Probing the structure and dynamics of end-grafted flexible polymer chain layers by combined atomic force-electrochemical microscopy. Cyclic voltammetry within nanometer-thick macromolecular poly(ethylene glycol) layers

The combined atomic force-electrochemical microscopy (AFM-SECM) technique was used in aqueous solution to determine both the static and dynamical properties of nanometer-thick monolayers of poly(ethylene glycol) (PEG) chains end-grafted to a gold substrate surface. Approach of a microelectrode tip from a redox end-labeled PEG layer triggered a tip-to-substrate cycling motion of the chains' free ends as a result of the redox heads' oxidation at the tip and re-reduction at the substrate surface. As few as approximately 200 chains at a time could be addressed in such a way. Quantitative analysis of the data, in the light of a simple model of elastic bounded diffusion SECM positive feedback, gave access to the end-tethered polymer layer thickness and the end-to-end diffusion coefficient of the chains. The thickness of the grafted PEG layer was shown to increase with the chain surface coverage, while the end-to-end diffusion coefficient was found to be constant and close to the one predicted by Rouse dynamics. At close tip-substrate separation, slowing of the chains' motion, as a consequence of their vertical confinement within the tip-substrate gap, was observed and quantitatively modeled.