Using surface plasmon resonance and the quartz crystal microbalance to monitor in situ the interfacial behavior of thin organic films

We have used a combination of surface plasmon resonance (SPR) and the quartz crystal microbalance (QCM) to monitor in situ the solution-phase adsorption of the perfluoropolyether lubricant Fomblin ZDOL onto a silver surface. This dual-probe technique was then extended in a novel way by the addition of electrochemical control and used to monitor the electrochemically induced solution-phase desorption of octadecanethiol (C18S) from a gold surface as well as the electrochemical polymerization of polypyrrole (PPY) on a gold surface. The experimental results obtained by the joint technique compare favorably with those obtained using SPR and QCM independently. The combination allows us to measure simultaneously the optical and acoustic properties of these materials as they interact with the metallic surface. While SPR and QCM have similar resolution and are both able to follow deposition in real time, there are distinct advantages to the simultaneous measurement. These advantages allow one to (1) test the validity of the governing equations often used to analyze data collected using the two techniques, bringing to light weaknesses in the assumptions inherent in these equations, (2) calculate interfacial density and refractive index values in a system where the bulk values are known and the physical state of the adsorbed material is similar to that of the bulk, (3) show that the viscoelastic properties of an adsorbed material change significantly as the material desorbs from an interface, and (4) observe the evolution in the electronic and chemical properties of a conducting polymer film as it is being deposited while precisely monitoring the mass of the deposited film.