An Atomic Insight into the Confusion on the Activity of Fe 3 O 4 Nanoparticles as Peroxidase Mimetics and Their Comparison with Horseradish Peroxidase.

Although Fe 3 O 4 nanoparticles were early reported to outperform horseradish peroxidase (HRP), recent studies suggested that this material bears a very poor activity instead. Here, we resolve this disagreement by reviewing the definition of descriptors used and provide an atomic view into the origin of Fe 3 O 4 nanoparticles as peroxidase mimetics. The redox between H 2 O 2 and Fe(II) sites on the Fe 3 O 4 surface was identified as the key step to producing OH radicals for the oxidation of colorimetric substrates. This mechanism involving free radicals is distinct from that of HRP oxidizing substrates with a radical retained on its Fe-porphyrin ring. Surprisingly, the distribution and chemical state of Fe species were found to be very different on single- and polycrystalline Fe 3 O 4 nanoparticles with the latter bearing not only a higher Fe(II)/Fe(III) ratio but also a more reactive Fe(II) species at surface grain boundaries. This accounts for the unexpected gap in the catalytic constant ( k cat ) observed for this material in the literature.