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Correlation of the surface physicochemical properties of nanoparticles with their interactions with biosystems provides key foundational data for nanomedicine. We report here the systematic synthesis of 2, 4, and 6 nm core gold nanoparticles (AuNP) featuring neutral (zwitterionic), anionic, and cationic headgroups. The cellular internalization of these AuNPs was quantified, providing a parametric evaluation of charge and size effects. Contrasting behavior was observed with these systems: with zwitterionic and anionic particles, uptake decreased with increasing AuNP size, whereas with cationic particles, uptake increased with increasing particle size. Through mechanistic studies of the uptake process, we can attribute these opposing trends to a surface-dictated shift in uptake pathways. Zwitterionic NPs are primarily internalized through passive diffusion, while the internalization of cationic and anionic NPs is …