Za. Shana et F. Josse, QUARTZ-CRYSTAL RESONATORS AS SENSORS IN LIQUIDS USING THE ACOUSTOELECTRIC EFFECT, Analytical chemistry, 66(13), 1994, pp. 1955-1964
Piezoelectric quartz crystal resonators (QCRs) have been investigated
as detectors in liquid environments. In all the applications, mass loa
ding and viscous coupling are the main interaction mechanisms which re
sult in changes in the QCR resonant frequency. However, other interact
ion mechanisms such as the acoustoelectric interaction due to fringing
fields at electrode ends arise which contribute to the total change i
n frequency, in particular, the parallel resonant frequency. In the pr
esent work, it is shown that by modifying the geometry of the electrod
e at the QCR surface in contact with the solution, a transition region
can be created in which the lateral decaying acoustic field is enhanc
ed. The electric field can then interact with an adjacent conductive/d
ielectric solution which will result in relatively large changes in th
e parallel resonance conditions of the QCR. An equivalent circuit is p
roposed to analyze the loaded QCR with a modified electrode geometry.
It is shown that this circuit is a general circuit which can be used t
o analyze all cases of a loaded QCR with one side in contact with a gi
ven viscous, conductive, or dielectric liquid. Especially, expressions
are obtained for the parallel resonant frequency of the loaded QCR in
terms of the solution dielectric constant and conductivity. It is sho
wn, using 11-MHz devices on AT-cut quartz, that the modified QCRs can
be used as effective and reliable detectors in conductive liquid envir
onments to detect ionic solutes and their dielectric properties. Other
applications are suggested.