In summary, the catalytic mechanism of the POD-like activity of Fe3O4 nanozymes is elucidated by characterizing the chemical composition and catalytic activity of the Fe3O4 NPs recycled from the long-term POD-like catalysis. These studies indicate that not only the surface Fe2+, but also the internal Fe2+ contribute to the POD-like activity of Fe3O4 nanozymes. The Fe2+ inside the particle can transfer electrons to the surface, regenerating the surface Fe2+ that is constantly involved in the sustained catalytic reaction. This process is usually accompanied by the outward migration of excess oxidized Fe3+ from the interior of the crystal, which is considered as a rate-limiting step. Analogous to the low-temperature oxidation of magnetite, Fe3O4 NPs participated in the POD-like reaction are eventually oxidized to γ-Fe2O3 NPs with a reduced POD-like capacity. Furthermore, this mechanism is well-validated on LiFePO4 NPs. This work reveals the depletable property of Fe3O4 nanozymes differing from natural enzymes and highlights the potential contribution of internal metal atoms in nanozymes-catalyzed reactions. Meanwhile, these findings provide a theoretical basis for the mechanistic study and rational design of other related nanozymes.
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