The alarming rise of indoor pollution and the need to combat the associated negative effects have promoted increasing attention in modernizing the chemical sensing technologies by newly designed materials with rich and tunable functionalities at atomic or molecular levels. With the appealing physical, chemical, optical, and electronic properties for various potential applications, the state-of-art gas-sensing nanomaterials and their future perspectives are well-documented and summarized in this paper. Specifically, the key performance attributes are addressed in detail such as the sensitivity, selectivity, reversibility, operating temperature, response time, and detection limit. As such, this review provides both critical insights in exploring and understanding various gas sensing nanomaterials and points out limitations and opportunities for further developments, such as morphology control, doping and surface alteration, atomic-scale characterization, and applications in different fields. Finally, the challenges and outlooks are discussed on the basis of the current developments.

The Pd–WO ₃ /m-CN sensor, exhibits enhanced response/recovery times to 25 ppm VOCs and formaldehyde under fluctuating humidity conditions.

Truxene is employed as a building block to successfully synthesize novel covalent organic frameworks (COFs). The condensation reaction between truxene (10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluorene, TX) and 1,4-phenylenediboronic acid (DBA) results in a crystalline COF with boron ester linkages (COF-TXDBA) and a surface area of 1526 m² g⁻¹, as confirmed by powder X-ray diffraction (PXRD) and Brunauer–Emmett–Teller (BET) surface area measurements. This is the first study where nanochannels generated by periodic COF planar layers are shown to ease the interactions of the boron ester linkages with the water molecules for efficient humidity sensing. The COF-TXDBA based % RH sensor exhibits a change of 3 orders of magnitude in impedance in the 11–98% RH range, with response and recovery times of 37 s and 42 s, respectively. The response transients measured by switching COF-TXDBA sensor back and forth in 4 loops of % RH range displays excellent reversibility of the sensor with a deviation of 2.3% in the switching process.

A highly sensitive and fast responsive relative humidity (% RH) sensor based on In–SnO₂ loaded cubic mesoporous graphitic carbon nitride (g-C₃N₄) has been demonstrated in this study. The mesoporous In–SnO₂/meso-CN nanohybrid was synthesized through template inversion of mesoporous silica, KIT-6, using a nanocasting process. Due to its 3D replicated cubic structure with ordered mesopores, the nanohybrid facilitates the process of adsorption, charge transmission and desorption of water molecules across the sensor surfaces. Consequently, the optimized In–SnO₂/meso-CN nanohybrid exhibits excellent response (5 orders change in impedance) in the 11–98% RH range, high stability, negligible hysteresis (0.7%) and superior real time % RH detection performance. Compared to traditional metal oxide based resistive sensors with unique mesoporous/hierarchical/sheet-like morphology, the 3D mesostructured In–SnO₂/meso-CN nanohybrid demonstrated a superfast response (3.5 s) and recovery (1.5 s) in the 11–98% RH range at room temperature. These results open the door for breath monitoring and show a promising glimpse for designing mesoporous 2D layered materials in the development of future ultra-sensitive % RH sensors.

Various MoO₃ nanostructures for gas sensing applications.