Response of a thickness-shear mode quartz crystal resonator upon hydrogel formation

The response of an AT-cut quartz crystal piezoelectric resonator operating in the thickness shear mode (TSM) during polymer gel formation has been examined by acoustic network analysis. The series and parallel resonant frequencies as well as the amplitude and phase of the impedance of the TSM resonator were monitored in real-time when a layer of polyacrylamide gel was formed on one side of the sensor. Various concentrations of radical initiator and crosslinking agents were used to control the rates of polymerization and crosslinking during the hydrogel formation. The increase in the magnitude of the minimum impedance and the decrease in the maximum phase angle were observed as a result of the damping of both the magnitude-frequency and phase-frequency curves, caused by the transformation of the contacting medium from a viscous liquid to a viscoelastic hydrogel. Significant difference was observed between the responses of series and parallel resonant frequencies. The series resonant frequency increases while the parallel resonant frequency decreases during gel formation, and the magnitude of change in the parallel frequency is several times that in the series frequency. Small variations in the gel-forming solutions such as the depletion of buffer and addition of double-stranded DNA cause distinct changes in the response profiles of the quartz resonator.