PTA Multiparametric measurement of changes occurring in polystyrene latex nanoparticles following treatment in protein-containing media (10% vol FBS in PBS) was carried out following an incubation duration of up to 24 hours at physiologically relevant temperature. Global patterns in nanoparticle parameter changes were tracked to map changes occurring as a function of incubation time and how these differ across different nanoparticle surface chemistries. Measured characteristics for each nanoparticle type at baseline was in the manufacturer specified size range (i.e., 100 nm diameter). In terms of nanoparticles incubated with proteincontaining media, the mean measured particle diameter significantly increased from baseline (0 hr) to 24 hr treatment for amine-, carboxylate-modified and untreated polystyrene latex nanoparticles with 126%, 36.2% and 28.7% increases in nanoparticle diameters measured by DLS (Figure ), respectively. The extent of measured nanoparticle diameter increase observed was most significant between 0-hour and 2-hour incubation measurements. This observed increase in mean particle diameter (69.8%) was accompanied by an increase in PDI for amine-modified nanoparticles, while the extent of diameter increase was to a lesser extent in the case of carboxylate-and unmodified nanoparticles. An increase in particle mean size was also observed in PTA-measured diameters, with a corresponding 36.3% (carboxylate), and 28.8% (unmodified) increase in particle size relative to control following incubation. PTA was used as an orthogonal higher resolution particle sizing technique for measuring changes in particle size following exposure to protein-containing media (Supplementary information, Table ).