Asymmetrically Coordinated CoN5 Sites with Enhanced Oxidase-like Activity for Dual-Mode Colorimetric Sensing of Nitrite

The design of efficient and user-friendly methods for nitrite detection is of great significance owing to its critical role in food safety and environmental protection. Herein, we report a novel cobalt single-atom nanozyme (CoN 5 SA) featuring a highly asymmetric CoN 5 coordination environment. This structural configuration stabilizes high-spin Co 2+ species and significantly enhances the oxidase-like activity. Leveraging this nanozyme, a dual-mode colorimetric sensing platform was developed integrating ratiometric detection and smartphone-based visual readout. The ratiometric method exhibited a wide linear range of 0–80 μM with a low detection limit of 0.52 μM, while the smartphone-assisted method showed a linear range of 0–70 μM and a detection limit of 0.87 μM. Both modes exhibited outstanding selectivity, rapid response, and satisfactory recoveries (92.6–97.6%) in real food samples. This work not only elucidates the structure–activity relationship of CoN 5 single-atom sites but also presents a robust and user-friendly strategy for on-site nitrite detection.