Impact of UV Aging and Casein Corona Formation on the Physicochemical Properties and Size-Dependent Cytotoxicity of Polystyrene Micro- and Nanoplastics

Abstract Micro- and nanoplastics (MNPs) undergo aging and interact with biomolecules before human ingestion, yet their combined effect on cytotoxicity remains poorly understood. This study investigated how UV-C aging and casein (CN) corona formation altered the physicochemical properties and cytotoxicity of polystyrene (PS) particles across three sizes (50 nm, 1 μm, and 90 μm). Physicochemical characterization revealed distinct responses across the three nominal particle sizes. Following UV aging, the mean hydrodynamic diameter of 50 nm PS nanoparticles (NPs) increased from 51 to 419 nm, while its carbonyl index (CI) increased approximately 6-fold. The CI of 1 μm PS significantly increased from 0.084 to 0.477, and its mean hydrodynamic diameter decreased by 10%. UV aging did not significantly change the mean diameter or CI of 90 μm PS. CN corona formation shifted particle ζ-potentials toward less negative values, particularly for UV-aged PS particles. In cytotoxicity assays (carried out over 48 h, with MNP concentration 100 μg/mL), 50 nm PS induced the strongest reduction in Caco-2 metabolic activity (32–43% loss) across all treatments, compared with 14–29% for the larger-particle groups. Transmission electron microscopy confirmed endosomal internalization of all 50 nm NPs. No significant membrane damage or oxidative stress was observed, suggesting a sublethal, nonlytic mechanism. These findings highlight that evaluating MNP surface properties, integrating photooxidative aging and protein interactions, is critical for assessing their biological responses.