Gravimetric and viscoelastic changes during the oxidation-reduction of layer-by-layer self assembled enzyme multilayers wired by an Os-containing poly(allylamine) polymer

The ellipsometric and mass increase during the oxidation-reduction cycles of self-assembled layer-by-layer osmium complex derivatized poly(allylamine) and glucose oxidase multilayers due to the exchange of anions and solvent with the electrolyte and viscoelastic changes of the surface layer have been studied using the electrochemical quartz crystal microbalance (EQCM) technique. From the measurement of the quartz crystal impedance spectrum of films deposited on a thiolate Au-quartz crystal immersed in viscous aqueous electrolyte, simultaneously with cyclic voltammetry and potential steps, the real (R_S) and imaginary components (X_LS=ωL_S) of the quartz complex impedance load have been derived. It has been shown that the quartz crystal impedance of the composite resonator (film and liquid) increases with the number of deposited layers due to increase of both the film thickness and the shear modulus. The reduced polymer multilayers, with ellipsometric thickness ranging from 200 to 600 nm behave as acoustically thin films, while in the oxidized Os(III) films viscoelastic characteristics are apparent at 10 MHz. Using a viscoelastic model for the enzyme/polymer film we conclude that the film acoustic impedance increases with the number of self-assembled layers due to the thickness increase, and also to the increase of the shear modulus due to oxidation. The values of the shear modulus and loss tangent were estimated from the acoustic impedance, the film density and the ellipsometric thickness; the latter follows a linear dependence with the degree of oxidation. The increase of loss modulus, and hence loss tangent, is more pronounced, the larger the degree of polymer oxidation.