With the rapid development of sensor networks in the Internet of Things (IoT) and the Industrial Internet of Things (IIoT), gas sensors that can convert information of gas compositions and concentrations into electrical signals or other signals have become an indispensable sensing technology. The emergence of gas sensors has made gas detection and analysis technology easier, faster and smarter. In recent years, numerous scientists and engineers have continuously developed new gas sensitive materials, device equipment and detection technologies, which made gas sensors play an increasingly important role in environmental monitoring, gas warning, hazardous chemicals detection, food quality monitoring, medical and health fields. This special issue presents novel results in gas sensors, including metal oxide hybrid materials for gas sensing, structural design of metal oxide gas-sensitive materials, non-metal oxide gas-sensitive materials, device fabrication processes, and non-resistive gas sensor technology.

Metal oxide semiconductors-based gas sensors are the most widely used gas sensors owning to their excellent gas sensing performance, wide sources and low cost. Compounding with other materials with different band gap is an effective way to improve the gas sensing performance of metal oxide semiconductors. Noble metal nanoparticles can be used as chemical sensitizers and electronic sensitizers to improve the gas sensitivity of metal oxides. Wang et al. demonstrated a microwave-assisted hydrothermal method to prepare Pt/SnO₂ for CO sensing [1]. Luo et al. reported Pd modified ZnO porous nanosheets with enhanced CO sensing properties [2]. Li et al. realized synthesis and in-situ noble metal modification of WO₃·0.33H₂O nanorods from a tungstencontaining mineral, which shows enhanced NH₃ sensing performance [3]. Zhao et al. developed a Pd/SnO₂ material for ethylene sensing, which exhibits great potential in fruit quality detection [4]. Alloy noble metals show more abundant properties with single component noble metals. Deng et al. used ultra-low content bimetallic PtCu to modify WO₃·H₂O hollow sphere for highly sensitive acetone gas sensor [5]. Constructing heterojunctions like p-n, n-n or p-p heterojunctionscan effectively control the thickness of the depletion layer and the carrier concentration, which in turn affects the gas sensing performance. Ding et al. designed and prepared ZnO-ZnS nano-heterojunction sensing material for a high-sensitivity H₂S gas sensor [6]. Shang et al. constructed 3D laminated CuO/SnO₂ hybrid materials via a simple self-assembly method to detect triethylamine [7]. Zhang et al. demonstrated that porous Zn-CoO_x materials exhibits alcohols sensing performance at relative low operating temperature [8]. With the deepening of graphene research in recent decades, combining metal oxides with graphene has also been found to significantly improve gas sensing performance. Chen et al. prepared N-GQDs/SnO₂ nanocomposite for a formaldehyde gas sensor [9]. Tian et al. fabricated a NH₃ gas sensor based on reduced graphene oxide-porous In₂O₃ nanocubes hybrid nanocomposites which can work at room temperature [10].

The optimization of the micro-nano structure can also improve the gas sensing performance of the metal oxides materials.The hierarchical structure is one of the ideal structures of gas-sensitive materials due to its large specific surface area and abundant channels. Zhang et al. reported an efficient formaldehyde gas sensor based on metal-organic frameworks-derived ZnO materials with hierarchical structures [11]. Yang et al. designed and synthesized hierarchical NiCo₂O₄ microspheres assembled by nanorods with p-type response for a triethylamine sensitivesensor [12]. Zheng et al. synthesized hierarchical shell-core SnO₂ microspheres and applied them in gas sensors [13]. Zhang et al. prepared ZnSnO₃ microspheres through microwave-assisted strategy and used them for the detection of ethanol [14]. Two-dimensional (2D) structure has become one of the popular choices for gas-sensitive material structures recently due to its anisotropy and large specific surface area. Ahmed et al. explored ZnO hexagonal nanotablets for amines sensing [15]. The exposure of the crystal plane is found to be related to the sensing performance of the material, and the researchers have also begun to focus on regulating gas-sensing performance through regulation of exposure surface. Tian et al. demonstrated that the most exposed surface of hexagonal WO₃ (001) surface is oxygen vacancy O-terminated surface [16].

Some new progress has also been made in the research of non-metal oxides gas sensitive materials. Sulfides such as two-dimensional MoS₂, sulfide quantum dots, exhibit great potential for gas sensing. Yu et al. prepared two-dimensional molybdenum disulfide for NO₂ detection at room temperature [17]. Tan et al. synthesized PbS@MoS₂ composites for NO₂ detection via combining the mechanical exfoliation method with the facile wet-chemical precipitation [18]. Moreover, Liu et al. synthesized novel Cu₁₂Sb₄S₁₃ quantum dots@reduced graphene oxide nanosheet composites with high ammonia sensitive ability at room temperature [19]. Ji et al. designed RhIr@MoS₂ nanohybrids based disposable microsensor for the point-of-care testing of NADH in real human serum [20]. The gas-sensing properties of polymers are worth exploring because they are quite different from inorganic materials in conductivity, hydrophilicity/hydrophobicity and so on. Chen et al. reported tadpole-shaped, polyhedral oligomeric silsesquioxane containing sulfonated block copolymer with humidity sensing properties [21]. Chen et al. designed and prepared superhydrophobic hierarchical porous divinylbenzene polymer for BTEX sensing and toluene/water selective detection [22]. In addition, some other materials have been studied for sensing-related applications. Jin et al. fabricated linaloolfunctionalized hollow mesoporous silica spheres nanoparticles for the application about bacterial [23]. Liu et al. demonstrated the design and sensing mechanism of novel calix[6]arene composite for sensitively detecting amine drugs [24].

In addition to sensitive materials, the manufacturing process of the gas sensor devices can affect its performance. Optimizing and developing new gas sensor devices is another way to develop highperformance sensors. Growing the material in situ on the ceramic tube can solve the problem of insensitive contact between the sensitive material and the substrate. Meng et al. realized in-situ growth of V₂O₅ flower-like structures on ceramic tubes for detection of trimethylamine [25]. Flexible sensors are one of the development directions of wearable devices. Li et al. constructed a flexible and high-performance trimethylamine sensor via depositing In₂O₃ nanofibers on polyimide substrates [26].

The development of new gas sensing technology can fill the deficiencies of resistive gas sensing technology, which promoted the rapid application of gas sensors. Peng et al. reported fiber-optic dual Fabry-Pérot interferometric carbon monoxide sensor with polyaniline/Co₃O₄/graphene oxide sensing membrane [27]. Zhu et al. prepared functional ordered mesoporous carbons and applied them as the QCM sensor with ultra-low humidity [28]. Tang et al. demonstrated a H₂S gas sensor based on integrated resonant dual-microcantilevers with high sensitivity and identification capability [29].

Developing gas sensors with high sensitivity, fast response and recovery, high stability and high selectivity remains a challenging field. This special issue introduces readers to some new results on sensitive materials, devices and detection technologies of gas sensors, and hopes to bring readers some new insights in relate areas.

Finally, we would like to extend our most sincere thanks to the authors, reviewers and the editorial team of Chinese Chemical Letters for this great special issue.

Declaration of competing interest

The authors declare that they have no known competing financial interestsor personal relationships that could have appeared to influence the work reported in this paper.