Axial modulation of Fe sites for boosted electrochemical oxidation.

Fe/NC single-atom catalysts have attracted extensive attention due to their maximal atomic utilization and tunable coordination environments. However, the structure-activity relationship of Fe single atoms in electrooxidation remains unclear. Herein, we report a defect engineering strategy to fine-tune the charge configuration of FeN₄ sites by introducing an axial N ligand and constructing FeN₅-Fe₁/NC. This asymmetric coordination environment enhances the catalytic activity for dopamine (DA) oxidation, delivering a 2.1-fold improvement over traditional Fe₁/NC. The FeN₅-Fe₁/NC biosensor exhibits a wide linear detection range of 0.05-500 μM with a low detection limit of 23 nM for DA. Additionally, theoretical calculations confirm that axial N coordination modulates the electronic structure of the Fe center, optimizes intermediate adsorption, and lowers the energy barrier for DA oxidation. This work provides valuable insights into the rational design of single-atom catalysts for high-performance electrochemical sensing and fundamental mechanistic studies at the atomic scale.