Unveiling the Role of Intracellular Dissolution Equilibria in the Antioxidant Mechanism of Ceria Nanoparticles

It is well-known that ceria nanoparticles (CNPs) exhibit significant antioxidant activity, offering potential applications in the treatment of ROS-related pathologies. This activity of CNPs as a nanozyme is typically interpreted by considering Ce(III)/Ce(IV) equilibria on the nanoparticles' surface. However, the validity of this mechanism has never been directly proven in a biological context. Furthermore, it is often overlooked that after endocytosis, CNPs are compartmentalized within endolysosomes, while ROS are primarily located in the cytoplasm, making their direct interaction difficult. This study presents chemical and biological evidence supporting an alternative mechanism of action. By utilizing synchrotron μXRF and μXANES analysis on individual cells, the study shows that the amount of Ce(III), the species responsible for the antioxidant activity, increases linearly with time within the endolysosomes, where CNPs are accumulated, and in their vicinity. Such an increase can be explained by the release of Ce³⁺ ions resulting from a partial reductive dissolution of CNPs in the acidic environment of the endolysosomes. The Ce³⁺ ions can then cross the endolysosomal membrane, reaching the cytosol, where they can exert their reducing activity on ROS. In fact, neutralizing the acidic endolysosomal pH results in a complete inhibition of the CNP activity. Consequently, CNP antioxidant activity should be regarded as the result of redox processes that extend beyond the nanoparticles surface but involve complex dissolution equilibria.