Bioaccumulation of differently-sized polystyrene nanoplastics by human lung and intestine cells

The bioaccumulation of nanoplastics (NPs) has been intensively examined using the fluorescence-labeling technique. As the fluorescence intensity per particle is different for NPs with different physicochemical properties, it's hard to directly compare their bioaccumulation based on fluorescence. Therefore, how physicochemical properties may affect NPs' bioaccumulation remains unclear. In the present study, we chose polystyrene NPs (PSNPs) with the primary particle size of 70 nm (PS70), 200 nm (PS200), and 500 nm (PS500), and examined their uptake by human lung and intestine cells. We found that PSNPs had low cytotoxicity, but could be taken up by both cell lines. The particle-mass-, particle-number-, and particle-surface-area-based accumulation of the differently-sized PSNPs were then compared. Smaller PSNPs showed lower particle-mass-based but higher particle-number-based uptake rate than the larger ones. Nevertheless, much less difference was observed when the unit of uptake rate was based on particle surface area, suggesting the critical role of surface area during PSNPs' interaction with the cell membrane. Additionally, all three PSNPs could enter the cells by phagocytosis and PS70 could also be internalized by clathrin- and caveolae-mediated endocytosis. Overall, the effects of size on the bioaccumulation of NPs need to be considered when evaluating their environmental and health risks.