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Gas detection and monitoring are critical to protect human health and safeguard the environment and ecosystems. Chemiresistive sensors are widely used in gas monitoring due to their ease of fabrication, high customizability, mechanical flexibility, and fast response time. However, with the rapid development of industrialization and technology, the main challenges faced by chemiresistive gas sensors are poor selectivity and insufficient anti-interference stability in complex application environments. In order to overcome these shortcomings of chemiresistive gas sensors, the pattern recognition method is emerging and is having a great impact in the field of sensing. In this review, we focus systematically on the advancements in the field of data processing methods for feature extraction, such as the methods of determining the characteristics of the original response curve, the curve fitting parameters, and the transform domain. Additionally, we emphasized the developments of traditional recognition algorithms and neural network algorithm in gas discrimination and analyzed the advantages through an extensive literature review. Lastly, we summarized the research on chemiresistive gas sensors and provided prospects for future development. [ABSTRACT FROM AUTHOR]

As a stable, low-cost, environment-friendly, and gas-sensitive material, semiconductor metal oxides have been widely used for gas sensing. In the past few years, single-atom catalysts (SACs) have gained increasing attention in the field of gas sensing with the advantages of maximized atomic utilization and unique electronic and chemical properties and have successfully been applied to enhance the detection sensitivity and selectivity of metal oxide gas sensors. However, the application of SACs in gas sensors is still in its infancy. Herein, we critically review the recent advances and current status of single-atom catalysts in metal oxide gas sensors, providing some suggestions for the development of this field. The synthesis methods and characterization techniques of SAC-modified metal oxides are summarized. The interactions between SACs and metal oxides are crucial for the stable loading of single-atom catalysts and for improving gas-sensitive performance. Then, the current application progress of various SACs (Au, Pt, Cu, Ni, etc.) in metal oxide gas sensors is introduced. Finally, the challenges and perspectives of SACs in metal oxide gas sensors are presented. [ABSTRACT FROM AUTHOR]

The highly sensitive detection of ppb-level NO 2 at room temperature (RT, 25 °C) remains a challenge hindering the detection of metal oxides due to their chemical inactivity. Herein, a strategy for constructing mixed metal oxides with high specific surface areas and highly active surfaces is proposed. In this work, we synthesized NiO-SnO 2 using the solvothermal and calcination methods with Ni/Sn bimetallic organic frameworks (Ni/Sn-BTC MOFs, BTC: benzene-1,3,5-tricarboxylic) as sacrificial templates. The characterization shows that NiO-SnO 2 has a high porosity (0.343 m3/g) and a large specific surface area (74.73 m2/g), which can produce abundant permeability pathways for NO 2 , maximize the number of active sites, and enhance the capacity for NO 2 capture. The sensing test results indicate that the NiO-SnO 2 sensor exhibits an excellent sensing response of 693.1% (ΔR/R a) to 150 ppb NO 2 , a fast response/recovery time (86.8/29.4 s), a distinct selectivity, a lower detection limit (25 ppb) and good long-term stability at RT and 60% RH (RH: relative humidity). The outstanding sensing performance can be attributed to the microstructure characteristics, oxygen vacancies and p-n heterojunctions formed between NiO and SnO 2. The current work provides a versatile platform for designing sensitive materials with heterogeneous structures and opens up a new pathway for the development of room temperature gas sensors based on metal oxide. [Display omitted] • NiO-SnO 2 hybrid was synthesized by pyrolysis of bimetallic Ni/Sn-BTC MOFs. • NiO-SnO 2 sensor exhibits extraordinary sensing performance for ppb-level NO 2 at RT. • Enhancement benefits from porous, heterojunction and oxygen vacancy of the composite. [ABSTRACT FROM AUTHOR]

