Wireless excitation of quartz crystals immersed in an aqueous fluid

Abstract

A novel toroidal coil geometry able to induce remote acoustic waves in quartz crystals has been evaluated for the development of (bio)sensors. Remote acoustic generation in air was obtained for two alternative toroidal coils, with corresponding electrical impedance changes of 40 Ω for a PDMS- and 140 Ω for a ferrite-supported toroid respectively. It was found that the range of remote acoustic generation relative to the spiral coil standard was much improved, increasing the axial separation of their resonant sensing element from 0.1 mm to 20 mm, thereby allowing electromagnetic wave penetration across glass walls and fluid media to be utilised. Consideration of the transduction mechanism, along with measured cyclic changes in acoustic signal as a function of rotation, indicated that the large PDMS toroidal coil produced an asymmetric electric field. It was shown for the first time that a quartz crystal blank fully immersed in an aqueous fluid could support chemically sensitive shear acoustic standing waves that were excited and detected remotely. A signal to noise ratio of 30 ∶ 1 at 20.13 MHz was achieved by placing a ferrite supported toroidal coil on the lower side of a glass beaker containing a 12 × 0.25 mm AT crystal blank and 1 mL of water. This discovery allows wireless shear acoustic wave measurements to be performed with total separation between the electronic detection system and assays undertaken in fluidic systems.