Unmodified, amine-modified, and carboxylate-modified polystyrene latex nanoparticles were incubated at 37 ˚C for 2 and 24 hours with protein media, and then isolated using the centrifugation-resuspension method. In parallel, additional samples were incubated with treatment media for 2 and 24 hours and analyzed with PTA without recovering the nanoparticles (in situ) from protein incubation medium (Figure ). For all polystyrene latex nanoparticle samples, the in-situ analysis of particle size distribution using PTA showed a reduction in nanoparticle concentration across all sample types following 2-hour and 24-hour incubation in protein media at 37℃. Consistent with this observed reduction in particle concentration, peak broadening of the size distribution to higher sizes was observed across all nanoparticle types examined. This observation is consistent with particle agglomeration, which resulted in lower particle concentration and the emergence of nanoparticles at higher size distributions. It is also likely that protein-particle agglomerates were formed and precipitated prior to measurement. This observed effect was absent for samples incubated in PBS without protein (supplementary information). When comparing the particle concentration detected by PTA between in situ measurements and nanoparticles recovered using the centrifugation-resuspension protocol, a notable reduction in particle concentration was observed across all samples for particles isolated from protein-containing treatment media. Of all polystyrene latex surface modifications examined in this work, aminemodified polystyrene nanoparticle size was the most susceptible to effects of the particle recovery protocol as can be seen from the flat size distributions (Figure ). This is likely caused by large agglomerates formed by nanoparticle-protein and protein-protein interactions which cannot be measured using PTA or may have precipitated.