Measurements using the quartz crystal microbalance technique of ferritin monolayers on methyl-thiolated gold: Dependence of energy dissipation and saturation coverage on salt concentration

The adsorption kinetics of ferritin as a function of ionic strength has bee n studied with a new quartz crystal microbalance technique, allowing simult aneous measurement of the frequency shift (proportional to the mass uptake under certain conditions) and of changes in the energy dissipation caused b y the adlayer. The measurements were performed with methyl-terminated (hydr ophobic) thiol-covered gold surfaces, at pH 7.0 and ionic strengths in the range 1-200 mM KCI. The saturation uptake increases rapidly with increasing ionic strength in the range 20-50 mM and is then independent of ionic stre ngth at > 100 mM. The dissipation factor reveals, in the low coverage regim e, distinct differences in the adlayer properties at low and high ionic str ength, respectively. These results are briefly discussed in terms of the sc reening properties of the solvent and its influence on the protein-protein interaction in solution and on the surface. (C) 1998 Academic Press.