter of 205 nm and 55 nm chemical bath deposition(CBD) and hydrothermal method (HD), respectively. Therefore, the fabricated ZnO nanorod based gas sensor device is suitable for human breath analysis.Capsule: This work investigates the fabrication of ZnO nanorods based gas sensor pattern.

Constructing a Resistive Gas Sensor Based on the Highly Stable Mil-53(Fe)/Ag/CNT Ternary Nanocomposite for Sensing Polar Volatile Organic Compounds Such as Methanol

In this study, MIL-53(Fe) nano-sized crystals were synthesized under microwave irradiation with various powers (80, 100 W) and ‎times (5, 10 min). Then, the crystals that had the best porous ‎structure were selected to prepare the MIL-53(Fe)/Ag/CNT ternary nanocomposite. Nano-sized crystals were characterized by scanning electron microscopy, Fourier transform infrared ‎spectroscopy, X-ray diffraction, and specific surface area analysis. Prepared nanocomposite was used to fabricate a resistive gas sensor device for detecting volatile organic compounds in environmental conditions (10% relative humidity and 25°C). The resistive gas sensor showed higher response to volatile organic compounds with higher polarity such as methanol. The lowest detectable concentration of methanol was 1.2 ppm. Linearity of the calibration curve was obtained in a wide range from 0 to 30 mg/L.

Fabrication Strategies and Measurement Techniques for Performance Improvement of Graphene/Graphene Derivative Based FET Gas Sensor Devices: A Review

In the ever-expanding arena of gas sensor research, graphene (or graphene derivative viz. reduced graphene oxide (rGO)) field-effect transistors (FET) are the most recent inclusions which have enormous potential to detect wide variety of oxidizing and reducing target species with high sensitivity. This review article aims to unify the various strategies adopted (during fabrication or during measurement) by the researchers to improve the performance index of graphene/rGO-FET gas sensor devices. These strategies can broadly be classified as; (i) fabrication/device level optimization and (ii) operational/measurement technique improvisation. In the first category, improvement of various sensing parameters of graphene/rGO FET through functionalization/surface modification/doping of channel, use of novel gate dielectric/ gate material and exploration of innovative device configurations (ionic top gate, single back gate or dual gate) have been discussed. While in the second category, optimized gate biasing, AC phase-sensitive measurement, quasi static/ real time conductance measurement and low-frequency noise spectra based measurement techniques have been elucidated. After critically comparing the performance improvement devoting the above two lines of approaches, with emphasis on the unique advantage and limitations of each of the schemes, the review concludes with the indication towards future research directions that lie ahead.

Application of a lab-made electronic nose for extra virgin olive oils commercial classification according to the perceived fruitiness intensity

An electronic nose, comprising nine metal oxide sensors, has been built aiming to classify olive oils according to the fruity intensity commercial grade (ripely fruity or light, medium and intense greenly fruity), following the European regulated complementary terminology. The lab-made sensor device was capable to differentiate standard aqueous solutions (acetic acid, cis-3-hexenyl, cis-3-hexen-1-ol, hexanal, 1-hexenol and nonanal) that mimicked positive sensations (e.g., fatty, floral, fruit, grass, green and green leaves attributes) and negative attributes (e.g., sour and vinegary defects), as well as to semi-quantitatively classify them according to the concentration ranges (0.05–2.25 mg/kg). For that, unsupervised (principal component analysis) and supervised (linear discriminant analysis: sensitivity of 92% for leave-one-out cross validation) classification multivariate models were established based on nine or six gas sensors, respectively. It was also showed that the built E-nose allowed differentiating/discriminating (sensitivity of 81% for leave-one-out cross validation) extra virgin olive oils according to the perceived intensity of fruitiness as ripely fruity, light, medium or intense greenly fruity. In conclusion, the gas sensor device could be used as a practical preliminary non-destructive tool for guaranteeing the correctness of olive oil fruitiness intensity labelling.

Experimental and Theoretical Advances in MXene-Based Gas Sensors.

