Measurement of viscoelastic changes at electrodes modified with redox hydrogels with a quartz crystal device

Abstract

A fast high-frequency ‘in situ’ transfer-function measurement with a quartz crystal microbalance (QCM) has been obtained by an ac voltage divider formed by the quartz crystal impedance (Z_Q) and a measuring resistor (R_m). The frequency is swept by a voltage-controlled oscillator (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 transfer-function modulus vs. frequency around the natural resonance frequency of the system in the 10 MHz range was performed with a Butterworth–Van Dyke equivalent circuit. Mass and viscoelastic components were evaluated with the Sauerbrey–Kanazawa–Bruckenstein equations.

The method was tested by varying the viscoelasticity of the quartz surface in sucrose solutions [i.e. by varying the quantity (ρ_Lη_L)^1/2] and mass sensitivity was calibrated by silver deposition.

Fast measurements in non-steady-state experiments were performed with redox hydrogels undergoing oxidation and reduction with anion and water exchange with the outside solution. Deconvolution of both rigid mass (Δm) and viscous resistance (R) was achieved and this explained the abnormal behaviour of the resonance-frequency shift (Δf) during cyclic voltammetry of ferrocene-modified poly(allylamine) hydrogels on a resonant electrochemical QCM.