Axial Nitrogen Coordination Engineering of Fe Single-Atom Catalyst for Enhanced Peroxidase-like Activity

Mimicking the hierarchical structure as well as the asymmetric Fe-N₅ sites in natural horseradish peroxidase (HRP) is of great importance in developing Fe₁/CN with high peroxidase-like (POD-like) activity. In this work, Fe₁/CN with an asymmetric FeN₅ moiety and ordered porous structure (FeN₅/CN) is fabricated by an ammonia-assisted redispersion strategy, which shows high structural similarity with HRP. Therefore, FeN₅/CN shows an excellent catalytic efficiency (specific activity = 117.9 U/mg, k_cat/K_m = 2185 mM^-1 s^-1) and selectivity (K_m = 0.059 mM) in a POD-like reaction. Based on the high catalytic properties of FeN₅/CN, a sensor for the detection of carbosulfan with a low limit of detection of 3.1 nM is assembled. Interestingly, FeN₅/CN activates H₂O₂ via a superoxide pathway, while ·OH, ¹O₂, and ·O₂^- can all be detected in the FeN₄/CN involved catalytic system. Mechanistic study by density functional theory calculations combined with experimental results illustrates that Fe-N₅ sites provide moderate adsorption of *OH, enlarging and decreasing the reaction energy to form ·OH and ·O₂^-, respectively, while Fe-N₄ sites exhibited higher affinity toward the OH* intermediate, resulting in the facile O-O bond cleavage from H₂O₂ molecule and prohibited the process of *OH desorption to ·OH.