Employing Two Different Quartz Crystal Microbalance Models To Study Changes in Viscoelastic Behavior upon Transformation of Lipid Vesicles to a Bilayer on a Gold Surface

By analyzing the viscoelastic properties of two distinct layers, a layer of “soft” vesicles and a “rigid” bilayer, we have created a model system to permit the study of film behavior in the region of nonlinear mass and frequency change (non-Sauerbrey). The structural transformation of lipid vesicles to a bilayer is shown to be accompanied by significant changes in their physical properties. After the adsorption and saturation of intact vesicles on gold surfaces, the adsorbed vesicle layer exhibits a soft, water−rich, viscoelastic state. The AH peptide, a vesicle-destabilizing agent, is then added to trigger the formation of a much thinner (∼5 nm), compact, and rigid bilayer. In this study, we used the quartz crystal microbalance with dissipation technique. Large non-Sauerbrey frequency and energy dissipation changes characterize the viscoelastic nature of adsorbed intact vesicle films thicker than ∼10 nm. Once the transformation is complete, the frequency changes along with zero energy dissipation for sufficiently thin films (t ∼ 5 nm) were effectively modeled with the Sauerbrey equation. Furthermore, we checked the validity of the Voigt−Voinova model in which the quartz substrate is treated as a Voigt element, which is beyond the Sauerbrey description. The calculations treating the film as having a constant viscosity agreed well with the Voigt−Voinova model. These results were compared to calculations done using the electromechanical (EM) model, which does not require a series expansion. The Voigt−Voinova results were in excellent agreement with the EM model, providing evidence that the expansion used in their study is quite accurate.