THE THEORY OF OPERATION OF PIEZOELECTRIC QUARTZ CRYSTAL SENSORS FOR BIOCHEMICAL APPLICATION

Piezoelectric quartz crystal shear wave generators have long been employed as mass detectors using the Sauerbrey equation to relate observed changes in resonant frequency to changes in mass. When these crystals are used in a more sophisticated way to detect the binding of an analyte by means of a sensor layer attached to the crystal, there is often considerable discrepancy between the mass change determined and that expected from the reaction. We re-examine the operation of the device in terms of the underlying shear wave processes and show that the viscoelastic properties of the layers on the crystal, rather than the simple mass, determine the change in resonant frequency. The theory is illustrated by examples of X-cut cylindrical and AT-cut planar crystal sensors for saccharides, based on interaction with concanavalin-A bound on the crystals. The correct emphasis on viscoelastic shear wave properties rather than mass change offers much better prospects for the successful use of such devices as biosensors.