SURFACE ACOUSTIC WAVE SENSORS FOR CHEMICAL APPLICATIONS 471 4.3. LIQUIDS The application of SAW devices in solutions is a somewhat neglected area, especially considering the bulk of research in the field of SAW gas sensors. This neglect may be partly due to the technical problems associated with their application in liquids. However, their potential cannot be ignored, as several groups have effi ciently used other than Rayleigh-type SAW devices in liquid-phase chemosensor applications (Josse et al. 2001; Leonte et al. 2006; Assouar 2009). Although SAW devices have not received a lot of attention for their applications in fluids, exam- ples can be cited where STW devices were successfully employed for liquid-phase sensing applications. Dickert and co-workers presented SAW sensors for the detection of polycyclic aromatic hydrocarbons (PAHs) incorporating a two-port 36° rotY,X-LiTaO 3 STW resonator coated with MIP­PAH-imprinted polyurethane (see Figure 10.10) (Dickert et al. 1999). Though selectivity is primarily determined by the size and orientation of the pattern molecules as indicated by the fluorescence waveguide intensi- ties, STW devices are particularly sensitive and render detection limits down to the parts-per-thousand range in aqueous phase. Figure 10.15 shows the sensor effect of a 428-MHz STW resonator to pyrene. Extreme sensitivities (in the picogram range) were also reported for the detection of yeast cells with SAW mass sensors based on 428-MHz LiTaO 3 STW resonators and bio-imprinted polyurethane layers (Greibl et al. 2002). 5. COMPARING SURFACE AND BULK ACOUSTIC WAVE DEVICES Various acoustic wave devices usually differ in the mode of acoustic wave propagation; unlike a Rayleigh- type SAW, a typical bulk acoustic wave (BAW) device resonates in thickness shear mode (TSM)--the so- called TSM resonator or quartz crystal microbalance (QCM). A thickness shear wave propagates within the complete piezoelectric substrate with maximum displacement at the upper and lower surfaces, mak- ing a QCM device sensitive to surface interactions. Additionally, QCMs can be easily employed in fluids due to very low damping of the thickness shear wave. However, the operating frequency of the QCM is directed by the thickness of the quartz plate, which consequently restricts working QCM frequencies to a few megahertz [commonly reported 10-MHz QCM sensors have a quartz plate thickness equal to 168 m, though higher-frequency QCMs have also been tested (Ogi et al. 2009)]. In contrast to QCMs, SAW devices allow substantially lower limits of detection and drastically enhanced sensitivity due to higher fundamental frequencies (reported up to 1 GHz). In principle, a SAW oscillator with a resonance frequency of 433 MHz produces 1875-fold superior sensor effect than a QCM operating at 10 MHz. Figure 10.16 clearly certifies this statement, showing sensor response of SAW and QCM devices, coated with hydrophobic cyclodextrins, to o-xylene vapors. 6. THE MARKET FOR SAW SENSORS In 2003, Harris reviewed the potential of SAW-based chemical sensors and stated that "SAW sensors occupy a small market but give other chemical vapor detectors a run for their money." This in fact gives