Formaldehyde, a probable carcinogen, is a ubiquitous indoor pollutant, but its highly selective detection has been a long-standing challenge. Herein, a chemiresistive sensor that can detect ppb-level formaldehyde in an exclusive manner at room temperature is designed. The TiO2 sensor exhibits under UV illumination highly selective detection of formaldehyde and ethanol with negligible cross-responses to other indoor pollutants. The coating of a mixed matrix membrane (MMM) composed of zeolitic imidazole framework (ZIF-7) nanoparticles and polymers on TiO2 sensing films removed ethanol interference completely by molecular sieving, enabling an ultrahigh selectivity (response ratio > 50) and response (resistance ratio > 1,100) to 5 ppm formaldehyde at room temperature. Furthermore, a monolithic and flexible sensor is fabricated successfully using a TiO2 film sandwiched between a flexible polyethylene terephthalate substrate and MMM overlayer. Our work provides a strategy to achieve exclusive selectivity and high response to formaldehyde, demonstrating the promising potential of flexible gas sensors for indoor air monitoring.

Noble metals or oxide catalysts have traditionally been loaded or doped to enhance the gas sensing properties of oxide semiconductor chemiresistors. However, the selective detection of various chemicals for a wide range of new applications remains a challenging problem. In this paper, we propose a novel bilayer design with an oxide chemiresistor sensing layer and nanoscale catalytic Au overlayer to provide high controllability for gas sensing characteristics. The Au nanocluster overlayer significantly enhances the methylbenzene response of a SnO2 thick film sensor by reforming gases into more reactive species and suppresses the responses to reactive interference gases through oxidative filtering, leading to excellent selectivity to methylbenzene. Gas sensing characteristics can be tuned by controlling the morphology, amount, and number density of Au nanoclusters through the variation in the Au coating thickness (0.5–3 nm) and thermal annealing conditions (0.5–4 h at 550 °C). Furthermore, the general validity of the proposed Au-coated bilayer sensor design was confirmed through the enhancement of response and selectivity toward methylbenzenes by coating Au nanoclusters onto ZnO and Co3O4 sensors. The sensing mechanism, advantages, and potential applications of bilayer sensors are discussed from the perspective of the separation of sensing and catalytic reactions, as well as the reforming and oxidation of analyte gases in association with the configuration of the sensing layer and Au catalytic overlayer.

Metal–organic frameworks (MOFs) with high surface area, tunable porosity, and diverse structures are promising platforms for chemiresistors; however, they often exhibit low sensitivity, poor selectivity, and irreversibility in gas sensing, hindering their practical applications. Herein, we report that hybrids of Cu 3 (HHTP) 2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) nanoflakes and Fe 2 O 3 nanoparticles exhibit highly sensitive, selective, and reversible detection of NO 2 at 20 °C. The key parameters to determine their response, selectivity, and recovery are discussed in terms of the size of the Cu 3 (HHTP) 2 nanoflakes, the interaction between the MOFs and NO 2 , and an increase in the concentration and lifetime of holes facilitated by visible-light photoactivation and charge-separating energy band alignment of the hybrids. These photoactivated MOF–oxide hybrids suggest a new strategy for designing high-performance MOF-based gas sensors.

A highly selective and sensitive detection of the plant hormone ethylene, particularly at low concentrations, is essential for controlling the growth, development, and senescence of plants, as well as for ripening of fruits. However, this remains challenging because of the non‐polarity and low reactivity of ethylene. Herein, a strategy for detecting ethylene at a sub‐ppm‐level is proposed by using oxide semiconductor chemiresistors with a nanoscale oxide catalytic overlayer. The SnO2 sensor coated with the nanoscale catalytic Cr2O3 overlayer exhibits rapid sensing kinetics, good stability, and an unprecedentedly high ethylene selectivity with exceptional gas response (Ra/Rg − 1, where Ra represents the resistance in air and Rg represents the resistance in gas) of 16.8 at an ethylene concentration of 2.5 ppm at 350 °C. The sensing mechanism underlying the ultraselective and highly sensitive ethylene detection in the unique bilayer sensor is systematically investigated with regard to the location, configuration, and thickness of the catalytic Cr2O3 overlayer. The mechanism involves the effective catalytic oxidation of interfering gases into less‐ or non‐reactive species, without limiting the analyte gas transport. The sensor exhibits a promising potential for achieving a precise quantitative assessment of the ripening of five different fruits.

