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Micro- and nano-sensors lie at the heart of critical innovation in fields ranging from medical to environmental sciences. In recent years, there has been a significant improvement in sensor design along with the advances in micro- and nano-fabrication technology and the use of newly designed materials, leading to the development of high-performance gas sensors. Advanced micro- and nano-fabrication technology enables miniaturization of these sensors into micro-sized gas sensor arrays while maintaining the sensing performance. These capabilities facilitate the development of miniaturized integrated gas sensor arrays that enhance both sensor sensitivity and selectivity towards various analytes. In the past, several micro- and nano-gas sensors have been proposed and investigated where each type of sensor exhibits various advantages and limitations in sensing resolution, operating power, response, and recovery time. This paper presents an overview of the recent progress made in a wide range of gas-sensing technology. The sensing functionalizing materials, the advanced micro-machining fabrication methods, as well as their constraints on the sensor design, are discussed. The sensors’ working mechanisms and their structures and configurations are reviewed. Finally, the future development outlook and the potential applications made feasible by each category of the sensors are discussed.

Room-temperature (RT) gas sensing is desirable for battery-powered or self-powered instrumentation that can monitor emissions associated with pollution and industrial processes. This review (with 171 references) discusses recent advances in three types of porous nanostructures that have shown remarkable potential for RT gas sensing. The first group comprises hierarchical oxide nanostructures (mainly oxides of Sn, Ni, Zn, W, In, La, Fe, Co). The second group comprises graphene and its derivatives (graphene, graphene oxides, reduced graphene oxides, and their composites with metal oxides and noble metals). The third group comprises 2D transition metal dichalcogenides (mainly sulfides of Mo, W, Sn, Ni, also in combination with metal oxides). They all have been found to enable RT sensing of gases such as NOx, NH 3 , H 2 , SO 2 , CO, and of vapors such as of acetone, formaldehyde or methanol. Attractive features also include high selectivity and sensitivity, long-term stability and affordable costs. Strengths and limitations of these materials are highlighted, and prospects with respect to the development of new materials to overcome existing limitations are discussed. Graphical Abstract The review summarizes the most significant progresses related to room temperature gas sensing by using hierarchical oxide nanostructures, graphene and its derivatives and 2D transition metal dichalcogenides, highlighting the peculiar gas sensing behavior with enhanced selectivity, sensitivity and long-term stability.

To meet requirements in air quality monitoring, sensors are required that can measure the concentration of gaseous pollutants at concentrations down to the ppb and ppt levels, while at the same time they exhibiting high sensitivity, selectivity, and short response/recovery times. Among the different sensor types, those employing metal oxide semiconductors (MOSs) offer great promises as they can be manufactured in easy/inexpensive ways, and designed to measure the concentration of  a wide range of target gases. MOS sensors rely on the adsorption of target gas molecules on the surface of the sensing material and the consequent capturing of electrons from the conduction band that in turn affects their conductivity. Despite their simplicity and ease of manufacturing, MOS gas sensors are restricted by high limits of detection (LOD; which are typically in the ppm range) as well as poor sensitivity and selectivity. LOD and sensitivity can in principle be addressed by nanostructuring the MOSs, thereby increasing their porosity and surface-to-volume ratio, whereas selectivity can be tailored through their chemical composition. In this paper we provide a critical review of the available techniques for nanostructuring MOSs using chemiresistive materials, and discuss how these can be used to attribute desired properties to the end gas sensors. We start by describing the operating principles of chemiresistive sensors, and key material properties that define their performance. The main part of the paper focuses on the available methods for synthesizing nanostructured MOSs for use in gas sensors. We close by addressing the current needs and provide perspectives for improving sensor performance in ways that can fulfill requirements for air quality monitoring. Graphical abstract

