Dual-Mechanism Insights into the Peroxidase-like Activity of Co3O4 Nanoparticles: Nonradical and Superoxide Radical Catalysis.

Abstract

Although the Co3O4 nanozyme has been reported to exhibit peroxidase (POD) mimicking activity, its explicit catalytic mechanism remains indefinable. This study systematically investigates the POD-like catalytic mechanism of Co3O4 nanoparticles (NPs) through integrated experimental and theoretical approaches. The results reveal that their catalytic activity originates from dual synergistic pathways: the nonradical and the radical pathways. In the nonradical pathways, Co3O4 NPs mediate electron transfer from the substrate (e.g., 3,3',5,5'-tetramethylbenzidine, TMB) to H2O2 through the Co(III)/Co(II) redox couple, as its redox potential lies between that of TMB and H2O2. During the radical pathways, electron paramagnetic resonance (EPR) and fluorescent or UV-vis probe experiments demonstrate that H2O2 preferentially decomposes into superoxide radicals (O2•-) over hydroxyl radicals (•OH). Furthermore, density functional theory calculations reveal that H2O2 exhibits a relatively lower activation barrier (0.78 eV) to generate O2•- on the Co3O4 (110) facet, compared to the higher barrier (1.72 eV) for •OH formation. Additionally, the distinct degradation behaviors of organic dyes provide further validation of the proposed mechanism. This research will encourage further exploration into the catalytic mechanisms of nanozymes, thereby facilitating their rational design and application.