Two-dimensional metal-organic framework nanosheets, as a new member of the two-dimensional family, are attracting increasing research interest in various fields. Herein, a uniform two-dimensional Zn-BTC MOF nanosheet with a thickness of 14 ± 2 nm was synthesized through a simple, efficient, and scalable sonication-assisted bottom-up strategy. The ZnO nanoparticles were used as the Zn source, which can be corroded by H 3 BTC to form Zn-BTC MOFs under sonication conditions. The morphologies of Zn-BTC MOFs can be modulated from nanosheets to nanorods by changing the content of H 3 BTC. The crystal growth process is dominated by the thermodynamics of sonication and the stoichiometric ratio of the generated Zn2+ and H 3 BTC, and the crystal growth mechanism was elaborated by the oriented attachment. Consequently, a chemiresistive gas sensor based on Zn-BTC nanosheet derivatives was fabricated to detect ppb-level acetone. Notably, the experimental detection limit of concentration could reach as low as 100 ppb of acetone due to the functional surface, and the dynamic response linearity reached 99 %. Our work explored a new method of synthesizing two-dimensional Zn-BTC MOF, and the fabricated gas sensing device demonstrated promising potential in gas sensing fields. [Display omitted] • The Zn-BTC MOF nanosheets are firstly prepared through a bottom-up sonication-assisted strategy. • A high-performance gas sensor with ppb-level acetone detection was designed. • The high-performance originate from the porous structure and functional surface of derivatives. [ABSTRACT FROM AUTHOR]

A strategy in solving moisture interference is proposed by coating hydrophobic fluorocarbon on metal oxide sensors. The CF layers largely enhance the anti-humidity performance of In 2 O 3 sensor, the response only varies 3.7% in high humidity. The sensor presents excellent sensitivity, stability and rapid response/recovery rates in high humidity. Nitrogen dioxide (NO 2) is one of the most dangerous pollutants, which extremely hazard our health and live environments. In 2 O 3 based chemiresistive gas sensors have been widely applicated to detect NO 2 because of their excellently selectivity and highly sensitivity. However, the performances of In 2 O 3 based NO 2 sensors are greatly perturbed by ambient moisture, which greatly limits their practical applications. In this work, because of the unique low surface energy and hydrophobic characters of fluorocarbon (CF), a layer of CF is modified on In 2 O 3 by CF plasma treatment to improve their anti-humidity properties in NO 2 sensing. The gas sensing performance measurements demonstrate that the CF modified In 2 O 3 (CF-In 2 O 3) sensors show good sensitivity and selectivity to NO 2. Most importantly, the modification of CF layer can largely enhance the anti-interference performance of CF-In 2 O 3 sensor to humidity; the response varies only 3.7 % (CV) in high humidity environment. Furthermore, the mechanisms of gas sensing and anti-interference to humidity have been explained based on the experimental results. This work makes a great advance in solving moisture interference and also makes a great significant to detect NO 2 in practical application. [ABSTRACT FROM AUTHOR]

The development of high-performance room temperature sensors for detecting trace nitrogen dioxide (NO 2) levels is critical for Internet of Things based environmental monitoring. We present a novel NO 2 gas sensor based on tin selenide (SnSe) nanosheet arrays films deposited on a variety of substrates using vacuum thermal evaporation. The nanosheets formed well-oriented arrays and had a thickness of about 10 nm and lateral size of 0.2–1 µm. The films' synthesis was well controlled, and the NO 2 gas sensors' performance was tuned by adjusting the microstructure of the films. The gas sensors were highly responsive to NO 2 concentrations ranging from 100 to 300 ppb, it can achieve a response of 155% towards 300 ppb concentration of NO 2 at room temperature; the gas sensing performance is affected by film thickness and microstructure. At a high NO 2 gas concentration of 10 ppm, a response time of 12 s and response rate of 700% were achieved at room temperature. Furthermore, the high selectivity and repeatability of the SnSe nanosheet arrays film gas sensor were demonstrated. Because of its ease of fabrication and high performance, the NO 2 gas sensor described here has promising applications. • A novel tin selenide (SnSe) nanosheet arrays film gas sensor has been fabricated. • The sensors show a high response for sensing NO 2 with ppb level at room temperature. • The sensors performance was optimized by adjusting the microstructures of the films. [ABSTRACT FROM AUTHOR]

