Study of layer-by-layer self-assembled viscoelastic films on thickness-shear mode resonator surfaces

We describe quartz crystal electroacoustic admittance studies in thickness shear mode resonators loaded with self-assembled multilayers composed of alternate layers of glucose oxidase (GOx) and poly(allylamine) covalently attached to [Os(bpy)₂CIPyCOH]^-, (PAH-Os), deposited on a 3-mercaptopropanesulfonic acid (MPS)-modified gold on the quartz crystal. The complex acoustic impedance parameters, R_S and X_LS of a lumped-element Butterworth-Van Dyke (BVD) resonator have been determined for organized thin films of different thickness obtained by varying the number of enzyme layers, n, in (PAH-Os)_n(GOx)_n structures. The ellipsometric film thickness and mass for dry enzyme multilayer films and films in contact with water were evaluated, and the average film density was estimated. By combination of the estimated film thickness and density, the expression for the surface mechanical impedance of the lumped-element modified resonator (Granstaff and Martin model), and the liquid density and viscosity, we simulate the layer-by-layer film growth on the basis of the measured electroacoustic impedance. The complex impedance X_LS and R_S increase with film thickness and the enzyme films can be regarded as acoustically thin in the reduced state for films thinner than 600 nm. We have also measured electroacoustic parameters for PAH-Os/GOx self-assembled multilayers under electrochemical perturbation in a buffer electrolytic solution. The electrostatically self-assembled multilayers behaved as lossy viscoelastic films at 10 MHz with G′_f and G^′_f on the order of 106 Pa. The films became viscoelastic upon oxidation to Os(III), resulting in an increase of R_S and X_LS in the oxidized state with the number of (PAHOs)(GOx) bilayers due to film swelling and an increase in the shear moduli during oxidation.