Single-Atom Iron Nanozymes Anchored on Graphitic N‑Doped Carbon for Visual Alkaline Phosphatase Detection

The types of nitrogen atoms in single-atom nanozymes are of paramount importance to their enzyme-like catalytic activity. Herein, we report that single-atom iron nanozymes anchored on graphitic nitrogen-doped porous carbon (g-FeN4) could serve as efficient oxidase mimics. The g-FeN4 was found to display 1.6 times higher catalytic activity than pyridinic nitrogen-dominated single-atom iron nanozymes (FeN4). Combined with experiments and theoretical simulations, it is shown that graphitic nitrogen could effectively regulate the charge distribution at the Fe active site, thereby accelerating O2 activation and thus enhancing its oxidase-like activity. As a concept verification application, we constructed an ultrasensitive alkaline phosphatase activity assay method, integrating a smartphone as a colorimetric reader, based on the superior oxidase activity of g-FeN4. This study not only unravels the significant effect of nitrogen types on the activity of single-atom nanozymes but also provides important guidance for designing highly efficient single-atom nanozymes.