MEASUREMENT OF VISCOELASTIC CHANGES AT ELECTRODES MODIFIED WITH REDOXHYDROGELS WITH A QUARTZ-CRYSTAL DEVICE

A fast high-frequency 'in situ' transfer-function measurement with a q uartz crystal microbalance (QCM) has been obtained by an ac voltage di vider formed by the quartz crystal impedance (Z(0)) and a measuring re sistor (R(m)). The frequency is swept by a voltage-controlled oscillat or (VCO) and the ratio of output (E(o)) and input (E(?)i) ac voltages gives the transfer function of the device. A non-linear fit of the tra nsfer-function modulus vs. frequency around the natural resonance freq uency of the system in the 10 MHz range was performed with a Butterwor th-Van Dyke equivalent circuit. Mass and viscoelastic components were evaluated with the Sauerbrey-Kanazawa-Bruckenstein equations. The meth od was tested by varying the viscoelasticity of the quartz surface in sucrose solutions [i.e. by varying the quantity (rho(L) eta(L))(1/2)] and mass sensitivity was calibrated by silver deposition. Fast measure ments in non-steady-state experiments were performed with redox hydrog els undergoing oxidation and reduction with anion and water exchange w ith the outside solution. Deconvolution of both rigid mass (Delta m) a nd viscous resistance (R) was achieved and this explained the abnormal behaviour of the resonance-frequency shift (Delta f) during cyclic vo ltammetry of ferrocene-modified poly(allylamine) hydrogels on a resona nt electrochemical QCM.