Mc Evoy Kevin

PhD project of Kevin Mc Evoy

Chemical and bio-industry engineer
PhD student since October 2007 (C. Dupont-Gillain)

Improving quantification of proteins at interfaces through a better and practical modeling 
of quartz crystal microbalance response

Quartz crystal microbalance with dissipation monitoring (QCM-D) consists of a probe, a quartz crystal between two gold electrodes, and of an electronic device measuring the resonance frequency (+ overtones) and the time needed to return to the static state after switching off the electrical field. Adding a supplementary mass to the crystal modifies the resonance frequency and, if the coupled mass has a viscous behavior, the energy dissipation too. This phenomenon is used to monitor mass exchanges between an interface and a supernatant medium. In case of a rigid film, mass sensitivity is a few tenths of ng.cm^-2 and the Sauerbrey model can be used to get the coupled mass from the resonance frequency shift:

Δm = C ∙ (Δf / n)

where m is the areal mass density (ng.cm^-2), f the resonance frequency (Hz), n the overtone number and C a constant value containing some physical properties of the quartz crystal at the current temperature (ng.Hz^-1.cm^-2). This equation is applicable only if the film has physical properties similar to those of the quartz.

Viscoelastic coupled layers are more difficult to weight because there is no proportional relation between frequency shift and areal mass density. Several physical models have been proposed to overcome this difficulty but a screening of the QCM-D literature reveals that these are seldom used and that QCM-D is mainly a qualitative method. This is probably due to a high level of complexity and to the need of input parameters not easily available (density of the layer, of the solvent, viscosities…).

Given that QCM-D is currently mostly a complementary technique, it is critical to evaluate the quantification potentialities of the device. The aim is to fill the gap between the physical models and biological systems, and to determine reliable quantification equations with defined application fields.

Because QCM-D is a real-time technique working under native conditions for biological molecules, a better interpretation of its response could greatly improve the characterization of biointerfaces. For example, it could help monitoring protein assemblies at interfaces and better controlling the grafting of biosensors such as AFM tips.