By collecting both the dissipation and the resonance frequency of a quartz crystal, 
QCM-D technology can be used to characterize the formation of thin films (nm) such as proteins, polymers and cells onto surfaces, in liquid. This is QCM-D, Quartz Crystal Microbalance with Dissipation monitoring, Q-Sense's proprietary sensor technology. 

In liquid, an adsorbed film may consist of a considerably high amount of water, which is sensed as a mass uptake by all QCMs. By measuring several frequencies and the dissipation it becomes possible to determine whether the adsorbed film is rigid or water-rich (soft) which is not possible by looking only at the frequency response. The amount of water in an adsorbed film can be as high as 95% depending on the kind of molecule and the type of surface you are studying. Picture elongated molecules - if they would adsorb flat on the surface, little water will be coupled to the molecules. However, if they adsorb standing up at the surface, lots of water will be coupled. With QCM-D the kinetics of both structural changes and mass changes are obtained simultaneously. 

The QCM-D Principle

Unlike all other QCMs, QCM-D monitors the response of the freely oscillating crystal, which is faster and more accurate than the usual frequency sweep principle. Here is a demonstration of the principles behind the QCM-D technology, click on the image below to start animation.

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QCM-D measures both mass and structural properties

The quartz crystal microbalance (QCM) has been used for a long time to monitor thin film deposition in vacuum or gas. After it was shown that the QCM may be used in the liquid phase, the number of applications for the QCM has increased dramatically.

A QCM consists of a thin quartz disc sandwiched between a pair of electrodes. Due to the piezoelectric properties of quartz, it is possible to excite the crystal to oscillation by applying an AC voltage across its electrodes.

The resonance frequency (f) of the crystal depends on the total oscillating mass, including water coupled to the oscillation. When a thin film is attached to the sensor crystal, the frequency decreases. If the film is thin and rigid the decrease in frequency is proportional to the mass of the film. In this way, the QCM operates as a very sensitive balance. The mass of the adhering layer is calculated by using the Sauerbrey relation:

C = 17.7 ng Hz-1 cm-2 for a 5 MHz quartz crystal.  n = 1,3,5,7 is the overtone number.

It is also possible to get an estimation of the thickness (d) of the adhering layer: 

where ρeff is the effective density of the adhering layer.

In most situations the adsorbed film is not rigid and the Sauerbrey relation becomes invalid. 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.

A soft film dampens the crystal's oscillation. The damping or, dissipation (D), of the crystal's oscillation reveals the the film's softness (viscoelasticity). D is defined as

where Elost is the energy lost (dissipated) during one oscillation cycle and Estored is the total energy stored in the oscillator.

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 Voigt 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. 

Read more about QCM-D in the technology note on your top right and in over 500 peer-reviewed publications citing the use of the technology.