Ellipsometry studies of the effects of surface hydrophobicity on protein adsorption

Abstract

The adsorption of some model proteins, human serum albumin (HSA), IgG, fibrinogen and lysozyme on silica surfaces was investigated with in situ ellipsometry and compared to previous results obtained for methylated silica surfaces. The adsorbed amount (Γ), the adsorbed layer thickness ( δ el) and the mean adsorbed layer refractive index ( n f) were obtained by a procedure involving studies of the bare substrate at two different ambient refractive indices, as well as four-zone averaging. The surface hydrophobicity strongly influenced the adsorption properties of all the proteins studied. For HSA, IgG and fibrinogen, the adsorbed amount was significantly lower on the hydrophilic surface than on the hydrophobic one, whereas the reverse was found for lysozyme. For fibrinogen, the adsorbed layer thickness on silica was smaller than that on methylated silica, whereas the adsorbed layer was more concentrated. For IgG, on the other hand, end-on adsorption was observed on both silica and methylated silica. For lysozyme, side-on adsorption in a dense monolayer was observed on silica, whereas on methylated silica, the adsorption occurs in two to three rather dilute molecular layers. Furthermore, the build-up of the adsorbed layers was studied. For fibrinogen, qualitatively the same behaviour was observed for silica and methylated silica, i.e. as the absorbed amount increases, both δ el and n f initially increase, while closer to adsorption saturation, n f levels off. However, at a given Γ, δ el was lower and n f higher on silica than on methylated silica. A similar finding was obtained for lysozyme. These findings are discussed in terms of adsorbed layer structure and formation.