Solidified liquid layer model makes quartz crystal microbalance a convenient molecular ruler

The applications of quartz crystal microbalance (QCM) in biointerfaces are limited by its quantitative ambiguities caused by viscoelasticity and solution effects. Although many studies clearly indicated that the quantitative interpretation of QCM data needed caution, none of those studies provided a practical solution that enabled general and quantitative interpretation of QCM data. Recently we proposed a "solidified liquid layer" model that enabled QCM to be used as a biomolecular ruler. Here we applied five kinds of proteins with significant differences in their sizes and shapes to further validate this model. The effective thickness (T(eff)) of surface immobilized, hydrated proteins were 10.2, 4.7, 1.8 and 4.8 nm for rabbit IgG, streptavidin, lysozyme, and bovine serum albumin, respectively. The critical number of stakes needed for the formation of a solidified liquid layer was found to be protein dependent. We believed this "solidified liquid layer" model will facilitate the popularization of QCM as a valuable tool in biointerface studies, such as protein adsorption process or the conformational change on surface.