Human serum albumin adsorption onto octadecyldimethylsilyl-silica gradient surface

The effects of long-range electrostatic repulsion and short-range hydrophobic attraction on human serum albumin adsorption were studied as a function of the surface density of octadecyldimethylsilyl chains (C18) on silica. A C18 surface density gradient was prepared on fused silica plates. The water contact angles increased smoothly from 12° to 105° in the 12.5 mm long gradient region. The maximal fractional surface coverage of the C18 chains was calculated to be 0.92. Fluorescein-5-isothiocyanate-labeled human serum albumin (FITC-HSA) adsorption from dilute buffer solution onto the C18 gradient surface was measured using spatially resolved total internal reflection fluorescence (TIRF) spectroscopy. FITC-HSA adsorbed progressively more onto the surfaces with higher hydrophobicity. When the fractional surface coverage of C18 chains was larger than 0.42 the adsorption saturation of FITC-HSA leveled off. The adsorption kinetics of FITC-HSA changed from a transport-limited process at moderate-to-high C18 surface coverages to an adsorption-limited process at lower C18 surface coverages. The kinetics of FITC-HSA adsorption, including the convective-diffusive transport of protein to the surface, were modeled as a simple binding process with a single forward and reverse rate. The apparent binding constant, derived from the initial forward and reverse binding rates, depended more strongly on the C18 surface coverage in 0.165 M buffer than in 0.025 M buffer.