Electromagnetic excitation of high frequency acoustic waves and detection in the liquid phase

Excitation of acoustic waves in a quartz disk has been instigated by exposing the piezoelectric substrate to the electromagnetic field of a planar spiral coil placed in close proximity to the disk. It is argued that reciprocally induced magnetic and electric fields lead to secondary electric fields which couple with the piezoelectric tensor. A comparison of acoustic resonance envelopes recorded in air and in electrolyte demonstrates that spurious acoustic modes are damped in the liquid medium, and that the dielectric properties at the device–solution interface contribute to the exciting electric field. In agreement with previous acoustic device work, increased viscosity of the surrounding fluid leads to a rise in damping which manifests itself in changes in sensor frequency, amplitude and quality factor. With respect to work at high frequencies it is possible to operate the device at as high a harmonic as the 75th, although at this level of frequency resonance begins to break down because of a significant reduction in acoustic Q value. Finally, the use of the system in the flow-injection mode has also been demonstrated through the on-line detection of the adsorption of the protein, neutravidin, to the device surface. The glycosylated parent molecule is very widely employed as a linker for the immobilization of biological macromolecules in bioanalytical chemistry.