Fluorescent Polystyrene Nanoparticles with Controlled Elasticity for Probing Cellular Internalization

Cellular uptake of nanoparticles (NPs) is influenced by their physicochemical properties, including particle size, surface charge, and mechanical stiffness. In this study, polystyrene (PSt) NPs with controlled particle sizes, surface charges, and elastic properties were prepared by emulsion polymerization. Particle stiffness was modulated by varying the amount of the crosslinking monomer divinylbenzene, yielding NPs with Young’s moduli of approximately 370 and 480 MPa while maintaining comparable particle sizes and surface charges. Fluorescent PSt-NPs encapsulating Nile Red exhibited comparable fluorescence intensities and negligible dye leakage. Fluorescence microscopy of cells showed consistently higher intracellular fluorescence for the stiffer NPs than for the softer NPs. These results indicate that differences in NP elasticity can influence cellular uptake even when particle size and surface charge are comparable. The developed fluorescent PSt-NPs therefore provide a useful model platform for investigating the relationship between NP mechanics and cellular internalization.