Volatile aromatic compounds are major air pollutants, and their health impacts should be assessed accurately based on the concentration and composition of gas mixtures. Herein, novel bilayer sensors consisting of a SnO2 sensing layer and three different xRh‐TiO2 catalytic overlayers (x = 0.5, 1, and 2 wt%) are designed for the new functionalities such as the selective detection, discrimination, and analysis of benzene, toluene, and p‐xylene. The 2Rh‐TiO2/SnO2 bilayer sensor shows a high selectivity and response toward ppm‐ and sub‐ppm‐levels of benzene over a wide range of sensing temperatures (325–425 °C). An array of 0.5Rh‐, 1Rh‐, and 2Rh‐TiO2/SnO2 sensors exhibits discrimination and composition analyses of aromatic compounds. The conversion of gases into more active species at moderate catalytic activation and the complete oxidation of gases into non‐reactive forms by excessive catalytic promotion are proposed as the reasons behind the enhancement and suppression of analyte gases, respectively. Analysis using proton transfer reaction‐quadrupole mass spectrometer (PTR‐QMS) is performed to verify the above proposals. Although the sensing characteristics exhibit mild moisture interference, bilayer sensors with systematic and tailored control of gas selectivity and response provide new pathways for monitoring aromatic air pollutants and evaluating their health impacts.

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Water poisoning, the dependence of gas‐sensing characteristics on moisture, in oxide chemiresistors remains a long‐standing challenge. Various approaches are explored to mitigate water poisoning but they are often accompanied by significant deterioration of sensing capabilities such as gas response deterioration, gas selectivity alteration, and sensor resistance increase up to unmeasurable levels. Herein, a novel sensor design with a moisture‐blocking Tb4O7 overlayer is suggested as a facile and universal strategy to remove moisture poisoning without sacrificing intrinsic sensing properties. A submicrometer‐thick coating of Tb4O7 overlayer on In2O3 sensors effectively eliminates the humidity dependence of the gas‐sensing characteristics without significantly altering the gas response, selectivity, and sensor resistance. Furthermore, the general validity of the water‐blocking effect using the Tb4O7 overlayer is confirmed in diverse gas sensors using SnO2, ZnO, and Pd/SnO2. The negligible moisture interference of the bilayer sensor is explained in terms of the hydrophobic nature of the Tb4O7 overlayer and the prevention of formation of the OH radical by the interaction between Tb4O7 and In2O3. A universal solution to design diverse humidity‐independent gas sensors with different gas selectivities can open up new pathways toward building accurate and robust gas sensors with new functionalities and high‐performance artificial olfaction.

The highly selective detection of trace gases using transparent sensors at room temperature remains challenging. Herein, transparent nanopatterned chemiresistors composed of aligned 1D Au–SnO2 nanofibers, which can detect toxic NO2 gas at room temperature under visible light illumination is reported. Ten straight Au–SnO2 nanofibers are patterned on a glass substrate with transparent electrodes assisted by direct‐write, near‐field electrospinning, whose extremely low coverage of sensing materials (≈0.3%) lead to the high transparency (≈93%) of the sensor. The sensor exhibits a highly selective, sensitive, and reproducible response to sub‐ppm levels of NO2, and its detection limit is as low as 6 ppb. The unique room‐temperature NO2 sensing under visible light emanates from the localized surface plasmonic resonance effect of Au nanoparticles, thereby enabling the design of new transparent oxide‐based gas sensors without external heaters or light sources. The patterning of nanofibers with extremely low coverage provides a general strategy to design diverse compositions of gas sensors, which can facilitate the development of a wide range of new applications in transparent electronics and smart windows wirelessly connected to the Internet of Things.