Here we present in situ observations of adsorption of bovine serum albumin (BSA) on citratestabilized
gold nanospheres. We implemented scattering correlation spectroscopy as a tool to
quantify changes in the nanoparticle Brownian motion resulting from BSA adsorption onto the
nanoparticle surface. Protein binding was observed as an increase in the nanoparticle
hydrodynamic radius. Our results indicate the formation of a protein monolayer at similar albumin
concentrations as those found in human blood. Additionally, by monitoring the frequency and
intensity of individual scattering events caused by single gold nanoparticles passing the
observation volume, we found that BSA did not induce colloidal aggregation, a relevant result
from the toxicological viewpoint. Moreover, to elucidate the thermodynamics of the gold
nanoparticle-BSA association, we measured an adsorption isotherm which was best described by
an anti-cooperative binding model. The number of binding sites based on this model was
consistent with a BSA monolayer in its native state. In contrast, experiments using poly-ethylene
glycol capped gold nanoparticles revealed no evidence for adsorption of BSA
