  Surface acoustic wave (SAW) based sensors for applications to gaseous
environments have been widely investigated since the last 1970s. More recently,
the SAW-based sensors focus has shifted towards liquid-phase sensing
applications: the SAW sensor contacts directly 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 and 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. The paper
provides an overview of different types of SAW sensors suitable for application
to liquid environments, and intents to direct the attention of the designers to
combinations of materials, waves nature and electrode structures that affect
the sensor performances.
