Selenium Hydride-Induced Oxidase-like Activity Inhibition\nof Amorphous/Crystalline Manganese Dioxide: Colorimetric Assay for\nSelenium Detection

Hydride\ngeneration-based optical sensors have achieved on-site\nvisual selenium (Se) determination with high anti-interference capability,\nyet they rely on the change of single-color intensity with a narrow\nlinear dynamic range. Herein, we combined selenium hydride (H<sub>2</sub>Se)-induced activity inhibition of a manganese dioxide (MnO<sub>2</sub>) nanozyme with different degrees of 3,3′,5,5′-tetramethylbenzidine\n(TMB) oxidation to realize sensitive multicolor visual detection of\nSe(IV). Due to its high oxidase-like (OXD-like) activity and sensitive\nresponse to H<sub>2</sub>Se, amorphous/crystalline manganese dioxide\n(<i>ac</i>-MnO<sub>2</sub>) was selected to form the headspace\nsingle droplet for microextraction and recognition. <i>Via</i> headspace redox reaction with H<sub>2</sub>Se, <i>ac</i>-MnO<sub>2</sub> was reduced into low valence accompanied by in situ\ngeneration of Se nanoparticles, leading to the formation of a Se-MnO<sub><i>x</i></sub> aggregate. The experimental results and\ntheoretical calculation indicated that, compared with MnO<sub>2</sub>, Se-MnO<sub><i>x</i></sub> had decreased active sites\nfor adsorbing O<sub>2</sub> to generate <sup>•</sup>O<sub>2</sub><sup>–</sup>, resulting\nin the nanozyme activity inhibition that was totally dependent on\nSe(IV) concentration. The implementation of this strategy enabled\naccurate Se(IV) detection with a linear range from 10 to 600 μg\nL<sup>–1</sup> and a limit of detection of 1.8 μg L<sup>–1</sup>. The portable smartphone-based detection for real\nsample analysis further demonstrated that this assay can be an easy,\nconvenient, and intelligent tool for on-site selenium determination.