Publication 580

J. Am. Chem., 126 (32) 10095 -10108, 2004 DOI: 10.1021/ja0493502 S0002-7863(04)09350-3
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

Contribution from the Laboratoire d'Electrochimie Moléculaire, (CNRS UMR 7591), Université Paris 7, 2 place Jussieu, Case 7107, 75251, Paris cedex 05, France. E-mail: demaille@paris7.jussieu.fr

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 ~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.