Disclosing the Origin of Transition Metal Oxides as Peroxidase (and Catalase) Mimetics

Since Fe₃O₄ was reported to mimic horseradish peroxidase (HRP) with comparable activity (2007), countless peroxidase nanozymes have been developed for a wide range of applications from biological detection assays to disease diagnosis and biomedicine development. However, researchers have recently argued that Fe₃O₄ has no peroxidase activity because surface Fe­(III) cannot oxidize tetramethylbenzidine (TMB) in the absence of H₂O₂ (cf. HRP). This motivated us to investigate the origin of transition metal oxides as peroxidase mimetics. The redox between their surface Mⁿ⁺ (oxidation) and H₂O₂ (reduction) was found to be the key step generating OH radicals, which oxidize not only TMB for color change but other H₂O₂ to produce HO₂ radicals for Mⁿ⁺ regeneration. This mechanism involving free OH and HO₂ radicals is distinct from that of HRP with a radical retained on the Fe-porphyrin ring. Most importantly, it also explains the origin of their catalase-like activity (i.e., the decomposition of H₂O₂ into H₂O and O₂). Because the production of OH radicals is the rate-limiting step, the poor activity of Fe₃O₄ can be attributed to the slow redox of Fe­(II) with H₂O₂, which is two orders of magnitude slower than the most active Cu­(I) among common transition metal oxides. We further tested glutathione (GSH) detection on the basis of its peroxidase-like activity to highlight the importance of understanding the mechanism when selecting materials with high performance.