As shown with previous measurements, this trend is accompanied by an increase in mean particle diameter and broadening peaks for the size distributions across all particles (unmodified, amine-modified, and carboxylate-modified). This observation likely arises due to particle-protein and protein-protein interactions and the formation of particle agglomerates. Particles incubated in PBS in the absence of protein did not show this effect (supplementary data). There was a further decrease in particle concentration detected by PTA measurements across all polystyrene latex nanoparticles (unmodified, amine-modified, and carboxylatemodified) following particle recovery using the centrifugation-resuspension method. This was once again most significantly observed for amine-modified nanoparticle.