APPRAISAL OF THE FUNDAMENTAL OPERATION OF QUARTZ-CRYSTAL SHEAR-MODE BIOSENSORS

The nature of the sensing responses of planar AT-cut quartz crystal shear-wave biosensors is examined and the close link between the shear-wave propagation constants and the viscoelastic compliance of the material on the crystal surface is emphasised. Change in the crystal resonance frequency can be ascribed ultimately to compliance changes in the elemental reaction volumes at the site of each sensor molecule. Experimentally observed crystal surface roughness is accommodated within the model as a composite layer of quartz, electrode metal, sensor molecules and ambient medium. This approach is able to account for the change in crystal resonance frequency when air is replaced by water as the ambient medium, for the changes which result from the detection of biotin by avidin immobilised on the surface and for those which accompany the build-up of successive surface layers by sequential reactions of dodecane bisbiotin and avidin.