Achieving highly sensitive and selective detection of trace-level As­(III) and clarifying the underlying mechanism is still a intractable problem. The electroanalysis of As­(III) relies on the electrocatalytic ability of the sensing interface. Herein, we first adopt single-atom catalysts as the electrocatalyst in As­(III) detection. Cobalt single-atoms anchored on nitrogen-doped carbon material (Co SAC) were found to have an extraordinary sensitivity of 11.44 μA ppb–1 with excellent stability and repeatability, which so far is the highest among non-noble metal nanomaterials. Co SAC also exhibited a superior selectivity toward As­(III) compared with some bivalent heavy metal ions (HMIs). Combining X-ray absorption spectroscopy (XAFS), density functional theory (DFT) calculation, and reaction kinetics simulation, we demonstrated that Co single atoms stabilized in N2C2 support serve as active sites to catalyze H3AsO3 reduction via the formation of Co–O hybridization bond, leading to a lower energy barrier, promoting the breakage of As–O bonds. Importantly, the first electron transfer is the rate-limiting step of arsenic reduction and is found to be more favorable on Co-SAC both thermodynamically and kinetically. This work not only expands the potential applicaiton of single-atom catalysts in the detection and treatment of As­(III), but also provides atomic-level catalytic insights into HMIs sensing interfaces.

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