Effects of Ultrasonic Irradiation and pH on the Enzymatic Activity and Biological Properties of Cerium Oxide Nanoparticles

Objective Cerium oxide nanoparticles (CeO 2 NPs), as metal oxide nanomaterials with multi-enzyme activities, are gradually being applied in the biomedical field. However, we have limited knowledge about the factors influencing the enzymatic activity and biological functions of cerium oxide nanoparticles. This study aims to investigate whether ultrasonic irradiation has a significant impact on their enzymatic activity, reactive oxygen species scavenging ability, and antibacterial activity, and to discuss the underlying mechanisms of these effects. Methods We first synthesized cerium oxide nanoparticles using a chemical precipitation method and characterized them. We set up a control group and irradiation groups with different ultrasonic parameters, and tested the mimicking activities of catalase, superoxide dismutase, and peroxidase of the cerium oxide nanoparticles using enzymatic activity assay kits. Then, we assessed the biocompatibility of the cerium oxide nanoparticles in macrophages. Subsequently, we examined their reactive oxygen species scavenging ability in both tumor cells and macrophages. Finally, we tested their inhibitory effect on Staphylococcus aureus . Results We found that cerium oxide nanoparticles exhibited enhanced enzymatic activity after ultrasound irradiation. The direction of their catalysis was regulated by pH: in environments with lower pH values, the peroxidase activity of the cerium oxide nanoparticles was stronger than that of catalase and superoxide dismutase, whereas in environments with higher pH values, the opposite was true. This pH-regulated multienzymatic activity resulted in significantly different reactive oxygen species (ROS) scavenging abilities between acidic tumor cells and slightly alkaline normal tissue cells: it demonstrated excellent ROS scavenging ability in macrophages but showed no ROS scavenging ability in tumor cells. Lastly, we discovered that the nanoparticles exhibited stronger antibacterial capabilities after ultrasound irradiation. Conclusion Ultrasound can enhance the enzymatic activity, ROS scavenging ability, and antibacterial capability of cerium oxide nanoparticles.