RESPONSE OF A THICKNESS-SHEAR MODE QUARTZ-CRYSTAL RESONATOR UPON HYDROGEL FORMATION

The response of an AT-cut quartz crystal piezoelectric resonator opera ting in the thickness shear mode (TSM) during polymer gel formation ha s been examined by acoustic network analysis. The series and parallel resonant frequencies as well as the amplitude and phase of the impedan ce of the TSM resonator were monitored in real-time when a layer of po lyacrylamide gel was formed on one side of the sensor. Various concent rations of radical initiator and crosslinking agents were used to cont rol the rates of polymerization and crosslinking during the hydrogel f ormation. The increase in the magnitude of the minimum impedance and t he decrease in the maximum phase angle were observed as a result of th e damping of both the magnitude-frequency and phase-frequency curves, caused by the transformation of the contacting medium from a viscous l iquid 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 frequ ency decreases during gel formation, and the magnitude of change in th e parallel frequency is several times that in the series frequency. Sm all variations in the gel-forming solutions such as the depletion of b uffer and addition of double-stranded DNA cause distinct changes in th e response profiles of the quartz resonator. (C) 1997 Elsevier Science B.V.