Gigahertz surface acoustic wave probe for chemical analysis

This paper considers the propagation of high frequency 0.1-2.6 GHz surface acoustic wave pulses in aqueous solutions of pure water, glycerol and prote in. The GHz frequency components of the pulse are used to provide the highe st operating frequencies so far reported and also to construct the first ac oustic absorption spectrum associated with the evanescent field. Acoustic g eneration is sourced from a single non-linear SAW device that provides a se ries of harmonic frequencies, simultaneously. The received power level is d etermined from digital samples of the received pulse waveform. The power le aked into glycerol solutions at the fundamental frequency was found to be 5 0% smaller for pulses, than for continuous acoustic waves, an effect that c ould be related to the equilibration of the evanescent field. Increasing th e concentration of the glycerol solutions or time exposed to the protein (I gG) solution, showed that the power losses from the surface acoustic wave p ulse were broadly consistent with the behaviour of transverse shear mode se nsors. Atomic force microscope measurements of the bare device revealed tha t the morphology of the silica overlayer was uniformly granular, whereas ad sorbed protein films formed non-contiguous islands. Confirmation of the pre sence of the IgG film was obtained from quantitative X-ray photoelectron sp ectroscopy. An 8 gigasample per second digitising oscilloscope running a fa st Fourier transform routine captured the acoustic absorption spectrum, and revealed a smooth characteristic for the glycerol and IgG, although for th e latter, frequencies beyond 500 MHz were associated with an irregular spec trum. These multiple frequency measurements of the solid-liquid interface p rovide evidence that when the penetration depth and film thickness are simi lar, disruption of the predicted exponential form of the evanescent wave oc curs, as indicated by the fluctuations seen in the absorption spectrum reco rded. These preliminary results have shown that multiple frequency operatio n of single non-linear SH-SAW devices is possible, and an evanescent interf acial absorption spectrum can be obtained. By extending the measurement tec hnique it may be possible to obtain additional information about the struct ure and composition of the solid-liquid interface.