As pioneering Fe₃O₄nanozymes, their explicit peroxidase (POD)-like catalytic mechanism remains elusive. Although many studies have proposed surface Fe²⁺-induced Fenton-like reactions accounting for their POD-like activity, few have focused on the internal atomic changes and their contribution to the catalytic reaction. Here we report that Fe²⁺within Fe₃O₄can transfer electrons to the surface via the Fe²⁺-O-Fe³⁺chain, regenerating the surface Fe²⁺and enabling a sustained POD-like catalytic reaction. This process usually occurs with the outward migration of excess oxidized Fe³⁺from the lattice, which is a rate-limiting step. After prolonged catalysis, Fe₃O₄nanozymes suffer the phase transformation to γ-Fe₂O₃with depletable POD-like activity. This self-depleting characteristic of nanozymes with internal atoms involved in electron transfer and ion migration is well validated on lithium iron phosphate nanoparticles. We reveal a neglected issue concerning the necessity of considering both surface and internal atoms when designing, modulating, and applying nanozymes.

Although Prussian blue nanozymes (PBNZ) are widely applied in various fields, their catalytic mechanisms remain elusive. Here, we investigate the long-term catalytic performance of PBNZ as peroxidase (POD) and catalase (CAT) mimetics to elucidate their lifespan and underlying mechanisms. Unlike our previously reported Fe₃O₄ nanozymes, which exhibit depletable POD-like activity, the POD and CAT-like activities of PBNZ not only persist but slightly enhance over prolonged catalysis. We demonstrate that the irreversible oxidation of PBNZ significantly promotes catalysis, leading to self-increasing catalytic activities. The catalytic process of the pre-oxidized PBNZ can be initiated through either the conduction band pathway or the valence band pathway. In summary, we reveal that PBNZ follows a dual-path electron transfer mechanism during the POD and CAT-like catalysis, offering the advantage of a long service life.

In this work, a very facile one-pot hydrothermal synthesis approach has been developed for the preparation of ultrathin magnetite nanoplates. The hydrothermal procedure was performed by aging ferrous hydroxide under anaerobic conditions, which is known as Schikorr reaction. Ethylene glycol (EG), which was introduced to the reaction as another solvent, played a critical role in the formation process of these nanoplates. Typically, hexagonal Fe₃O₄ nanoplates with a thickness of 10 to 15 nm and a side length of 150 to 200 nm have been synthesized with EG/H₂O = 1:1 in experiments. Our data suggest that the thickness of Fe₃O₄ nanoplates decreases, and the shape of the nanoplate becomes more irregular when the concentration of EG increases. The as-prepared Fe₃O₄ nanoplates were highly crystallized single crystals and exhibited large coercivity and specific absorption rate coefficient.