Measurements Using the Quartz Crystal Microbalance Technique of Ferritin Monolayers on Methyl-Thiolated Gold: Dependence of Energy Dissipation and Saturation Coverage on Salt Concentration

Abstract

The adsorption kinetics of ferritin as a function of ionic strength has been studied with a new quartz crystal microbalance technique, allowing simultaneous measurement of the frequency shift (proportional to the mass uptake under certain conditions) and of changes in the energy dissipation caused by the adlayer. The measurements were performed with methyl-terminated (hydrophobic) thiol-covered gold surfaces, at pH 7.0 and ionic strengths in the range 1–200 mM KCl. The saturation uptake increases rapidly with increasing ionic strength in the range 20–50 mM and is then independent of ionic strength at >100 mM. The dissipation factor reveals, in the low coverage regime, distinct differences in the adlayer properties at low and high ionic strength, respectively. These results are briefly discussed in terms of the screening properties of the solvent and its influence on the protein–protein interaction in solution and on the surface.