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Rapid bio-interface characterisation with a quartz crystal microbalance with dissipation monitoring Simultaneous measurement of thickness, mass, and viscosity of adsorbed soft layers

With the QCM-D (Quartz Crystal Microbalance with Dissipation monitoring) technology from Q-Sense (Gothenburg, Sweden) it is possible to determine the physical properties of soft and elastic adsorbed layers like polyelectrolyte multilayers or bio molecules in air as well as in liquids.

The core of a quartz crystal micro balance is a thin slice if quartz with electrodes on its surfaces. Is an alternating voltage of certain frequency applied to those electrodes, a resonant oscillation is induced. The resonance frequency of the quartz crystal is conditioned by the total mass of that crystal. As long as a thin and rigid film, which follows the oscillation perfectly, is attached, the Sauerbrey equation (Δm = - k Δf) for easy determination of the mass of the adsorbed film is valid. But when you need to measure “soft” films this simple technique is unusable. A film that is soft (viscoelastic) will not fully couple to the oscillation of the crystal, hence the Sauerbrey relation will underestimate the mass at the surface. As a soft film dampens the crystal's oscillation this damping or, dissipation (D), of the crystal's oscillation reveals the film's softness (viscoelasticity).

The dissipation of the crystal is measured by recording the response of a freely oscillating crystal that has been vibrated at its resonance frequency. This also gives the opportunity to jump between the fundamental frequency and overtones (e.g. 15, 25 and 35 MHz). By measuring at multiple frequencies and applying a viscoelastic model (the so called Voight model) incorporated in Q-Sense software QTools, the adhering film can be characterized in detail; viscosity, elasticity and correct thickness may be extracted even for soft films when certain assumptions are made.

There are 2 different systems available: the E4 with 4 sensor crystals for 4 separate measurements. In addition a compact E1 (for one sensor crystal) was developed which in combination with a window cell enables microscopy and QCM-D measurements at the same time.