On the mass sensitivity of acoustic-plate-mode sensors

The mass sensitivity of acoustic plate modes (APMs) in general and in particular APMs on ZX-LiNbO3 is investigated for gas and liquid phase-sensing applications. It is shown that, unlike pure shear horizontal APMs, the modes on ZX-LiNbO3 consist of both propagating bulk-wave components (with a linear frequency dependence of mass sensitivity) and evanescently trapped surface components from the pseudo surface acoustic wave (PSAW) (with f(2) frequency dependence of mass sensitivity). Thus, for a given plate thickness, the overall frequency dependence of mass sensitivity depends on the frequency range of operation. Theoretical and experimental results are in good agreement. Results are also given for actual immunosensor experiments. The results show that, in sensor applications, an f(2) dependence at relatively high frequencies, and hence higher mass sensitivity, is achievable. This will require using thinner plates and the dominant PSAW-derived APM.