Gigahertz surface acoustic wave probe for chemical analysis

Abstract

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 protein. The GHz frequency components of the pulse are used to provide the highest operating frequencies so far reported and also to construct the first acoustic absorption spectrum associated with the evanescent field. Acoustic generation is sourced from a single non-linear SAW device that provides a series of harmonic frequencies, simultaneously. The received power level is determined from digital samples of the received pulse waveform. The power leaked into glycerol solutions at the fundamental frequency was found to be 50% smaller for pulses, than for continuous acoustic waves, an effect that could be related to the equilibration of the evanescent field. Increasing the concentration of the glycerol solutions or time exposed to the protein (IgG) solution, showed that the power losses from the surface acoustic wave pulse were broadly consistent with the behaviour of transverse shear mode sensors. Atomic force microscope measurements of the bare device revealed that the morphology of the silica overlayer was uniformly granular, whereas adsorbed protein films formed non-contiguous islands. Confirmation of the presence of the IgG film was obtained from quantitative X-ray photoelectron spectroscopy. An 8 gigasample per second digitising oscilloscope running a fast Fourier transform routine captured the acoustic absorption spectrum, and revealed a smooth characteristic for the glycerol and IgG, although for the latter, frequencies beyond 500 MHz were associated with an irregular spectrum. These multiple frequency measurements of the solid–liquid interface provide evidence that when the penetration depth and film thickness are similar, disruption of the predicted exponential form of the evanescent wave occurs, as indicated by the fluctuations seen in the absorption spectrum recorded. These preliminary results have shown that multiple frequency operation of single non-linear SH-SAW devices is possible, and an evanescent interfacial absorption spectrum can be obtained. By extending the measurement technique it may be possible to obtain additional information about the structure and composition of the solid–liquid interface.