Deciphering the Catalytic Mechanism of Peroxidase-like Activity of Iron Sulfide Nanozymes.

Iron sulfide nanomaterials represented by FeS 2 and Fe 3 S 4 nanozymes have attracted increasing attention due to their biocompatibility and peroxidase-like (POD-like) catalytic activity in disease diagnosis and treatments. However, the mechanism responsible for their POD-like activities remains unclear. Herein, taking the oxidation of 3,3,5,5-tetramethylbenzidine (TMB) by H 2 O 2 on FeS 2 (100) and Fe 3 S 4 (001) surfaces, the catalytic mechanism was investigated in detail using density functional theory (DFT) calculations and experimental characterizations. Our experimental results showed that the catalytic activity of FeS 2 nanozymes was significantly higher than that of Fe 3 S 4 nanozymes. Our DFT calculations indicated that the surface iron ions of iron sulfide nanozymes could effectively catalyze the production of HO• radicals via the interactions between Fe 3d electrons and the frontier orbitals of H 2 O 2 in the range of -10 to 5 eV. However, FeS 2 nanozymes exhibited higher POD-like activity due to the surface Fe(II) binding to H 2 O 2 , forming inner-orbital complexes, which results in a larger binding energy and a smaller energy barrier for the base-like decomposition of H 2 O 2 . In contrast, the surface iron ions of Fe 3 S 4 nanozymes bind to H 2 O 2 , forming outer-orbital complexes, which results in a smaller binding energy and a larger energy barrier for the base-like decomposition of H 2 O 2 . The charge transfer analysis showed that FeS 2 nanozymes transferred 0.12 e and Fe 3 S 4 nanozymes transferred 0.05 e from their surface iron ions to H 2 O 2 , respectively. The simulations were consistent with the experimental observations that the FeS 2 nanozymes had a greater affinity for H 2 O 2 compared to that of Fe 3 S 4 nanozymes. This work provides a theoretical foundation for the rational design and accurate preparation of iron sulfide functional nanozymes.