Characterizing the phosphatase mimetic activity of cerium oxide nanoparticles and distinguishing its active site from that for catalase mimetic activity using anionic inhibitors

Cerium oxide nanoparticles (CeNPs) are potent reactive oxygen and nitrogen species scavengers and demonstrate beneficial antioxidant properties in both cell culture and animal studies. However, their environmental fate, particularly in animals, is still under investigation. Studies have shown that CeNPs at very high doses can be retained briefly in organs such as the liver and in the bone marrow. The interaction of these nanoparticles with their local environment plays a major role in their distribution and long-term stability. We have previously shown that CeNPs with a low 3+/4+ cerium oxidative state ratio exhibit both catalase and phosphatase mimetic activities. Here, we aimed at further characterizing the active site(s) involved in these catalytic activities using potentially inhibitory anions. Results indicated that tungstate and molybdate inhibited the phosphatase activity without altering the oxidative state of cerium atoms but were ineffective against catalase activity. This suggests that distinct chemistry and active sites are involved in these two catalytic activities. Additionally, it was observed that CeNPs in aqueous environments were more active, strongly suggesting that water plays an important role in the phosphatase activity. Given the abundance of phosphate and other metal anions in both tissues and the environment, studying the nature of catalytic activities of CeNPs and the surface chemistry involved will help us form a stronger understanding of their environmental fate and thus qualify their biomedical applications.