SH-SAW sensor

Josse F., Bender F., Cernosek R. W., and Zinszer K., Guided SH-SAW sensors for liquid-phase detection, presented at IEEE International Frequency Control Symposium and PDA Exhibition, June 6-8, 2001, Seattle, Washington, USA. 

Sehra G., Cole M., and Gardner J. W., Miniature taste sensing system based on dual SH-SAW sensor device An electronic tongue. Sens. Actuators B, 103(1-2), 233, 2004. 

Figure 3.1 Schematic sketches of the four types of acoustic sensors, (a) Thickness-Shear Mode (TSM) resonator (b) Surface-Acoustic-Wave (SAW) sensor, (c) Shear-Horizontal Acoustic-Plate-Mode (SH APM) sensor, and (d) Flexural-Plate-Wave (FPW) sensor.

Another less-utilized transduction mechanism for biosensors involves the acoustoelectric effect. In principle, any biochemical process that produces a change in the electrical properties of the solution, can be monitored by observing changes in the frequency and/or attenuation of the device if its surface is not metallized. For example, a SH-SAW device has been reported for the detection of pH changes associated with the enzyme-catalyzed hydrolysis of urea [235]. Using an immobilized urease membrane on the sensor surface, it was anticipated that urea concentrations as small as 3 /u.M could be reliably detected. 

FIGURE 4.9 Permittivity-conductivity chart to derive electrical properties from SH-SAW responses. Kondoh J. and Shiokawa S Shear Horizontal Surface Acoustic Wave Sensors. Sensors Update. 2001. 6. Copyright Wiley-VCH Verlag GmhH Co. KGaA. Reproduced with permission. 

An electronic tongue based on dnal shear horizontal surface acoustic wave (SH-SAW) devices was developed to discriminate between the basic tastes of sour, salt, bitter, and sweet [57]. Sixty MHz SH-SAW delay line sensors were fabricated and placed below a miniature PTFE housing containing the test liquid. All the tastes were correctly classified without the need for a selective biological or chemical coating. 

It was shown that eqn [10] can be employed to describe the sensor response of both SH-SAW and SH-APM resonators all with acoustic waveguides. In conclusion, the slope at the operating point zq of the dispersion curve determines the sensitivity of the device. 

The second type utilizes acoustic waves confined to the surface of the piezoelectric material and are known as surface acoustic wave (SAW) devices. Both BAW and SAW sensors use longitudinal waves or shear waves (SH). Surface acoustic waves are generated by converse piezoelectric effect at the input interdigital transducer (IDT), and they are converted back into an electric signal by direct piezoelectric effect. In their basic form, they consist of a piezoelectric substrate, on top of which two metallic interdigital transducers (IDTs) are patterned to form a delay line structure, as shown in Fig. 13.3. This is the most commonly utilized structure for gas-sensing applications with the sensitive layer normally deposited in between the two IDT ports. 

Most immunochemically based sensors to date have been developed for liquid-phase measurements thus, the TSM resonator has been the device of choice. Of course, other plate-mode devices (SH-APM, FPW) would be equally well suited for liquid-phase detection and may have advantages in terms of sensitivity. A low-frequency (20 MHz) SAW liquid-phase immunoassay device has been reported [27], but operation of SAWs of higher frequencies in liquids is not feasible due to excessive attenuation of the SAW by the liquid. An alternative to in-situ detection is to expose a protein-coated AW device to a liquid-phase sample for a period of time, then dry it [226] the observed frequency shift is proportional to analyte concentration. When using this technique, it is crucial that careful control experiments in the absence of analyte be performed to obtain an accurate idea of the reproducibility of the baseline oscillation frequency throughout the procedure. 

Chemical sensors based on acoustic wave (AW) devices have been studied for a number of sensing applications, the majority of which fall in the category of gas and vapor detection (1-8). Recently, the use of these sensors in liquid environments has been explored (9-13). AW sensors utilize various types of acoustic waves, including the surface acoustic wave (SAW), the shear-horizontal acoustic plate mode (SH-APM) (10-13), and the Lamb wave (also a plate mode) (3.14). Even though most studies of these piezoelectric sensors have centered on SAW devices (1.2.4-8), differences in the propagation characteristics of the various acoustic modes make some better suited than others for a given sensing application. 

An interdigital transducer on the surface of a piezoelectric material can excite and detect waves which propagate along the surface (e.g. the SAW) or through the bulk (e.g. the Lamb wave and the SH-APM) of the substrate. AW sensors typically include an input transducer to generate the wave, an interaction region in which the propagating wave is affected by its environment, and an output transducer to detect the wave. Thus, unlike the quartz crystal microbal-... 

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