Aging amplifies the combined toxic effects of polystyrene nanoplastics and norfloxacin on human intestinal cells

Abstract

Micro- and nanoplastics (MNPs) serve as both intrinsic toxicants and vectors for environmental pollutants such as antibiotics, raising concerns about potential synergistic toxic effects. However, the mechanistic role of MNPs in mediating combined toxicity remains insufficiently understood. This study examines how simulated environmental aging affects the toxicity of polystyrene nanoplastics (PS-NPs), both alone and in combination with norfloxacin (NOR), in human intestinal Caco-2 cells. Aging significantly altered PS-NPs physicochemical properties, including enhanced surface oxidation, increased hydrophilicity, and reduced particle size. These changes substantially enhanced cytotoxicity: at 400 μg mL⁻¹, cell viability dropped to 50.7% for aged PS-NPs compared to 74.4% for virgin particles. Co-exposure with NOR (5 μg mL⁻¹) further exacerbated this effect. Counterintuitively, this increased toxicity did not result from improved NOR carrier capacity. Cellular NOR uptake showed no significant difference between aged and virgin PS-NPs groups, and aged PS-NPs exhibited significantly lower cellular uptake (p < 0.001) than virgin particles at 200–400 μg mL⁻¹. Mechanistic analysis revealed that intensified toxicity originates from heightened intrinsic reactivity of aged particles, leading to elevated oxidative stress (higher ROS levels at 400 μg mL⁻¹) and enhanced inflammatory responses (increased TNF-α release). At high concentrations, the dominant intrinsic toxicity of aged PS-NPs masked synergistic effects with NOR. In summary, environmental aging critically amplifies nanoplastics hazards by enhancing their intrinsic toxicity rather than carrier capacity, emphasizing the urgent need to revise risk assessment frameworks to account for aging-induced changes in plastic pollution.