Guided acoustic wave sensors for liquid environments

Surface acoustic wave (SAW)-based sensors for applications to gaseous environments have been widely investigated since the 1970s. More recently, the SAW-based sensors focus has shifted towards liquid-phase sensing applications: the SAW sensor directly contacts the solution to be tested and can be utilized for characterizing physical and chemical properties of liquids, as well as for biochemical sensor applications. The design of liquid-phase sensors requires the selection of several parameters, such as the acoustic wave polarizations (i.e. elliptical, longitudinal or shear horizontal), the wave-guiding medium composition (i.e. homogeneous or non-homogeneous half-spaces, finite thickness plates or composite suspended membranes), the substrate material type and its crystallographic orientation. This paper provides an overview of different types of SAW sensors suitable for application to liquid environments, and intends to direct the attention of the designers to combinations of materials, wave nature and electrode structures that affect the sensor performances.