Single-atom Co-N-C nanozyme with enhanced oxidase-like activity for smartphone-assisted colorimetric detection of sulfite.

A smartphone-assisted colorimetric sensing platform was developed for the sensitive detection of sulfites based on a cobalt single-atom nanozyme (Co-N-C). The nanozyme was synthesized via high-temperature pyrolysis of bimetallic Zn/Co-ZIF precursors, resulting in atomically dispersed Co active sites anchored on a nitrogen-doped carbon framework. Benefiting from the well-defined catalytic centers, Co-N-C exhibits exceptional oxidase-like activity, efficiently catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) into a blue product (oxTMB). The presence of sulfites triggers rapid discoloration through a dual mechanism involving reactive oxygen species (ROS) scavenging and the direct reduction of oxTMB. Under optimized conditions, the proposed assay demonstrates a wide linear range of 0.5–80 µM with a low detection limit of 0.066 µM. To eliminate potential interference from biothiols such as glutathione, a selective masking strategy using N-(2-hydroxyethyl)maleimide (HEMI) was introduced. Furthermore, integration with smartphone-based RGB analysis enables portable and instrument-free quantification. The platform was successfully applied to real food samples (white sugar, liquor, and bean vermicelli), yielding satisfactory recoveries of 92.6%-104.0%. This work highlights the potential of single-atom nanozymes in developing low-cost, portable sensing platforms for practical food safety monitoring.