HighlightsRecent progress in noble metal-decorated (NM-D) semiconducting metal oxides (SMOs) gas sensors are summarized.Gas sensing mechanisms related to noble metal decoration are carefully discussed.Crucial challenges facing the development of NM-D SMOs gas sensors are analyzed.Highly sensitive gas sensors with remarkably low detection limits are attractive for diverse practical application fields including real-time environmental monitoring, exhaled breath diagnosis, and food freshness analysis. Among various chemiresistive sensing materials, noble metal-decorated semiconducting metal oxides (SMOs) have currently aroused extensive attention by virtue of the unique electronic and catalytic properties of noble metals. This review highlights the research progress on the designs and applications of different noble metal-decorated SMOs with diverse nanostructures (e.g., nanoparticles, nanowires, nanorods, nanosheets, nanoflowers, and microspheres) for high-performance gas sensors with higher response, faster response/recovery speed, lower operating temperature, and ultra-low detection limits. The key topics include Pt, Pd, Au, other noble metals (e.g., Ag, Ru, and Rh.), and bimetals-decorated SMOs containing ZnO, SnO2, WO3, other SMOs (e.g., In2O3, Fe2O3, and CuO), and heterostructured SMOs. In addition to conventional devices, the innovative applications like photo-assisted room temperature gas sensors and mechanically flexible smart wearable devices are also discussed. Moreover, the relevant mechanisms for the sensing performance improvement caused by noble metal decoration, including the electronic sensitization effect and the chemical sensitization effect, have also been summarized in detail. Finally, major challenges and future perspectives towards noble metal-decorated SMOs-based chemiresistive gas sensors are proposed.

This work is a comprehensive review of sensing materials, which interact with several target gases pertinent to agricultural monitoring applications. Sensing materials which interact with carbon dioxide, water vapor (relative humidity), hydrogen sulfide, ethylene and ethanol are the focus of this work. Performance characteristics such as dynamic range, recovery time, operating temperature, long-term stability and method of deposition are discussed to determine the commercial viability of the sensing materials considered in this work. In addition to the sensing materials, deposition methods are considered to obtain the desired sensing material thickness based on the sensor’s mechanism of operation. Various material classes including metal oxides, conductive polymers and carbon allotropes are included in this review. By implementing multiple sensing materials to detect a single target analyte, the issue of selectivity due to cross sensitivity can be mitigated. For this reason, where possible, it is desirable to utilize more than one sensing material to monitor a single target gas. Among those considered in this work, it is observed that PEDOT PSS/graphene and TiO2-coated g-C3N4 NS are best suited for CO2 detection, given their wide dynamic range and modest operating temperature. To monitor the presence of ethylene, BMIM-NTf2, SWCNTs and PtTiO2 offer a dynamic range most suitable for the application and require no active heating. Due to the wide dynamic range offered by SiO2/Si nanowires, this material is best suited for the detection of ethanol; a gas artificially introduced to prolong the shelf life of the harvested crop. Finally, among all other sensing materials investigated, it observed that both SWCNTs and CNTs/SnO2/CuO are most suitable for H2S detection in the given application.

In recent decades, chemiresistive gas sensors (CGS) have been widely studied due to their unique advantages of expedient miniaturization, simple fabrication, easy operation, and low cost. As one ubiquitous interference factor, humidity dramatically affects the performance of CGS, which has been neglected for a long time. With the rapid development of technologies based on gas sensors, including the internet of things (IoT), healthcare, environment monitoring, and food quality assessing, the humidity interference on gas sensors has been attracting increasing attention. Inspiringly, various anti-humidity strategies have been proposed to alleviate the humidity interference in this field; however, comprehensive summaries of these strategies are rarely reported. Therefore, this review aims to summarize the latest research advances on humidity-independent CGS. First, we discussed the humidity interference mechanism on gas sensors. Then, the anti-humidity strategies mainly including surface engineering, physical isolation, working parameters modulation, humidity compensation, and developing novel gas-sensing materials were successively introduced in detail. Finally, challenges and perspectives of improving the humidity tolerance of gas sensors were proposed for future research.

HighlightsThe types, working principles, advantages and limitations of pattern recognition methods based on chemiresistive gas sensor array are reviewed and discussed comprehensively.Outstanding and novel advancements in the application of machine learning methods for gas recognition in different important areas are compared, summarized and evaluated.The current challenges and future prospects of machine learning methods in artificial olfactory systems are discussed and justified.As information acquisition terminals for artificial olfaction, chemiresistive gas sensors are often troubled by their cross-sensitivity, and reducing their cross-response to ambient gases has always been a difficult and important point in the gas sensing area. Pattern recognition based on sensor array is the most conspicuous way to overcome the cross-sensitivity of gas sensors. It is crucial to choose an appropriate pattern recognition method for enhancing data analysis, reducing errors and improving system reliability, obtaining better classification or gas concentration prediction results. In this review, we analyze the sensing mechanism of cross-sensitivity for chemiresistive gas sensors. We further examine the types, working principles, characteristics, and applicable gas detection range of pattern recognition algorithms utilized in gas-sensing arrays. Additionally, we report, summarize, and evaluate the outstanding and novel advancements in pattern recognition methods for gas identification. At the same time, this work showcases the recent advancements in utilizing these methods for gas identification, particularly within three crucial domains: ensuring food safety, monitoring the environment, and aiding in medical diagnosis. In conclusion, this study anticipates future research prospects by considering the existing landscape and challenges. It is hoped that this work will make a positive contribution towards mitigating cross-sensitivity in gas-sensitive devices and offer valuable insights for algorithm selection in gas recognition applications.

