We develop a general model that describes the electrical responses of thickness-shear mode resonators subject to a variety of surface conditions. The model incorporates a physically diverse set of single-component loadings, including rigid solids, viscoelastic media, and fluids (Newtonian or Maxwellian). The model allows any number of these components to be combined in any configuration. Such multiple loadings are representative of a variety of physical situations encountered in electrochemical and other liquid-phase applications, as well as gas-phase applications. In the general case, the response of the composite load is not a linear combination of the individual component responses. We discuss application of the model in a qualitative diagnostic fashion to gain insight into the nature of the interfacial structure, and in a quantitative fashion to extract appropriate physical parameters such as liquid viscosity and density and polymer shear moduli.

The electrical response of polymer-coated acoustic wave sensors depends on changes in the surface mass loading and changes in viscoelastic properties of the coating material. In this paper we consider the acoustic behaviour and the electrical response of a thickness-shear mode resonator on changes in shear parameters of the coating material at its fundamental frequency as well as its third and fifth harmonics. The changes in material properties were induced by temperature changes. Both a glassy and a rubbery polymer were investigated. The complex shear parameter and dynamic glass transition temperature were calculated from impedance measurements.

The design and performance of guided shear horizontal surface acoustic wave (guided SH-SAW) devices on LiTaO3 substrates are investigated for high-sensitivity chemical and biochemical sensors in liquids. Despite their structural similarity to Rayleigh SAW, SH-SAWs often propagate slightly deeper within the substrate, hence preventing the implementation of high-sensitivity detectors. The device sensitivity to mass and viscoelastic loading is increased using a thin guiding layer on the device surface. Because of their relatively low shear wave velocity, various polymers including poly(methyl methacrylate) (PMMA) and cyanoethyl cellulose (cured or cross-linked) are investigated as the guiding layers to trap the acoustic energy near the sensing surface. The devices have been tested in biosensing and chemical sensing experiments. Suitable design principles for these applications are discussed with regard to wave guidance, electrical passivation of the interdigital transducers from the liquid environments, acoustic loss, and sensor signal distortion. In biosensing experiments, using near-optimal PMMA thickness of approximately 2 microm, mass sensitivity greater than 1500 Hz/(ng/mm2) is demonstrated, resulting in a minimum detection limit less than 20 pg/mm2. For chemical sensor experiments, it is found that optimal waveguide thickness must be modified to account for the chemically sensitive layer which also acts to guide the SH-SAW. A detection limit of 780 (3 x peak-to-peak noise) or 180 ppb (3 x rms noise) is estimated from the present measurements for some organic compounds in water.

The radial dependence of mass sensitivity of the sensing surface is analytically calculated for two examples of “modified-electrode” quartz crystal resonators (QCR). The term “modified-electrode” QCR is used here with respect to the conventional QCR which has two identical circular and concentric electrodes. For these QCRs, the sensing surface is divided into a fully electroded, a partially electroded, and an unelectroded region, and the efficiency of each region is evaluated in terms of the electrode mass loading factor. Such QCRs are typically investigated for sensor applications in which the electrical properties of the liquid load or the coating deposited on the sensing surface (electroded and partially electroded regions) are being measured in addition to mass loading. While modified-electrode QCRs can be viewed as a simple capacitance sensor in those applications, the use of a piezoelectric crystal resonator in the narrow range of frequencies near resonance and antiresonance allows for a direct measurement of the capacitance through the antiresonant frequency, provided that the device damping (motional resistance) is not too high or that the resonance quality factor, Q, is high enough for a stable vibration under the load. It is shown that, for some values of the electrode mass loading factor, the off-electrode efficiency (partially electroded and unelectroded region efficiency) can still have a significant contribution to the overall surface area mass sensitivity. Knowledge of the efficiencies is needed to determine the loading area required for stable QCR sensor operation. This is because additional dissipation of energy into the load can occur, especially for cases where the sample load extends to the unelectroded surface, which has a nonnegligible particle displacement amplitude. It is also shown that, for some applications involving a liquid load and for some values of the electrode thickness, the shear particle displacement profile is such that compressional wave generation can contribute significantly to device damping, thus making the device unstable. Experimental measurements of the mass sensitivity profile on the surface are also performed for those QCRs and compared to theory.

We derive a lurnped-elemen~equivalent-circuit model for the thickness shear mode (TSM) resonator with a viscoelastic film. This modified ButterWorth-Van Dyke model includes in the motional branch a series LCR resonator, representing the quartz resonance, and a parallel LCR resonator, representing the film resonance. This model is valid in the vicinity of film resonance, which occurs when the acoustic phase shift across the film is an odd multiple of 7d2radians. This model predicts accurately the frequency changes and darnping that arise at resonance and is a reasonable approximation away fi-om resonance. The elements of the model are explicitly related to film properties and can be interpreted in terms of elastic energy storage and viscous power dissipation.The model leads to a simple graphical interpretation of the coupling between the quartz and film resonances and facilitates understanding of the resulting responses. These responses are compared with predictions fi-omthe transmission-line and the Sauerbrey models. KEYWORDSThickness-shear mode resonatoq quartz crystal microbakmce; viscoelasticity; film resonance; equivalent circuit; lumped-element model; transmission-line model.

Both a transmission-line model and its simpler variant, a lumped-element model, can be used to predict the responses of a thickness-shear-mode quartz resonator sensor. Relative deviations in the parameters computed by the two models (shifts in resonant frequency and motional resistance) do not exceed 3% for most practical sensor configurations operating at the fundamental resonance. If the ratio of the load surface mechanical impedance to the quartz shear characteristic impedance does not exceed 0.1, the lumped-element model always predicts responses within 1% of those for the transmission-line model.

Direct chemical sensing in liquid environments using polymer-guided shear horizontal surface acoustic wave sensor platforms on 36° rotated Y-cut LiTaO3 is investigated. Design considerations for optimizing these devices for liquid-phase detection are systematically explored. Two different sensor geometries are experimentally and theoretically analyzed. Dual delay line devices are used with a reference line coated with poly (methyl methacrylate) (PMMA) and a sensing line coated with a chemically sensitive polymer, which acts as both a guiding layer and a sensing layer or with a PMMA waveguide and a chemically sensitive polymer. Results show the three-layer model provides higher sensitivity than the four-layer model. Contributions from mass loading and coating viscoelasticity changes to the sensor response are evaluated, taking into account the added mass, swelling, and plasticization. Chemically sensitive polymers are investigated in the detection of low concentrations (1−60 ppm) of toluene, ethylbenzene, and xylenes in water. A low-ppb level detection limit is estimated from the present experimental measurements. Sensor properties are investigated by varying the sensor geometries, coating thickness combinations, coating properties, and curing temperature for operation in liquid environments. Partition coefficients for polymer−aqueous analyte pairs are used to explain the observed trend in sensitivity for the polymers PMMA, poly(isobutylene), poly(epichlorohydrin), and poly(ethyl acrylate) used in this work.

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