Designing Hierarchically Porous Single Atoms of Fe-N5 Catalytic Sites with High Oxidase-like Activity for Sensitive Detection of Organophosphorus Pesticides

Recently, single-atom catalysts (SACs) have been used to construct biosensors for the determination of organophosphorus pesticides (OPs). However, most nanozymes including SACs are peroxidase-like enzymes and require highly toxic and unstable hydrogen peroxide (H₂O₂) as a co-reactant to generate reactive oxygen species. Inspired by the heme site of cytochrome c oxidases (Ccos), the construction of Fe-N₅-coordinated SACs by introducing axial N ligands is expected to bind O₂ to generate active metal-oxygen intermediates. Herein, a SAC with an Fe-N₅ active center confined by hierarchically porous carbon nanoframes (Fe SAs/N₅-pC-4) was prepared by a polymerization-pyrolysis-evaporation-etching strategy, and its underlying enzyme-like mechanism was uncovered through experiments and density functional theory calculations. The 100% metal atom utilization, increased accessible active sites, accelerated mass transfer, excellent hydrophilicity, and an electron-driven mechanism of axial N endow the SAC with enhanced oxidase-like activity. Notably, its catalytic rate constant (0.398 s^-1) is 569 times greater than that of the commercial Pt/C catalyst. Similar to the catalytic mechanism of Ccos, O₂ can be converted into reactive oxygen species, avoiding the use of co-reactant H₂O₂ effectively. In addition, based on the inhibitory effect of thiols on the active site of Fe SAs/N₅-pC-4, a biosensor was constructed and applied to the colorimetric analysis of OPs. This provides a facile, cost-effective method for efficient OP screening at sites to help control their contamination.