QCM

A quartz crystal microbalance, or QCM, is an acoustical transducer that converts mass changes on the surface of an oscillating quartz crystal resonator into an electronic signal.  First introduced by Sauerbrey in 1959 QCM has been widely used in such areas as vapor deposition of metals, and over the last decade it has become more widely used in biosensor applications.  Often called quartz crystal resonators, the basic design of a QCM is a thin quartz crystal wafer that has electrodes deposited on each face. Primarily used to apply an electric potential across the crystal, when made of the proper metal the electrodes also become a linker chemistry friendly sensor surface.

Sauerbrey discovered that a linear relationship exists between the mass absorbed onto the surface of a QCM and its resonance frequency, and this relationship can be described by:

where f₀ is the resonance frequency, ρ_q is the density of the quartz, v_qis the shear wave velocity in quartz, A is the electrode area, and Δm is the sample mass on the quartz surface.  As the frequencies of oscillation for QCM’s are extremely high, even mass changes on a molecular level occurring at the crystal surface can be measured with great accuracy.

In practice a constant electrical voltage is applied across the quartz crystal causing it to oscillate at a specific frequency.  Mass added to, or lost from the crystal surface will change its oscillation frequency.  By monitoring frequency changes of the crystal it is possible to characterize mass change at the crystal surface.  In biosensor applications QCM is used to monitor mass changes resulting from molecules binding to, and dissociating from the crystal surface or molecules attached to the surface.  As molecules bind to crystal surface the frequency of oscillation decreases, as molecules dissociate from the surface the frequency of oscillation increases.

A detected change in frequency can signify that a specific binding event has occurred, and the level of frequency change can be related to the concentration or mass of the molecule binding.  By monitoring frequency changes over time, binding parameters such as kinetic rates and affinity can be determined.

As the QCM measurement technique is a direct detection process, only those molecules actually binding to the crystal surface are detected.  Unlike optical techniques that measure local refractive index at a sensor surface, impurities and solvent in the tested sample will have minimal impact on the QCM detection process.

In addition to measuring frequency changes, it is also possible to measure changes in resistance at the surface of the quartz resonator.  By measuring resistance changes at the crystal surface during molecular binding events there is also the potential to characterize structural changes occurring to the interacting molecules.