THE MASS-LOADING SENSITIVITY OF ACOUSTIC LOVE WAVE BIOSENSORS IN AIR

The theoretical mass-loading sensitivity of Love wave biosensors compo sed of thin layers of SiO2 on ST-cut quartz is compared with measured sensitivities. The comparison presented here was prompted by earlier r esults in which theoretical predictions showed reasonable agreement fo r the mass-loading sensitivity, over a limited range of SiO2 layer thi cknesses, but were unable to predict the velocities of the Love waves, and could not reproduce the rapid loss of sensitivity as the layer th ickness increased beyond its optimum value. In the earlier work, the t heory was based on wave propagation in isotropic, non-piezoelectric, l ayered materials, combined with perturbation theory to predict the eff ect on wave velocity of a thin, solid, mass-loading layer. We therefor e wished to determine whether the previous discrepancies between theor y and experiment arose because of the use of isotropic theory to descr ibe the material properties of the layers. Further theory was therefor e performed, in which the anisotropic and piezoelectric nature of the layers was included. We show in this paper that the full theory gives an improved prediction of the velocities of the guided Love waves, aga in predicts the trend in the variation of mass-loading sensitivity wit h SiO2 layer thickness, measured using sputtered gold as the mass-load ing layer, and correctly predicts the optimum layer thickness at which maximum sensitivity occurs. However, the theory underestimates the ma ximum sensitivity and again does not predict the rapid decrease in sen sitivity beyond this maximum. The reason for this discrepancy is still , therefore, unclear. One possible explanation, that electrical effect s were partially responsible for the frequency changes recorded in the experiments, is discounted as the fuller theory shows that any such e ffects should be negligible for the Love waves being considered. We co nclude that a theory based on a homogeneous guiding layer perfectly ad hered to a piezoelectric substrate is not adequate to describe the mea sured sensitivities of the Love wave devices.