Quartz crystal microbalance has become instrumental in life sciences and biosensor research. It is widely used for monitoring interfacial process, including protein adsorption and conformational changes in the protein adsorbed films,¹ liposome-surface interactions and supported bilayer formation,² and in the development of biosensor platforms.

            The technique is based on monitoring, on several overtones, resonance frequency and bandwidth of a thin quartz plate oscillating in a shear-thickness mode, e.g., as material adsorbs to the crystal surface. Changes in resonance frequency relative to that of the bare crystal in the same liquid are independent of the overtone order and reflect the mass of the adsorbed material if the resulting film is thin, homogeneous, and rigid. Changes in the bandwidth associated with this process are negligible. In most cases, however, a non-zero change in bandwidth and a dependence of the frequency shift on the overtone order are observed. Typically, this kind of response is interpreted in terms of the layer viscoelastic properties according to equation (1):³

,                                                     (1)

where f and G are frequency and bandwidth, respectively, Z_q is the acoustic impedance of quartz (a real number), h, r, h, and J are respectively thickness, density, viscosity, and complex shear compliance, and subscripts q, f, and liq refer to quartz, film, and liquid, respectively.

Equation (1) was derived for homogeneous films. Its applicability to films comprised of isolated entities (protein molecules, liposomes, colloidal spheres) is debatable. In this study, we use a combination of atomic force microscopy and quartz crystal microbalance to investigate the process of ferritin adsorption on gold. We show, that while ferritin film formation from non-purified preparations is associated with strong changes in bandwidth and an overtone dependence of the frequency shift Df, those formed from purified preparations exhibit neither of these characteristics. The former preparation contained low molecular weight contaminants as well as high-molecular weight aggregates and resulted in non-homogeneous films (as viewed by AFM). We conclude, that in the case of ferritin adsorption on gold, the changes in bandwidth and frequency dependence of Df reflect sample heterogeneity or dissipation at the protein-surface contact, and not the viscoelastic properties of protein layer per se.

¹ F. Hook, B. Kasemo, T. Nylander et al., Analytical Chemistry 73 (24), 5796 (2001).

² C. A. Keller and B. Kasemo, Biophysical Journal 75, 1397 (1998).

³ D. Johannsmann, in Piezoelectric Sensors, edited by C. Steinem and A. Janshoff (Springer, 2007), pp. 49