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Abstract
Cerium oxide nanoparticles (nanoceria) have recently been shown to protect cells against
oxidative stress in both cell culture and animal models. Nanoceria has been shown
to exhibit superoxide dismutase (SOD) activity using a ferricytochrome C assay, and
this mimetic activity that has been postulated to be responsible for cellular protection
by nanoceria. The nature of nanoceria's antioxidant properties, specifically what
physical characteristics make nanoceria effective at scavenging superoxide anion,
is poorly understood. In this study electron paramagnetic resonance (EPR) analysis
confirms the reactivity of nanoceria as an SOD mimetic. X-ray photoelectron spectroscopy
(XPS) and UV-visible analyses of nanoceria treated with hydrogen peroxide demonstrate
that a decrease in the Ce 3(+)/4(+) ratio correlates directly with a loss of SOD mimetic
activity. These results strongly suggest that the surface oxidation state of nanoceria
plays an integral role in the SOD mimetic activity of nanoceria and that ability of
nanoceria to scavenge superoxide is directly related to cerium(III) concentrations
at the surface of the particle.