Chemical liquid-phase detection using guided SH-SAW: theoretical simulation and experiments

Results are presented for direct chemical sensing in liquid environments using guided shear horizontal surface acoustic wave (SH-SAW) sensor platforms on 36° rotated Y-cut LiTaO₃. Two different sensor geometries are theoretically analyzed. Complex bulk and shear moduli are utilized to represent the viscoelastic properties of the polymers and estimate their influence on the velocity shift and attenuation change, hence on the sensor characteristics. Experimental results are presented and discussed for dual delay line devices with a reference line coated with PMMA and a sensing line coated with a chemically sensitive polymer, which acts as both a guiding layer and a sensing layer. Various chemically sensitive polymers are investigated, and the tested analytes include toluene, ethylbenzene and xylene. Analytes in the low concentration (1 ppm to 60 ppm) range in aqueous solutions are tested. Stability, sensitivity and partial selectivity are investigated by varying the coating thickness and curing temperature for the chemically sensitive layer. Partition coefficients for polymer-aqueous analyte pairs are used to explain the observed trend in sensitivity. Both mass loading and the coating viscoelasticity change influence the sensor response. A low ppb level detection limit is estimated from the present experiment measurement.