Surface-Mediated Production of Complexed •OH Radicals and Fe=O Species as a Mechanism for Iron Oxide Peroxidase-Like Nanozymes

Fe₃O₄ nanoparticles (NPs) with intrinsic peroxidase-like properties have attracted significant interest, although limited information is available on the definite catalytic mechanism. Here, it is shown that both complexed hydroxyl radicals (•OH) and high-valent Fe-O species are attributed primarily to the peroxidase-like catalytic activity of Fe₃O₄ NPs under acid conditions rather than only being caused by free •OH radicals generated through the iron-driven Fenton/Haber–Weiss reactions as previously thought. The low energy barrier of O-O bond dissociation of H₂O₂/•OOH (0.14 eV) and the high oxidation activity of surface Fe-O (0 eV) due to the reduced state of Fe on the surface of Fe₃O₄ NPs thermodynamically favor both the •OH and Fe-O pathways. By contrast, high-valent Fe-O species are the key intermediates in the catalytic cycles of natural peroxidase enzymes. Moreover, it is demonstrated that the enzyme-like activity of Fe₃O₄ NPs can be rationally regulated by modulating the size, surface structure, and valence of active metal atoms in the light of this newly proposed nanozyme catalytic mechanism.