Surface reactivity of plastic particles increases with size reduction during mechanical fragmentation: Implications for hazard potential

Toxicity studies of microplastics and nanoplastics have largely relied on primary plastic particles with smooth surfaces. However, these model particles may not accurately represent the majority of plastic particles present in food and environmental matrices, to which humans are predominantly exposed. This study demonstrates the increasing surface reactivity and its toxicological consequences in particles generated through mechanical fragmentation process. Mechanically generated high-density polyethylene (HDPE) microplastics (MPs) and polyethylene terephthalate (PET) (100–350 μm) and nanoplastics (NPs) (< 1000 nm) were evaluated for physicochemical properties, surface chemistry, radical associated activity, abiotic reactive oxygen species (ROS) generation, and membrane lysis. Molecular fingerprinting confirmed that the parent polymer identity was retained after fragmentation. However, smaller particles showed greater surface roughness, altered surface charge, reduced crystallinity, chemical heterogeneity, and enhanced surface oxidation. HDPE NPs remained mainly hydrocarbon rich but structurally disordered whereas PET NPs became oxygen enriched, with increased C-O and O-C = O contributions. Electron paramagnetic resonance and chemical probe assays showed stronger radical associated and oxidative activity in NPs than in MPs. NPs also induced greater erythrocyte membrane lysis and intracellular oxidative stress in Caco-2 cells. Bovine serum albumin (BSA) corona formation attenuated cellular oxidation, indicating that accessible particle surfaces contribute to biological reactivity.