Figure 1. Hypothesized Mechanism of Action of CeONPs.

In a given CeONP, the cerium atom exists in the 3+ and 4+ valence states, bound to oxygen and containing oxygen vacancies (O_v). When exposed to a superoxide radical, it exhibits SOD-mimetic activity, and Ce3+ is converted to Ce4+, with a corresponding change in oxygen vacancies. There is also likely a contribution to this reaction from the hydration shell around the CeONP. Superoxide is converted to H₂O₂. Via a catalase-mimetic activity involving Ce4+, H₂O₂ is converted to O₂ + 4H⁺, and cerium valence to +3 (with corresponding changes in oxygen vacancies), regenerating the origin CeONP state. Again, there is a likely contribution from ions present in the water hydration shell. In the biological milieu, this action exists in a continuous cycle, depending on the ionic species exposed to the CeONPs, the hydration shell, oxygen partial pressure, and any surrounding ionic species. Although we utilized superoxide and H₂O₂ as examples, radicals scavenged could be any number of biologically relevant free radicals.