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2018) PbSe quantum dotsbased chemiresistors for room-temperature NO2 detection. Sensors and Actuators B: Chemical, 256. AbstractColloidal quantum dots (CQDs) are promising building blocks for low-cost and high-performance gas sensors due to their excellent solution processability and extremely small size. Among chalcogenide CQDs, PbSe has a large exciton Bohr radius and exhibits strong confinement energies, facilitating the fast charge-carrier transport. However, CQDs-based devices are susceptible to degrade due to the poor stability of CQDs. Here, in order to obtain air-stable PbSe CQDs for gas sensing application, we synthesized PbSe CQDs using a cation exchange method with in situ chloride and cadmium passivation. The sharp absorption peak in UV-vis absorption spectra confirmed strong quantum confinement in the PbSe CQDs and their average diameter was estimated to be 2.87 ± 0.23 nm. To construct gas sensors, PbSe CQDs were spin-coated onto ceramic substrates and then Pb(NO 3 ) 2 treatment was carried out to remove the long-chain ligands surrounding PbSe CQDs. At 25 °C, the sensor was highly sensitive and selective to NO 2 with a response of 22.3 at 50 ppm and a fast response time of 7 s. Moreover, the sensor response showed a 85.2% stability as the time increased up to 20 days, suggesting the potential applications of PbSe CQDs for NO 2 monitoring at room temperature.Highly sensitive gas sensors for detecting tiny amounts of toxic and hazardous gases play a vital role in our daily life for public safety, industrial processes, medical diagnosis, and continuous monitoring of environmental pollution [1,2]. To date, worldwide effort has been made to develop reliable gas sensors with high sensitivity and selectivity, and rapid progress has been achieved driven by the emerging nanoscience and nanotechnology [3][4][5].Recently, low-dimensional nanostructured materials with tailored structures have expressed great potential for serving as the sensing layer of gas sensors [6][7][8][9]. Their performance can be significantly enhanced through reducing the dimension of the gas-sensing materials. Specifically, zero-dimensional nanoparticles were extensively used to make gas sensors because of their unique electrical and optical properties. For example, Pd nanoparticles [10] were used in surface acoustic wave H 2 sensor, ZnO [11] and BaTiO 3 [12] nanoparticles were used for detection of formaldehyde and H 2 S, respectively. However, one of the major challenges faced in the development of nanoparticlebased gas sensors is the difficulty in obtaining various sizes of nanoparticles below 10 nm as well as the lack of precise control of their size distribution, because smaller particles are expected to have more gas adsorption sites.Colloidal quantum dots (CQDs), semiconductor nanoparticles synthesized and suspended in the solution phase, display certain merits due to their special structures and properties. First, being highly tunable material, CQDs possess flexible adjustability of sizes, with advantages of extremely small siz...