Insight into Several Factors that Affect the Conversion between Antioxidant and Oxidant Activities of Nanoceria

Many conflicting results have been reported related to the antioxidant and oxidant activities of nanoceria. On the basis of this research, many factors might affect the antioxidant activity of nanoceria. However, all of the factors reported only affect the antioxidant activity of nanoceria to a limited extent or cause the antioxidant activity to be lost. We found that several factors can induce conversion between the protective effect and toxicity of nanoceria. At low concentrations of hydroxyl radicals (•OH) and nanomaterials, nanoceria exhibited antioxidant activity but could produce greater amounts of •OH at higher •OH or nanomaterial concentrations and subsequently exhibit oxidant activity. Moreover, the morphology and size of nanoceria can also affect this conversion. We found that high concentrations of •OH and nanoceria could introduce a high amount of Ce<sup>3+</sup> in the system, which might be the reason that nanoceria converted from exhibiting antioxidant to oxidant activity. Under this condition, nanoceria act as a catalyst similar to Fe<sup>2+</sup> to promote •OH production in a Fenton system and also as a catalyst promoter to boost Fe<sup>2+</sup> production of additional •OH during the redox reaction. These conclusions support a better understanding of conflicting reports on medicinal applications for nanoceria and promote their practical application.