  Protein-surface interactions cause the desirable effect of controlled protein
adsorption onto biodevices as well as the undesirable effect of protein
fouling. The key to controlling protein-surface adsorptions is to identify and
quantify the main adsorption mechanisms: adsorptions that occur (1) while
depositing a protein solution onto dry surfaces and (2) after the deposition
onto wet surfaces. Bulk measurements cannot reveal the dynamic protein
adsorption pathways and thus cannot differentiate between the two adsorption
mechanisms. We imaged the interactions of single streptavidin molecules with
hydrophobic fused-silica surfaces in real-time. We observed both adsorbed
proteins on surfaces and diffusing proteins near surfaces and analyzed their
adsorption kinetics. Our analysis shows that the protein solution deposition
process is the primary mechanism of streptavidin adsorption onto surfaces at
the sub-nanomolar to nanomolar protein concentrations. Furthermore, we found
that hydrophilic fused-silica surfaces can prevent the adsorption of
streptavidin molecules.
