An equivalent circuit model for a liquid loaded lateral-field excited acoustic wave sensor

The lateral field excited (LFE) acoustic wave sensor element has been shown to be more sensitive to mass, viscous, and electrical loading than the quartz crystal microbalance. Much of this advantage is due to the bare sensing surface of the LFE sensor element, which has its exciting electrodes on the crystal face that is not exposed to the analyte. The objective of this work is to present an equivalent circuit model of the LFE sensor element loaded with a Newtonian liquid. The circuit model will predict the sensor response near resonance to mechanical and electrical property changes in the liquid or sensing film due to mechanical and electrical perturbations. The equivalent circuit model developed results in an accurate, within ±5%, admittance near the resonant frequency for LFE sensors in deionized water. The model predicts the frequency shift of the LFE sensor to perturbations in the density, viscosity, permittivity, and conductivity of the contacting liquid. For example, the predicted frequency shift is within ±5 ppm for viscous loading and ±50 ppm for dielectric loading when compared to measured frequency shifts.