Adsorption of viologen-based polyelectrolytes on carboxylate-terminated self-assembled monolayers

The adsorption of viologen-containing polyelectrolytes on the surface of carboxylate-terminated, self-assembled monolayers was investigated by voltammetric methods. The two polyelectrolytes used had either butyl (VC4) or undecyl (VC11) linkages between the repeating viologen (4,4'-bipyridinium subunits. Both polyelectrolytes were found to adsorb at neutral pH on monolayers prepared by the self-assembly of either 3-mercaptopropionic (HSC2COOH) or 8-mercaptooctanoic (HSC7COOH) acids on gold. However, the adsorption was optimized by using the more hydrophobic polyelectrolyte (VC11) and the longer thiol (HSC7COOH). The gradual deprotonation of the HSC7COOH monolayer in the pH range 6-11 enhances the adsorption of the VC11 polyelectrolyte. From these data, an apparent pK(a) of similar to 8 for the monolayer -COOH groups was obtained. The overall degree of packing of the viologen subunits at the monolayer-solution interface was found to be rather poor. The combined monolayer-polyelectrolyte interfacial structures did not block effectively the reduction of dissolved Ru(NH3)(6)(3+) ions on the underlying gold electrode.