MXenes, two-dimensional (2D) transition metal carbides and nitrides, have been arousing interest lately in the field of gas sensing thanks to their remarkable features such as graphene-like morphology, metal-comparable conductivity, large surface-to-volume ratio, mechanical flexibility, and great hydrophilic surface functionalities. With tunable etching and synthesis methods, the morphology of the MXenes, the interlayer structures, and functional group ratios on their surfaces were effectively harnessed, enhancing the efficiency of MXene-based gas-sensing devices. MXenes also efficiently form nanohybrids with other nanomaterials, as a practical approach to revamp the sensing performance of the MXene sensors. This Mini-Review summarizes the recent experimental and theoretical reports on the gas-sensing applications of MXenes and their hybrids. It also discusses the challenges and provides probable solutions that can accentuate the future perspective of MXenes in gas sensors.

Urchin like CuO hollow microspheres for selective high response ethanol sensor application: Experimental and theoretical studies

Herein, the synthesis, characterizations and gas sensor application of nano-textured urchin like CuO hollow microspheres (HM) were reported. The detailed characterizations confirmed the uniform growth and nanocrystalline monoclinic structure of urchin like CuO HM. The synthesized urchins like CuO HM were utilized to fabricate resistive sensor devices which exhibited elevated response towards ethanol with high selectivity. The optimization of operating temperature was carried out for the highest ethanol response and the effect of different concentrations were also recorded. The sensor device showed 1241% response at an operating temperature of 225 °C for 100 ppm ethanol concentration. The lowest ethanol was detected at 25 ppm having the response of 49% by the fabricated CuO sensor device. Transient responses as well as stability of the sensor were analyzed. The selectivity of CuO sensor device was studied for NO2, CO2, CH4 and H2 gases and remarkably it was seen that the developed gas sensor devices demonstrated outstanding selectivity towards ethanol gas. Finally, the density functional theory (DFT) computational quantum modelling was utilized in order to understand the adsorption phenomenon of ethanol and oxygen molecules in the sensing application. An ethanol sensing mechanism using CuO sensor was also elucidated.

Gas sensor device for high-performance ethanol sensing using α-MnO2 nanoparticles

α-MnO2 nanoparticles were hydrothermally synthesized and characterized for their various physicochemical properties using different techniques. The average diameter of the nanoparticles was observed to be 70 ± 5 nm along with well-resolved inter-planar spacing of 0.241 nm corresponding to (211) plane for α-MnO2 crystal phase. Working electrode for ethanol gas sensing fabricated by using α-MnO2 nanoparticles showed gas response of 30.6 at optimized temperature of 300 °C. The corresponding response and recovery times were 30 s and 40 s, respectively for 200 ppm ethanol at 300 °C.

Nanoparticles of CuO thin films for room temperature NO2 gas detection: Annealing time effect

In this work, the effect of annealing time on sensing properties of cupric oxide nanoparticles deposited on black silicon (CuO/BSi) as a gas sensor device was studied. CuO nanoparticles were prepared using pulsed laser ablation (PLA) beyond which, the prepared precursor was deposition using the spin coating deposition technique. The deposited CuO thin films were annealed in air for 1, 2, and 3 h at 400 °C. The influence of annealing time on the structural, morphological, and sensing performance at room temperature of prepared CuO/BSi was investigated in details. XRD patterns revealed nanocrystal of CuO with the crystallite size increased from 17 to 25 nm when that annealing time increased from 1 h to 3 h. The FESEM images revealed a nanostructured stone-like structure of different sizes. The average diameter increased with 1 h annealing time. The appearance of the porous surface of the annealed sample for two hours, while the porosity on the surface of the annealed sample for three hours decreased. A strong, sharp emission in the Visible-region was observed in the spectrum of CuO nanostructures, revealed by PL measurements at room temperature. The PL spectrum also showed a shift towards a shorter wavelength in the visible region that refers to the decreasing of energy gap from 1.73 eV to 1.71 eV. The prepared films were exposed to NO2 to detect the sensitivity of the nanostructured CuO/BSi thin films. It was found that the annealing time had a major role in increased sensitivity. The sensitivity of the annealed sample increased from 77% to 90% when the annealing time increased from one to two hours, while the sensitivity of the third model decreased to 4% at a three-hour annealing time. The reason for this behavior has been also discussed.

