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There is considerable interest in the pH-dependent, switchable, biocatalyticproperties of cerium oxide (CeO2) nanoparticles (CeNPs) in biomedicine, wherethese materials exhibit beneficial antioxidant activity against reactive oxygenspecies (ROS) at basic physiological pH but cytotoxic prooxidant activity inacidic cancer cell pH microenvironment. While the general characteristics ofthe role of oxygen vacancies are known, the mechanism of their action at theatomic scale under different pH conditions has yet to be elucidated. Thepresent work applies density functional theory (DFT) calculations to interpret,at the atomic scale, the pH-induced behavior of the stable {111} surface ofCeO2 containing oxygen vacancies. Analysis of the surface-adsorbed mediaspecies reveals the critical role of pH on the interaction between ROS and thedefective CeO2 {111} surface. Under basic conditions, the superoxide dismutase(SOD) and catalase (CAT) biomimetic reactions can be performed cyclically,scavenging and decomposing ROS to harmless products, making CeO2 an excellentantioxidant. However, under acidic conditions, the CAT biomimetic reaction ishindered owing to the limited reversibility of Ce3+ and Ce4+ and formation andannihilation of oxygen vacancies. A Fenton biomimetic reaction is predicted tooccur simultaneously with the SOD and CAT biomimetic reactions, resulting inthe formation of hydroxyl radicals, making CeO2 a cytotoxic prooxidant.

DFT Investigation of Biocatalytic Mechanisms from pH-Driven, Multi-Enzyme, Biomimetic Behavior in CeO2