Advanced Chemosensors for Gas Detection

1. Introduction

The exploration of gas sensing technologies lies at the forefront of modern scientific and technological advancements [1,2,3,4,5]. Chemosensors are powerful and indispensable tools that enable the detection and quantification of chemical substances in diverse gas environments, serving as tools for environmental monitoring, industrial safety, healthcare, food sanitation supervision, and more [6,7,8]. The demand for advanced chemosensors capable of detecting trace gases with high sensitivity, selectivity, and real-time responsiveness is ongoing. In recent years, a large number of advanced materials and techniques have been developed in this field [9,10,11,12,13,14].

2. The Special Issue

This Special Issue comprises fifteen high-quality original research papers and two comprehensive review papers, all focused on the latest advances and innovative applications of chemosensors for gas detection. In brief, it covers novel gas-sensitive materials (inorganic, organic, hybrid, composite, etc.), together with their synthesis and characterization, advanced sensor processing technology, special architectural or theoretical design, multi-dimensional performance evaluation and optimization, and their applications or even promising prospects for the future.

On the one hand, the design of inorganic semiconductor composite material systems, especially employing metal-based oxides and heterostructure architectures [15,16,17,18], holds significant value and abundant use for the detection of gaseous analytes by chemosensors. Optimized designs based on composition, form, morphology, etc., could enhance sensor performance (such as increasing sensitivity, improving selectivity, accelerating response speed, and enhancing stability), broaden the application scope to accommodate various gaseous analytes, or meet different environmental conditions. For example, Xiaofeng Wang et al. [19] prepared a n-p type SnS₂ nanosheet/LaFeO₃ nanoparticle composite using a hydrothermal method together with the sol–gel technique. The optimized chemosensor can be operated at 140 °C and demonstrated strong stability, selectivity, and long-term durability for triethylamine (TEA) vapor detection. Hui Xu et al. [20] constructed a 2D core–shell TiO₂@MoS₂ composite with n-n heterostructures. This chemosensor can monitor trace toluene at 240 °C, exhibiting a superior response (Ra/Rg = 9.8 to 10 ppm), rapid response/recovery kinetics (9 s/16 s), a low detection limit (50 ppb), and excellent selectivity against interfering gases and moisture. Larissa Egger et al. [21] reported a complementary metal oxide semiconductor (CMOS)-integrated SnO₂ thin film gas chemosensor functionalized with mono-, bi-, and trimetallic (Ag-, Pd-, and Ru-) nanoparticles. It can detect carbon monoxide (CO) and a specific hydrocarbon mixture in a concentration range of 5–50 ppm. Moreover, the use of CMOS chips gives such low-power and integrated sensors a chance to be applied in cell phones, watches, etc., which can be used for air quality monitoring at different temperatures and humidity levels. Takeo Hyodo et al. [22] introduced an adsorption/combustion-type microsensor using 5 wt% Pt-loaded aluminas (α-Al₂O₃ and γ-Al₂O₃) with catalytic effect and thermal conductivity to determine the dynamic response of ethanol and toluene vapor in air. Xavier Vilanova and Eduard Llobet et al. [23] studied a selective chemosensor based on reduced graphene oxide (GO)@MnO₂ deposited on different substrates. The sensor deposited on Kapton showed the highest response of 6.6% towards 1 ppm of NO₂ under dry conditions at room temperature, while the sensor on silicon showed the highest response of 18.5% towards 50 ppm of NH₃ under 50% relative humidity (RH) at room temperature. Cristian Eugen Simion et al. [24] realized the sensitive CO₂ response under in-field-like conditions by using SnO₂-deposited Al₂O₃ substrates with Pt-based interdigital electrodes with gaps of 100 µm and 30 µm. Chris Blackman et al. [25] prepared a Ti-doped p-type Cr₂O₃ thin film sensor via an aerosol-assisted chemical vapor deposition method. It can monitor the flammable isobutylene (C₄H₈) and toxic NH₃, showing a reversible response and good sensitivity, which may be explained by the variations in the oxygen vacancy concentration. Héctor Guillén-Bonilla, et al. [26] emphasized the bifunctional use (gas sensing and photocatalytic dye degradation) of ZnSb₂O₆ pellets. Khursheed Ahmad and Tae Hwan Oh et al. [27] provided a special review on layered double hydroxide (LDH)-based gas and electrochemical sensors that are capable of monitoring toxic and hazardous gases/compounds due to their layered structure, larger surface area, decent conductivity, and excellent electrochemical properties.

