Abstract
Cerium oxide nanoparticles (CeNPs) possess multiple redox enzyme mimetic activities in scavenging reactive oxygen species (ROS) as a potential biomedicine. These enzymatic activities of CeNPs are closely related to their surface oxidation state. Here we have reported a synthetic method to modify CeNPs' surface oxidation state by changing the conformation of the poly(acrylic acid) (PAA) polymers adsorbed onto the CeNP surface. The synthesized PAA-CeNPs exhibited the same core size, morphology, crystal structure, and colloidal stability, with the only variation being their surface oxidation state (Ce3+ percentage). The modification mechanism can be attributed to the polymers chemisorbed onto the metal oxide surface forming a metal complexation structure. Such adsorption further modified CeNPs' surface oxidation state in a temperature-dependent manner. The series of PAA-CeNPs exhibited multiple redox enzyme mimetic activities (superoxide dismutase, catalase, peroxidase, and oxidase) directly related to their surface oxidation state. In vitro experiments showed no cytotoxic effect of these PAA-CeNPs on the osteoblastic cell line SAOS-2 at high loadings. Microscopic images confirmed the internalization of PAA-CeNPs in the cells. All tested PAA-CeNPs can reduce the basal and hydrogen peroxide-induced intracellular ROS level in the cells, indicating their effective intracellular ROS scavenging effect. However, we did not observe a positive correlation between the CeNP surface oxidation state and their capacities to reduce the intracellular ROS levels. We propose that CeNPs can maintain a dynamic state of Ce3+/Ce4+ during their catalytic activities, exhibiting a non-linear correlation between the CeNP surface oxidation state and their effect on regulating the intracellular ROS level.
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