Chapter 21
Acoustic-Wave Sensors
Mass change is the most straightforward modification caused by the recognition event in a chemical sensor. Mass-change assessment appears therefore as a very advantageous transduction method, particularly as it does not involve labels. However, extremely sensitive mass transducers are required for this purpose as the mass change can be below the μg level. Such ultrasensitive transducers are based on vibrating piezoelectric crystals.
Acoustics is the study of mechanical vibrations in gases, liquids, and solids. As vibrations are perceived as sound, the name acoustic-wave sensor is ascribed to the devices presented in this chapter An acoustic wave can develop either simply at the surface of a vibrating device or it can expand to its whole volume. The first case refers to surface-acoustic waves, whereas the second refers to bulk acoustic waves.
Applications of acoustic waves in chemical sensors rely on the interaction of the analyte with an adjacent recognition layer, with resulting modifications in the wave frequency and other wave parameters.
As the key physical phenomenon in an acoustic-wave sensor is piezoelectricity, the first part of this chapter introduces the basic concept of this physical effect. The second part presents the principles of the thickness–shear mode vibrating piezoelectric crystal that is currently the most widely used mass transducer in chemical sensors. Subsequent sections address a series of applications of this transducer ...