On the other hand, the organic–inorganic composite materials combine the characteristics of organic and inorganic chemosensing materials, enabling more effective capture of gaseous analytes and generating stronger signal responses, thereby enhancing their response sensitivity [28]. Organic materials also can enhance their ability to recognize target gases through molecular design, and the combination of the organic and inorganic feature can reduce signal interference from other gases, achieving selective detection of specific analytes. Moreover, the interaction between the organic and inorganic components in the composite material can promote charge transfer and substance transport, accelerating its response speed to gaseous analyte and improving the real-time monitoring capability. By reasonably designing the composition and structure of the composite material, it can be optimized for different gaseous analytes, enabling the detection of multiple gases and meeting the requirements of different application scenarios and environmental conditions. For example, Thomas Strunskus, Franz Faupel, and Stefan Schröder et al. [29] demonstrated the optimized selective sensing performance and long-term stability (exhibiting high reliability even for more than 427 days) of TiO₂-coated ultra-thin copolymer films of poly(trivinyltrimethylcyclotrisiloxane-co-tetrafluoroethylene) (P(V3D3-co-TFE)), even in a high humid environment and under an optimized operating temperature of 300 to 350 °C. Sadam Hussain and Mujahid Mehdi et al. [30] detected various alcohol species (methanol, ethanol, propanol, butanol, and pentanol) with a twisted polymer optical fiber (POF) sensor. Such a refractive index sensor represents a significant leap forward in optical sensing technology. Yunbo Shi et al. [31] reported polyaniline (PANI)/black phosphorus (BP), which can sensitively monitor NO₂ within 2–60 ppm at room temperature.

In addition to the research on material systems, researchers are also constantly improving the design, manufacturing, and application technologies of chemosensors for gas detection. For instance, Domenico Suriano et al. [32] integrated a miniaturized chemosensor with a smartphone for mobile monitoring of the frequently changing atmospheric environment, including CO and typical indicators of air quality. The maximum response value for the indoor CO and NO₂ concentrations was 12.3 ppm and 64 ppb, respectively, while for mobile measurements, the maximum concentrations were 8.3 ppm and 38 ppb, respectively. Cristian Viespe et al. [33] proposed a model to assess the frequency–amplitude characteristics of surface acoustic wave (SAW) oscillator, aiming to increase the robustness and interpretability of the sensing behavior.

Moreover, breath analysis technology, typically electronic noses (e-noses), has advantages for obtaining key information of gaseous analytes through non-invasive, real-time, and convenient methods, with great significance in multiple aspects, including medical health, environmental monitoring, public security, and other fields [34,35,36,37]. Compared to traditional gas analysis methods, e-noses are more efficient and accurate when using sensor array technology and pattern recognition algorithms [38]. For example, Bingqiang Cao and Chenyu Jiang et al. [39] investigated an integrated breath gas detection system based on cavity ring-down spectroscopy (CRDS). The ring-down time, detection sensitivity, and stability of this system for breath acetone were 1.068 μs, 1 ppb, and 0.13%, respectively. Sonia Freddi et al. [40] fabricated graphene-based sensors functionalized with unconventional in-house synthesized zinc and copper octyl-pyrazinoporphyrazines and commercially available zinc phthalocyanine, which demonstrated excellent performance for detecting NH₃, benzene, and H₂S as a single sensor. After being assembled into an e-nose, it showed remarkable discrimination capability of single gases and their mixtures. Wufan Xuan and Shuai Chen et al. [41] summarized the progress on screening and diagnosis technologies for pneumoconiosis, known as one of the most serious global occupational diseases, emphasizing the application prospect of chemosensing strategies like e-noses compared to conventional and wide-used imaging analysis ways.

In summary, these of chemosensors can provide highly sensitive and selective response to diverse gaseous chemicals via optical, electrical, electrochemical signals, etc. With the advancement of material science, processing technology, miniaturized or integrated devices, Internet of Things (IoT), emerging artificial intelligence (AI) and machine learning (ML) concepts, etc., gas chemosensors will usher in a future of growth. This Special Issue may promote increased interest or research in related fields, bringing together emerging sensing material, device innovation, and commercial applications.