A comparative PM-IRRAS and ellipsometry study of the adsorptive behaviour of bovine serum albumin on a gold surface

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and ellipsometry were used ex situ to investigate adsorption of bovine serum albumin (BSA) on a gold surface, in terms of the adsorption equilibrium and kinetics. The aim of the work was to examine if the two different techniques give similar/complementary results under the same experimental conditions employing the same protein/surface system, and thus validate the use of the techniques for the investigation of protein/surface interactions under the applied experimental conditions, in general. It was found that the adsorption of BSA on gold follows Type I isotherm, which can be described by the Freundlich isotherm. The initial BSA adsorption kinetics was found to be very fast, and the results were modelled using a two-step kinetic model. The first step represents reversible BSA adsorption that yields BSA adsorbed in a native configuration (θ1) that is not thermodynamically stable. The second step represents the irreversible transformation of this protein configuration into a thermodynamically stable surface-adsorbed configuration (θ2). It was found that the θ2/θ1 ratio increased with time. Finally, the comparison of the results obtained by the two techniques showed a very good agreement.