Chemical Sensors Volume 1: Fundamentals of Sensing Material: General Approaches

BASIC PRINCIPLES OF OPERATION OPERATION

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 Application of piezoelectric sensors in liquids is much more difficult than working with gases, since the surrounding liquid liqu id phase acts strongly, damping the oscillating crystal. Although the t he problems have been solved, real analytical applications are rarely encountered even today. The best-investigated applications are antibody layers immobilized on quartz crystals, cr ystals, which are used in immunoassays. Reaction Reaction with the corresponding antigen is highly selective, and the resulting mass change is measurable. Piezoelectric sensors in liquid phase do not obey the Sauerbrey equation. 10.2. SURFACE ACOUSTIC WAVE SENSORS Surface acoustic wave ( SAW) SAW) or Rayleigh sensors consist of a piezoelectric substrate with electrodes patterned on the surface in the form of interdigital transducers ( IDTs), IDTs), as shown in Figure 1.20. 1 .20.  This is the most commonly utilized structure for gas-sensing applications, with the sensitive layer normally deposited in between the two IDT ports. However, if the layer is of low conductivity, it may  be deposited over the entire device. This configuration generates the Rayleigh-mode SAW, which predominantly has two particle displacement components in the sagittal plane. Surface particles move in elliptical paths with a surface-normal and a surface-parallel component. The surface-parallel component is parallel to the direction of propagation. The electromagnetic field associated with the acoustic wave travels in the same direction. The wave velocity is determined by the substrate material and the cut of  the crystal. Most of the energy of the SAW is confined to a few wavelengths depth of the substrate (Wohltjen 1984). The acoustic wavelength is determined by the IDT periodicity. These sensors are normally operated in a “delay line” configuration, with one set of IDTs acting as a transmitter and the other set acting as a receiver. These devices work at higher frequencies (a typical frequency of operation being 30–500 MHz) and in comparison to TSM resonators have higher sensitivity, because the wave propagates not through the bulk of the device but near the surface. The use of Rayleigh SAW sensors is applicable only to gas media, as the Rayleigh wave is severely attenuated in liquid media (Calabrese et al. 1987).  The basic principle in a SAW-based gas sensor is the detection of small deviations in acoustic  wave propagation characteristics, which which are caused by perturbations on the active surface of the device.  The most common technique is to measure the deviation in propagation velocity of the surface wave as a change in resonant frequency of the device. However, perturbations that affect acoustic phase

Sensitive film

Wave propagation

IDT  Piezoelectric substrate Figure 1.20. Schematic view of a SAW sensor .

IDT