Dissolved gas analysis (DGA) is considered to be the most convenient and effective approach for transformer fault diagnosis. Due to their excellent performance and development potential, chemiresistive gas sensors are anticipated to supersede the traditional gas chromatography analysis in the dissolved gas analysis of transformers. However, their high operating temperature and high power consumption restrict their deployment in battery-powered devices. This review examines the underlying principles of chemiresistive gas sensors. It comprehensively summarizes recent advances in low-power gas sensors for the detection of dissolved fault characteristic gases (H2, C2H2, CH4, C2H6, C2H4, CO, and CO2). Emphasis is placed on the synthesis methods of sensitive materials and their properties. The investigations have yielded substantial experimental data, indicating that adjusting the particle size and morphology structure of the sensitive materials and combining them with noble metal doping are the principal methods for enhancing the sensitivity performance and reducing the power consumption of chemiresistive gas sensors. Additionally, strategies to overcome the significant challenge of cross-sensitivity encountered in applications are provided. Finally, the future development direction of chemiresistive gas sensors for DGA is envisioned, offering guidance for developing and applying novel gas-sensitive sensors in transformer fault diagnosis.

Various catalysts are developed to improve the performance of metal oxide semiconductor gas sensors, but achieving high selectivity and response intensity in chemiresistive gas sensors (CGSs) remains a significant challenge. In this study, an in situ‐annealing approach to synthesize Cu catalytic sites on ultrathin WO 2.72 nanowires for detecting toluene at ultralow concentrations ( R a / R g = 1.9 at 10 ppb) with high selectivity is developed. Experimental and molecular dynamic studies reveal that the Cu single atoms (SAs) act as active sites, promoting the oxidation of toluene and increasing the affinity of Cu single‐atom catalysts (SACs)‐containing sensing materials for toluene while weakening the association with carbon dioxide or water vapor. Density functional theory studies show that the selective binding of toluene to Cu SAs is due to the favorable binding sites provided by Cu SAs for toluene molecules over other gaseous species, which aids the adsorption of toluene on WO 2.72 nanowires. This study demonstrates the successful atomic‐level interface regulation engineering of WO 2.72 nanowire‐supported Cu SAs, providing a potential strategy for the development of highly active and durable CGSs.

What are the main findings? A novel percolation-driven sensing mechanism in Sn/SnO nanostructures at room temperature is identified. Tin- and tin oxide-based sensors are ultra-sensitive to NO2, with the lowest tested concentration of 100 parts-per-billion at room temperature. What are the implications of the main findings? The sensor demonstrates parts-per-billion sensitivity while operating at room temperature. Sensitivity can be further improved by controlling the particle size in percolation-driven gas sensing. Monitoring air quality is crucial for understanding and improving public health. There is interest in developing ultra-sensitive, low-power, cost-effective sensors. This work demonstrates that structural modulation of Sn nanoparticles through controlled deposition and oxidation enables a transition from metallic to semiconducting percolative networks, significantly enhancing NO2 sensing performance at room temperature. The proposed percolation-driven sensing mechanism provides a new framework for understanding charge transport and gas interaction in nanostructured metal oxide systems. The nanoparticles are deposited near the percolation threshold for electrical conduction and, upon exposure to air, consist of a tin core and an amorphous Sn3O4 surface. Post-deposition heating in air at 320 °C for two hours forms SnO and Sn3O4 on top of the gold electrodes and polycrystalline SnO in the tetragonal litharge phase, known as Romarchite, on the glass between the electrodes. Both as-deposited and heat-treated sensors were capable of detecting NO2 at room temperature, with a limit of detection in the parts-per-billion range. A percolation model is used to explain their operating currents, in which NO2 reacts at nanoparticle gaps and intra-grain boundaries to form charge-depletion regions that primarily determine their resistance. Heat treatment has also been found to cause disproportionation of SnO, resulting in tin-rich precipitates and increasing the operating current to the milliampere range. These precipitates, although oxidized on their surfaces when exposed to air, may serve as bridges that reduce the total resistance of the percolating paths.