Complex gas environments and the demand for room temperature detection will provide challenges for chemo-resistive-type gas sensors in a variety of applications. In this work, 2D/0D SnSe 2 /TiO 2 composite is used as a sensing material, displaying good sensitivity and specificity to ppb-level NO 2 at ambient temperature (25 °C). The response value and time of the SnSe 2 /TiO 2 sensor are 207 % and 139s when exposed to 100 ppb NO 2 , as opposed to 62 % and 183s for the pristine SnSe 2 sensor. The composite sensor also offers great long-term stability for NO 2 and good gas selectivity. Notably, the SnSe 2 /TiO 2 sensor's mixed gases (NO 2 +H 2 S) measurement is done to examine its NO 2 detection capacity in the presence of interfering gas. The constructed composite sensor can accurately detect NO 2 even if the concentration of the reductive interfering H 2 S is 66.7 times higher than that of the oxidative target NO 2. The effective usage of SnSe 2 modified by TiO 2 for low-concentration, room-temperature NO 2 detection in a complex gas environment is shown by the composite sensor's pleasing gas detection results. • 2D/0D SnSe 2 /TiO 2 hybrids are obtained by hydrothermal and ultrasonic methods. • SnSe 2 /TiO 2 can detect low concentrations of NO 2 at room temperature. • SnSe 2 /TiO 2 exhibits higher NO 2 gas response than SnSe 2 (∼3.3 times). • SnSe 2 /TiO 2 can accurately and qualitatively identify NO 2 under the interference of high concentrations of H 2 S. [ABSTRACT FROM AUTHOR]

Real-time gas sensors with high response and selectivity are highly desirable for hydrogen sulfide (H 2 S) monitoring concentration changes in the environment, especially at the ultra-trace level. Herein, cobalt-doped indium oxide (In 2 O 3) with shorter-length nanorings was synthesized by foreign metal cobalt ions. By controlling the morphology of the material and calcinating the indium-based metal-organic frameworks (In-MOFs), the nanostructures of the obtained materials have a larger specific surface area, which further improves the sensing performance. The sensor prepared with a suitable amount of cobalt doping materials can exhibit the best gas-sensing performance. The Co-doped In 2 O 3 sensor shows a high response to 2 ppm H 2 S (∼12.6) at 225 °C, low detection limits (100 ppb), and excellent selectivity, which is 5.34 times higher than pure In 2 O 3 at 300 °C. Moreover, the potential growth mechanism speculates that the presence of Cobalt ions leads to a change in material morphology from nanorods to nanorings. This work provides an implementable method for controlling the morphology of the materials, which could facilitate the development of high-performance gas sensors. • Metal ions change the morphology of nanomaterials. • In-O bonds are easily broken due to cobalt doping. • The specific surface area increases due to shorter indium oxide nanorod length. • Hydrogen sulfide gas sensor with excellent selectivity and low detection limits. • Decrease in operating temperature of cobalt-doped indium oxide sensors. [ABSTRACT FROM AUTHOR]

NiO-CuO hydrangeas-like composite is synthesized by a solvothermal reaction and subsequent pyrolysis of bimetallic Ni-Cu-BTC in air. The NiO-CuO-based sensor not only exhibits recoverable characteristic to H 2 S gas but also possess a wide linear detection range at room temperature. The low detection limit of 50 ppb indicates that the NiO-CuO-based sensor has potential applications in the Internet of Things. [Display omitted] • NiO-CuO hydrangeas-like hybrid was synthesized by pyrolysis of Ni/Cu bimetallic MOFs. • No poisoning phenomenon occurs in detecting H 2 S at room temperature. • NiO-CuO sensor exhibits a low detection limit and a wide linear detection range. • Enhancement benefits from porous, heterojunctions and synergy between NiO and CuO. Highly sensitive and selective detection of trace-level concentration H 2 S remains a major challenge for metal oxide semiconductor gas sensor due to sulfuration-induced poisoning. Herein, a strategy to adjust the reaction site by constructing heterojunction with mixed-metal oxide is proposed. In this work, NiO-CuO hydrangeas-like composite is prepared by a solvothermal reaction and subsequent pyrolysis of bimetallic Ni-Cu-BTC in air. The characterization demonstrates that NiO-CuO hydrangeas has a large specific surface area (27.85 m2/g) and porosity (0.14 m3/g), which can provide abundant adsorption sites and diffusion pathways for gas molecules. Moreover, the sensing test results indicate that NiO-CuO sensors exhibit recoverable characteristic and superior sensing performance. The response value of 2NiO-CuO sensor to 1 ppm H 2 S in dry air at room temperature (25℃) is 3.99, which can be improved to 10.77 in 40 % humidity condition, making it suitable for practical application. Particularly, the sensor has a low detection limit of 50 ppb, a wide linear detection range (50 ppb-2 ppm), good repeatability, selectivity and long-term stability. The improvement of gas sensing response can be attributed to the porous structure, multiple p-p heterojunctions and synergistic effects between NiO and CuO. [ABSTRACT FROM AUTHOR]

