Depletable peroxidase-like activity of Fe3O4 nanozymes accompanied with separate migration of electrons and iron ions

As pioneering Fe 3 O 4 nanozymes, their explicit peroxidase (POD)-like catalytic mechanism remains elusive. Although many studies have proposed surface Fe 2+ -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 Fe 2+ within Fe 3 O 4 can transfer electrons to the surface via the Fe 2+ -O-Fe 3+ chain, regenerating the surface Fe 2+ and enabling a sustained POD-like catalytic reaction. This process usually occurs with the outward migration of excess oxidized Fe 3+ from the lattice, which is a rate-limiting step. After prolonged catalysis, Fe 3 O 4 nanozymes suffer the phase transformation to γ-Fe 2 O 3 with 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.