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

As pioneering Fe3O4 nanozymes, 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 Fe3O4 can 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, Fe3O4 nanozymes suffer the phase transformation to gamma-Fe2O3 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. The mechanism of peroxidase-like Fe3O4 nanozymes remains elusive. Here, the authors show the electron transfer mechanism of Fe(II) ions to regenerate surface Fe(II) and the related phase transformation and depletion of activity.

Automated summary

The explicit peroxidase-like catalytic mechanism of Fe3O4 nanozymes remains unclear. In this study, the authors demonstrate the electron transfer mechanism of Fe(II) ions, which regenerate surface Fe(II) and are crucial for sustaining the peroxidase-like catalytic reaction. They also identify the phase transformation and depletion of activity after prolonged catalysis, highlighting the importance of considering both surface and internal atoms in the design and application of nanozymes.