There has been growing interest in the development of integrated acoustic sensors in the past two decades. Most of them are based on bulk acoustic waves (BAW) [1-14], surface acoustic waves (SAW) including shear horizontal and Love types [15-41], plate acoustic waves including Lamb and shear horizontal types [42-54], and most recently thin rod acoustic waves [55-60]. These sensors are used for chemical sensing in either gases or liquids, to determine concentrations of certain chemical and biological substances [6, 7, 9, 19, 12, 17, 18, 20, 24-28, 35, 39, 40,51,60,61], as well as for physical property monitoring such as film thickness [1-4], temperature [15,18,34], pressure [15,16,30], viscosity [8, 13, 16, 34, 36, 52,54], acceleration [29], and voltage [21], etc.

When additional mass is loaded onto the sensing surface of a device, it causes changes in the acoustic properties. In an acoustic delay line, a perturbation leads to a change of the wave velocity and thus a change of the time for the waves traveling from the transmitter to the receiver. In a resonator, the resonant frequency shifts from the unloaded resonant frequency, due to the loaded mass. A sensor coated with chemical  or biological selective layer(s) may be able to bind  particular particles of interest onto its surface. By  measuring and analyzing the change of the acoustic  parameters due to the mass loading, the so called  gravimetric effect, information about the concentration  of the attached substances in the medium (gas or liquid)  may be obtained. A review article [61] has outlined  several basic requirements of such chemically selective  coatings. In order to reduce the sensor noise caused by  environmental fluctuations such as temperature and  pressure, in general a dual channel configuration  [19,22] was used.

Many recent studies have been reported on membranes and fibres, which have considerably higher sensitivity than traditional BAW and SAW devices [46-58]. In general, the mass sensitivity is inversely proportional to the mass per unit area of a characteristic layer in the thickness direction of the device substrate. Physically this layer is the active region of the device that stores elastic energy and is perturbed by changes of surface mass. For the BAW and SAW sensors the thickness of this layer is approximately the operating wave length, and for-the plate and thin rod devices it is the plate thickness or the rod diameter. To increase the sensitivity of the BAW or SAW sensors, the wavelength must be decreased, or equivalently the frequency must be increased. By using very thin plates or fibres, the sensitivity can be substantially increased independent of the wavelength and thus the wave velocity. In addition, for a given acoustic wavelength, in the lowest antisymmetric (A₀) mode the phase velocity decreases as the plate thickness or the rod diameter reduces. Therefore, these sensors can work at a very low frequency and velocity with a high sensitivity. Furthermore, the sensitivity of a lowest symmetric (S₀) or shear-horizontal (SH₀) plate acoustic wave sensor and that of a lowest order extensional or torsional fibre acoustic wave sensor are also inversely proportional to their thickness/diameter, when the product of the frequency and the thickness (or diameter) is very small [50-64]. In addition to the high sensitivities, thin plate acoustic wave devices offer a significant advantage in that the sensing region and the device electronics are separated by the thin plate [42,45-48,50,51].

 The previous discussion assumed that the  perturbation of the sensor by the medium is uniquely  due to mass loading. In fact it is now well known [65,  66] that many parameters of the environmental medium have a direct effect on sensor response. These include liquid density and viscosity, as well as the elastic modulus and electrical conductivity of the coating layer. These effects are by no means negligible in specific cases, in fact they may well may be dominant. For example, Ricco et al [67] found in NO₂ molecular solutions that the conductivity or acousto-electric term was of the order of a thousand  times greater than the mass loading effect. Grate et al [68] showed that for poly (isobutylene) coated sensors, swelling induced modulus changes produced sensor responses four to six times greater than those due to mass loading. Nevertheless, in a large number of cases the mass loading effect is the principle or unique cause of the sensor response, and we restrict our attention to this case in the present report.