Hydrogen sulfide (H 2 S), as a representative food spoilage gas produced by the breakdown of sulfur-containing proteins, is highly irritant and toxic. Designing gas sensors sensitive to H 2 S for food spoilage evaluation is of great significance for automated quality control in the modern food industry. However, humidity in the environment can generally deteriorate the detection performance of conventional metal oxide semiconductor gas sensors. Therefore, it is critical to develop H 2 S sensors based on novel materials that can operate effectively in atmospheric humidity conditions. In this work, tin diselenide (SnSe 2) nanosheets decorated with tungsten trioxide (WO 3) nanoparticles are synthesized by a one-step ultrasound strategy, and used as sensing layers for H 2 S gas sensors. The water molecule activated charge-transfer of the SnSe 2 /WO 3 composite boosts H 2 S sensing ability at high relative humidity (RH) levels. At room temperature (25 °C), the SnSe 2 /WO 3 composite sensor with optimal mass ratio exhibits enhanced H 2 S response value (33.80%@10 ppm) compared with that of the pristine SnSe 2 sensor (14.29%@10 ppm) at 82% RH. In addition, the fabricated sensor is proven to be practically effective for evaluating the spoilage of eggs by comparing the sensing response of a fresh egg with that of a spoiled egg. This study offers possibilities for evaluating the spoilage of foods in poultry, aquatic, and food processing industries using a rapid and sensitive humidity-activated H 2 S gas sensor. • SnSe 2 /WO 3 was synthesized successfully with facile ultrasonication techniques. • The SnSe 2 /WO 3 -50% enhances 2.37 times more response to 10 ppm H 2 S than pure SnSe 2. • The heterojunction accelerates electron migration and boosts the response to H 2 S. • The SnSe 2 /WO 3 sensor was used in the application of the egg spoilage evaluation. [ABSTRACT FROM AUTHOR]

The gaseous volatile organic compounds (VOCs) sensors with high-selectivity and low-power consumption have been expected for practical applications in environmental monitoring and disease diagnosis. Herein, we demonstrate a room-temperature VOCs gas sensor with enhanced performance based on Ti 3 C 2 T x -TiO 2 nanocomposites. The Ti 3 C 2 T x -TiO 2 nanocomposites with regular morphology are successfully synthesized via a facile one-step hydrothermal synthesis strategy by using Ti 3 C 2 T x itself as titanium source. Attributed to the formation of interfacial heterojunctions and the modulation of carrier density, the Ti 3 C 2 T x -TiO 2 sensor exhibits about 1.5–12.6 times enhanced responses for the detection of various VOCs at room temperature than pure MXene sensor. Moreover, the nanocomposite sensor has better response to hexanal, both an air pollutant and a typical lung cancer biomarker. The gas response of the Ti 3 C 2 T x -TiO 2 sensor towards 10 ppm hexanal is about 3.4%. The hexanal gas sensing results display that the nanocomposite sensor maintains a high signal-to-noise ratio and the lower detection limit to hexanal gas is as low as 217 ppb. Due to the low power consumption and easy fabrication process, the Ti 3 C 2 T x -TiO 2 nanocomposite sensor is promising for application in IoT environmental monitoring as well as real-time health monitoring. [Display omitted] • Ti 3 C 2 T x -TiO 2 nanocomposites were synthesized by one-step hydrothermal strategy. • The nanocomposite sensor exhibited enhanced responses for various VOCs. • Ti 3 C 2 T x -TiO 2 sensor showed good selectivity to hexenal with 217 ppb detection limit. • The enhancement is attributed to the Schottky junction and synergy of the hybridization. • The elaborated gas sensor can operate at room temperature. [ABSTRACT FROM AUTHOR]