Transition metal-N-doped carbon has been demonstrated to mimic natural enzyme activity; in this study, cobalt–nitrogen co-doped carbon (Co-N-C) nanomaterial was developed, and it could be an oxidase mimic. Firstly, Co-N-C with oxidase-like activity boosts the chromogenic reaction of 3,3′,5,5′-tetramethylbenzidine (TMB) to produce the oxidized TMB (oxTMB). And the aromatic primary amino group of oxTMB reacts with nitrite (NO2−) to form diazo groups. Based on this background, we developed a cascade system of a Co-N-C-catalyzed oxidation reaction and a diazotization reaction for nitrite determination. The low detection limit (0.039 μM) indicates that Co-N-C is superior compared with the vast majority of previously reported nitrite assays. This study not only provides a novel nanozyme with sufficiently dispersed active sites, but it also further applies it to the determination of nitrite, which is expected to expand the application of nanozymes in colorimetric analysis.

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