Modification of commercially available low-loss SAW devices towards an immunosensor for in-situ measurements in water

An acoustoelectric immunosensor which closely resembles the “classic” surface acoustic wave (SAW) technique for gas detection is presented. The device operates at frequencies above 100 MHz and allows in-situ measurements in water with a typical additional attenuation of only 4 dB compared to that in air. This is achieved by using commercially available low-loss SAW devices operating with horizontally polarized surface transverse waves (STW) on a lithium tantalate (LiTaO₃) substrate. The devices were used as the frequency determining element of an oscillator circuit. Modifications of the devices towards an immunosensor are done in three steps: Precoating with a shielding layer of polyimide to prevent corrosion of the aluminium transducers, chemical activation of the polyimidized surface and subsequent covalent precipitation of an antibody layer. The immuno-specific adsorption of antigens from solution can be monitored directly by the decrease in resonant frequency arising from the surface mass loading