Liquid Metal-Based Epidermal Flexible Sensor for Wireless Breath Monitoring and Diagnosis Enabled by Highly Sensitive SnS2 Nanosheets.

Real-time wireless respiratory monitoring and biomarker analysis provide an attractive vision for noninvasive telemedicine such as the timely prevention of respiratory arrest or for early diagnoses of chronic diseases. Lightweight, wearable respiratory sensors are in high demand as they meet the requirement of portability in digital healthcare management. Meanwhile, high-performance sensing material plays a crucial role for the precise sensing of specific markers in exhaled air, which represents a complex and rather humid environment. Here, we present a liquid metal-based flexible electrode coupled with SnS2 nanomaterials as a wearable gas-sensing device, with added Bluetooth capabilities for remote respiratory monitoring and diagnoses. The flexible epidermal device exhibits superior skin compatibility and high responsiveness (1092%/ppm), ultralow detection limits (1.32 ppb), and a good selectivity of NO gas at ppb-level concentrations. Taking advantage of the fast recovery kinetics of SnS2 responding to H2O molecules, it is possible to accurately distinguish between different respiratory patterns based on the amount of water vapor in the exhaled air. Furthermore, based on the different redox types of H2O and NO molecules, the electric signal is reversed once the exhaled NO concentration exceeds a certain threshold that may indicate the onset of conditions like asthma, thus providing an early warning system for potential lung diseases. Finally, by integrating the wearable device into a wireless cloud-based multichannel interface, we provide a proof-of-concept that our device could be used for the simultaneous remote monitoring of several patients with respiratory diseases, a crucial field in future digital healthcare management.

Organic Thin-Film Transistors as Gas Sensors: A Review.

Organic thin-film transistors (OTFTs) are miniaturized devices based upon the electronic responses of organic semiconductors. In comparison to their conventional inorganic counterparts, organic semiconductors are cheaper, can undergo reversible doping processes and may have electronic properties chiefly modulated by molecular engineering approaches. More recently, OTFTs have been designed as gas sensor devices, displaying remarkable performance for the detection of important target analytes, such as ammonia, nitrogen dioxide, hydrogen sulfide and volatile organic compounds (VOCs). The present manuscript provides a comprehensive review on the working principle of OTFTs for gas sensing, with concise descriptions of devices’ architectures and parameter extraction based upon a constant charge carrier mobility model. Then, it moves on with methods of device fabrication and physicochemical descriptions of the main organic semiconductors recently applied to gas sensors (i.e., since 2015 but emphasizing even more recent results). Finally, it describes the achievements of OTFTs in the detection of important gas pollutants alongside an outlook toward the future of this exciting technology.

Adsorbed of toxic gas molecules (CO, H2S, and NO) on alkali-metal-doped g-GaN monolayer

Using the first-principles calculation, based on density functional theory, the geometric structures, electronic, magnetic, and optical properties of CO, H2S, and NO toxic gas molecules adsorbed on alkali metals (Li, Na, K, Rb, and Cs) doped g-GaN monolayer have been investigated. The adsorbed systems have induced magnetic property except H2S adsorbed on Na doped g-GaN monolayer, the total magnetic moment of CO, H2S, and NO adsorbed on alkali metals doped g-GaN are 2.0, 2.0, and 1.0 μB, respectively, and the materials are spin semiconductors. The charge transfer mainly occurs between gas molecules, doped atoms, and Ga atoms. The adsorbed behaviors of CO, H2S, and NO adsorbed on intrinsic g-GaN are physisorption, H2S adsorbed on Na doped g-GaN monolayer and NO adsorbed on Li, Na, K, Rb, and Cs doped g-GaN monolayer are chemisorption, while CO adsorbed on alkali metals doped g-GaN monolayer are physisorption. The results show that the adsorptive properties can be enhanced dramatically by doping with alkali metals, and the materials can be used in gas sensor devices and detector devices. In addition, the adsorbed coefficients in ultraviolet and visible indicate the materials promising the application of violet and UV optoelectronic devices.