SAW sensor

There are adsorption layers which react with the sample irreversibly, e.g. ammonia with a mixture of ascorbic acid and silver nitrate or hydrogen sulphide with silver acetate. 

Selectivities of BAW sensors differ according to their acceptor layer. Whereas palladium is highly selective for hydrogen, organic amines are only weakly selective for sulphur dioxide, and carbowaxes (well known as stationary phase in chromatography) interact with numerous non-polar gases, hence they are non-selective sensors for hydrocarbons. Response time also varies within a wide range. 

In electrochemistry, the piezoelectric BAW sensor is known as an electrochemical quartz crystal microbalance (EQCM). This device plays an important role in electrochemical research. Metal deposition, corrosion and formation of passive layers have been studied successfully. 

Piezoelectric sensors, where surface acoustic waves (SAW sensors) are used, came to the fore in recent years. The function principle is demonstrated in Fig. 4.2. 

For SAW sensors, all the materials given in Table 4.1 except those with metallic conductivity can be used as chemically sensitive coatings. 

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Pohl, A., A review of wireless SAW sensors, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 47,317, 2000. 

Liron, Z. Kaushansky, N. Frishman, G. Kaplan, D. Greenblatt, J., The polymer coated SAW sensor as a gravimetric sensor, Anal. Chem. 1997, 69, 2848 2854... 

Changes produced by the measurand on the properties of the CIM can affect both the phase velocity and the propagation loss of the acoustic wave. There are examples of SAW sensors based on the measurements of the changes in the phase velocity. 

The model immunoassay is the enzyme-linked immunosorbent assay (ELISA) in which a non-specific capture antibody is bound to a surface, such as a multi-well plate or small tube [13]. In the basic form of ELISA, a second antibody tagged with an enzyme interacts specifically with the analyte. The enzyme assay produces a colored product that is read with a spectrophotometer. There are many variations on the basic immunoassay format that serve to increase sensitivity, specificity, linear range, and speed. Many commercial instruments have been developed to take advantage of various technologies for reporter molecules. The immunoassay may be coupled to an electronic sensor and transducer, such as a surface acoustical wave (SAW) sensor. Electrochemiluminescence (ECL) is a method in which the detector antibody is tagged with a ruthenium-containing chelate [13-15]. When the tag is... 

Garcia-Gonzalez, D.L., Barie, N. Rapp, M., Aparicio, R. (2004) Analysis of virgin olive oil volatiles by a novel electronic nose based on a miniaturized SAW sensor array coupled with SPME enhanced headspace enrichment. J. Agric. Food Chem. 52 7475-7479. 

The simplest SAW sensor is a two-terminal transmission (delay) line in which the acoustic (mechanical) wave is piezoelectrically launched in one oscillator, called the transmitter. It travels along the surface of the substrate and is then transformed back into an electrical signal by the reverse piezoelectric effect at the receiving oscillator (Fig. 4.18). 

Most SAW sensors employ a resonator arrangement a sensor delay line and the reference line are combined on one substrate with one generator in the center and two receivers at the ends (D Amico et al., 1982/83 Fig. 4.20) terminated by two reflectors. 

An example of a hydrogen SAW sensor is shown in Fig. 4.20, where the area of Ls is coated with 3,000 A of Pd. The substrate is LiNbC>3. There are five finger pairs and the device is operated at Fq = 75 MHz. The width of the acoustic beam is 100 acoustic wavelengths (4.1). The sensor responds to change of hydrogen concentration in the range 50-10,000 ppm. 

Various thin selective layers, as discussed in Chapter 1, can be used to provide chemical selectivity. As with piezoelectric crystals, the acoustic properties of these films affect the performance of the SAW sensor in different ways by the change of... 

A comparative study of the readout options for the SAW sensor with additional film has shown that for a single SAW sensor the highest signal-to-noise ratio is obtained from the amplitude measurement (Wohltjen and Dessy, 1979). Voltage output related to the phase-shift as discussed above works well for dual delay lines. There are also inherent advantages in measurement of the change of the resonant frequency. The frequency shift due to deposited film of low elastic shear modulus p is... 

Thus, the addition of a 1 pm thick polymer film coated onto a quartz SAW sensor operating at 31 MHz will cause a AF = —130 kHz shift. 

Gravimetric Gardner (UK) Surface acoustic wave (SAW) sensors [20]... 

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.

The simplest interaction, and the one most utilized for SAW sensor applications, is the response due to changes in the arealAnass density (mass/area) on the device surface. The harmonic motion of the crystal surface caused by the passing surface wave causes particles bound to the surface to be translated in an elliptical orbit in synchronism with the SAW surface displacement. The effect on wave velocity and attenuation of this interaction may be derived firom eneigy considerations. 

These considerations may also explain the nonmcmotonic frequency responses with vapor partial pressure observed for adsorption of non-polar molecules onto the quartz surface of a SAW sensor [114]. Since it is well known that surface coverage increases monotonically with partial pressure, these trends are inconsistent with a simple mass-loading interpretation. In this paper, the authors no-posed a coverage-dependent SAW-adsorbate interaction to explain the anomalous results for the weaker binding non-polar s )ecies (no anomalous trends were observed with polar adsorbates). 

The single designation indicates that the response is from an individual SAW sensor (i.e., no reference sensor was employed). (Reprinted with permission. See Ref, [671. 1985 Elsevier F lblishers.)... 

The AW velocity in piezoelectric media is sensitive to changes in temperature. For most practical sensors, this fact necessitates some means of temperature control and/or compensation, as discussed in detail in Chapter 6. For most chemical sensing applications, substrates such as ST-quartz for SAW sensors and AT-quartz for TSM sensors, which have small temperature coefficients, are selected. 

Adherence to this model is indicated if there is a linear relationship between the logarithm of the rate of the chemical reaction and the logarithm of adsorbate concentration. Application of the LH and power-law models to respcmses from reagent-coated SAW sensors has been described by 2 11ers et al. [108]. 

Figure 5.13 Linear dynamic range for irreversible response (—Hz/min) of the PEM-coated SAW sensor to cyclopentadiene.

Unfortunately, in many instances the materials employed as sensor coatings are nonvolatile solids (polymers) for which 6 values cannot be calculated directly. Solubility parameters for these materials can be estimated, however, by immersion testing [172b], inverse gas chromatography [173,174] or ftom coated-SAW sensor responses [166]. In inverse chromatography, the polymeric coating material is used as a stationary phase on a GC column, and the specific retention volumes (V ) for several solutes are determined. Since the Vg is directly related to, Kc, the solubility parameter for the polymer coating can be derived from relationships similar to Equation 5.32. A similar approach is used to derive S, from SAW sensor response data [166]. 

The models discussed previously by no means represent an exhaustive list. In addition to the solubility models, approaches using molecular orbital computations have been used to study hydrogen-bonding mechanisms and to compare the results with SAW sensor data [187]. These ab-initio computations have been used successfully, but can currently only be applied to molecules of limited size because of the high cost and long computing time involved. 

The concepts underlying these methods can be extended to permit identification of both individual vapors and the components of vapor mixtures from the sensor response patterns [271]. A useful feature of this extended disjoint principal components regression (EDPCR) method is the integration of the qualitative and quantitative aspects of the sensor responses. Implementation of EDPCR has been demonstrated for arrays of polymer-coated SAW sensors exposed to a range of vapors and vapor mixtures [92a,92c,271]. Accurate identification and quantification of individual vapors and vapor-mixture components was achieved. 

Basics of Acoastk-Wave Sensor Design and Fabrication 341 Table 6.2 IDT Design Parameters for ST -Quartz-Based SAW Sensor Devices... 

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