Depletable peroxidase-like activity of Fe(3)O(4) 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.