  There is considerable interest in the pH-dependent switchable biocatalytic
properties of cerium oxide nanoparticles (CeNPs) in biomedicine, where these
materials exhibit beneficial antioxidant activity against reactive oxygen
species at neutral and basic physiological pH but cytotoxic prooxidant activity
at acidic pathological pH. Oxygen vacancies play a key role in such
biocatalytic activities. While the general characteristics of the role of
oxygen vacancies are known, the mechanism of their action at the atomic scale
under different pH conditions has yet to be elucidated. The present work
applies density functional theory (DFT) calculations to interpret the
pH-induced behavior of the stable {111} surface of CeO2 at the atomic scale.
Analysis of the surface-adsorbed media species reveals the critical role of pH
on the reversibility of the Ce3+ and Ce4+ redox equilibria and the formation
and annihilation of the oxygen vacancies. Under acidic conditions, this
reversible switching is hindered owing to incomplete volumetric filling of the
oxygen vacancies by the oxygen in the water molecules, hence effective
retention of the oxygen vacancies, and consequent inhibition of redox
biomimetic reactions. Under neutral and basic conditions, the capacity for this
reversible switching is preserved due to complete filling of the oxygen
vacancies by the OH ions owing to their ready size accommodation, thereby
retaining the capacity for performing redox biomimetic reactions cyclically.
