Boosted peroxidase-like activity of metal-organic framework nanoparticles with single atom Fe(Ⅲ) sites at low substrate concentration

Single atom nanomaterials possess catalytic activity like natural enzymes are termed as SAzymes which have gained great attention during last two years because of the maximal utilization of atoms and the benefit of understanding structure-property relationship. However, most of SAzymes are fabricated based on hydrophobic carbon, which disperse poorly in water and exhibit inferior affinity towards substrates, which may limit their biomedical applications. Here, we report a peroxidase-like SAzyme through the post-modification route based on hydrophilic defective metal-organic frameworks. Hydrochloric acid (HCl) is employed as ligand modulator to fabricate defective NH₂-UiO-66 nanoparticles (HCl-NH₂-UiO-66 NPs). Compared with the NPs fabricated through acetic acid modulation method (Ac-NH₂-UiO-66 NPs), HCl-NH₂-UiO-66 NPs have more missing linkers. Hence, more Fe(Ⅲ) ions can be successfully doped onto Zr₆ clusters in HCl-NH₂-UiO-66 NPs in a single atom state via formation of Fe-O-Zr bridge. The HCl-NH₂-UiO-66 NPs doped with Fe(Ⅲ) ions (Fe-HCl-NH₂-UiO-66 NPs) possess higher peroxidase-like activity than Fe-Ac-NH₂-UiO-66 NPs due to the higher loading amount of Fe. Besides, both Fe-HCl-NH₂-UiO-66 NPs and Fe-Ac-NH₂-UiO-66 NPs exhibit lower Michaelis-Menten constants (K_m) for hydrogen peroxide (H₂O₂) than most reported nanomaterials, indicating their higher affinity to H₂O₂. Due to their excellent catalytic activity to low concentration of substrates, Fe-HCl-NH₂-UiO-66 NPs can detect H₂O₂ with a limit of detection (LOD) of 1.0 μM. Thus, our system can be used to detect the low cellular H₂O₂ concentration. With high peroxidase-like activity induced by plenty of single atom Fe(Ⅲ) sites, Fe-HCl-NH₂-UiO-66 NPs can also find wide applications in other fields including nanomedicine, pollution degradation and catalysis.