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The operational modes of lateral field excited (LFE) quartz crystal microbalances (QCMs) under various electrical boundary conditions have been under investigation for use in sensing applications. The present results indicate connections between the behaviors of acoustic plate modes (APMs) and LFE-QCM modes. The influences of deposited thin conducting and semiconducting films on the mode responses of LFE-QCMs strongly agree with reported effects on APMs. Thus the main operating modes of LFE devices are concluded to be laterally varying APMs, which may be close in character to thickness modes. Mode response changes caused by slowly varying surface curvature are explained from this perspective. The consistency of the usual LFE thickness mode coupling formalism is evaluated, and the superposition of partial waves method is found to be more appropriate for analyzing LFE devices. This view of LFE devices operating through APMs supports improved sensing applications and investigations through access to existing APM knowledge. 

The most common bulk acoustic wave device used in biosensing applications is the quartz crystal microbalance (QCM), in which a resonant pure shear acoustic wave is excited via electrodes on both major faces of a thin AT-cut quartz plate. For biosensing, the QCM is used to detect the capture of a target by a target-capture film. The sensitivity of the QCM is typically based solely on the detection of mechanical property changes, as electrical property change detection is limited by the electrode on its sensing surface. A modification of the QCM called the lateral field excited (LFE) QCM (LFE-QCM) has been developed with a bare sensing surface as both electrodes are now on a single face of the quartz plate. Compared to the QCM, the LFE-QCM exhibits significantly higher sensitivity to both electrical and mechanical property changes. This paper presents theoretical and experimental aspects of LFE-QCMs. In particular, the presence and strength of the usual and newfound LFE-QCM modes depend on the electrical properties of the film and/or sensing environment. This work also presents examples of experimental setups for measuring the response of an LFE-QCM, followed by results of LFE-QCMs used to detect liquid electrical and mechanical properties, chemical targets, and biological targets. Finally, details are given about the attachment of various target-capture films to the LFE-QCM surface to capture biomarkers associated with diseases such as cancer.