GaPO4-based SHAPM sensors for liquid environments

Sensors based on the propagation of acoustic waves use a detection mechanism based on the perturbation of the acoustic waves characteristics. Any acoustic wave device is potentially a sensor: when the acoustic wave propagates on the surface of the device material, any perturbation that affects the propagating medium (i.e. temperature, pressure, relative humidity, mass loading, electric loading, viscosity loading, and so on) result in a change of the acoustic wave velocity and/or attenuation. Acoustic wave devices are described by the characteristics of the wave propagation, i.e. the wave velocity and the particle displacement components. Many combinations of polarization and velocity are possible, depending on the acoustic waveguide material types, its crystallographic orientation, the wave propagation direction, and the boundary conditions. Longitudinal waves are polarized parallel to the propagation direction, while shear horizontal and shear vertical waves are polarized parallel and normal to the propagating surface. All the acoustic wave sensors are able to work in gaseous environment, regardless of their polarization: only a subset of them can be used for the design of biosensors working in liquid environment. Thus biosensors based on electroacoustic devices require a careful design to be able to work in liquid environment and to show high sensitivity to the liquid properties (such as viscosity and conductivity). Sensors based on the propagation of in-plane polarized waves do not radiate appreciable energy into the liquids contacting the sensor surface: Love waves, surface transverse waves (STW), shear horizontal acoustic plate modes (SHAPM) and shear horizontal surface acoustic waves (SHSAW) are examples of acoustic waves whose shear horizontal polarization ensures no coupling between the liquid and the elastic propagating medium. On the contrary, electroacoustic devices based on the propagation of waves with a shear vertical displacement component are not suitable for liquid application, since they radiates compressional waves into the liquid, thus causing excessive damping. An exception to this rule occurs for devices based on the propagation of the elliptically polarized fundamental anti-symmetric Lamb mode, A0, when it propagates at a velocity lower than the sound velocity in the liquid. Among the piezoelectic materials, the most commonly used are quartz (SiO2), lithium tantalate (LiTaO3), and lithium niobate (LiNbO3). Each has specific advantages and disadvantages, which include temperature coefficient of delay (TCD), electromechanical coupling efficiency K2, and propagation velocity. Quartz shows specific cut angle and wave propagation direction that are suitable to obtain a low or high first order temperature dependence of the wave velocity. The latter condition is suitable when an acoustic wave temperature sensor has to be designed. On the contrary, LiNbO3 and LiTaO3 don't show any temperature stable cut but exhibit larger electroacoustic coupling efficiency than quartz. Gallium orthophosphate, GaPO4, is a relatively new and still poorly explored piezoelectric material that has the unique advantage to be able to withstand temperature as high as 900°C without losing its piezoelectric properties. Its chemical inertness makes it suitable for the implementation of sensors able to work in harsh environment and in extreme conditions. Recently this material has been studied for biosensing applications: results in the development of a GaPO4 micro balance for thermo gravimetric analysis and for affinity sensor applications in aqueous liquids, such as biosensors, are described in reference [1], while in reference [2] the creation and arrangement of biomolecule-binding sensors based on TiO2 nanofibrous scaffold grown on GaPO4