Free reactive oxygen species-independent dual enzymatic activity of iron single-atom catalyst for hydrogel-assisted portable visual analysis.

Since the enzymatic-like activity of FeO was reported, research on iron-based nanozymes has undergone vigorous development. However, most of previously reported iron-based nanozymes, including iron single-atom nanozymes, always rely on free reactive oxygen species (ROS) to exert their catalytic effects, especially the OH derived from Fenton-like reaction mediated by HO. In this study, we present an iron single-atom nanozyme (SA-FeNC) with catalytic mechanisms akin to that of natural cytochrome c oxidase and horseradish peroxidase. This nanozyme catalyzes the oxidation of substrates via a surface Fe(IV) = O intermediate pathway, without generating free ROS. Notably, SA-FeNC demonstrates exceptional catalytic activity in the absence of HO, and high concentration of HO is crucial for exhibiting peroxidase-like activity, which complements the toolbox of oxidase mimics. Leveraging this remarkable oxidase-like activity, colorimetric ascorbic acid assay with excellent analytical performance was established and further engineered into a portable gel/smartphone sensing platform, rendering it an attractive option for point-of-care detection of total antioxidant capacity detection. Furthermore, the development of an acid phosphatase detection-initiated colorimetric NAND logic gate is anticipated. This work not only opens up new horizons for the exploration of oxidase-like activities of Fe-based single-atom nanozymes, but also provides new ideas for the construction of portable gel sensing platforms and the coupling of nanozymes with information technology.