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Abstract: In the current work, CuO nanoparticles were synthesized in a facile way, and characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetry. Using these CuO nanoparticles, CuO functionalized QCM resonators were fabricated and explored for HCN sensing. The sensing properties were studied by a sensor characterization system coupled with a mass spectrometer. The sensor response to HCN (+347Hz) was found to be in an opposite direction as compared with other common volatile substances (ether: −1230Hz; water: −1815Hz; n-hexane: −2100Hz; benzene: −3410Hz; acetic acid: −4840Hz; ethanol: −6270Hz), offering excellent selectivity for HCN detection. In addition, the sensitivity (15.1Hz/μg) was very high, and the response (30s) and recovery (750s) were very fast. A sensing mechanism was proposed based on experimental results, in which a surface redox reaction occurs between CuO and Cu2O on the nanoparticle reversibly upon contact with HCN and air, respectively. The current results would provide an exciting alternative to fast, sensitive and selective detection of trace HCN, which would be of particular benefit in the area of public security and environmental applications. [Copyright &y& Elsevier]