Electrochemically controlled binding and release of protonated dimethyldopamine and other cations from poly( N-methyl-pyrrole)/polyanion composite redox polymers

Abstract

The composite polymer, poly( N-methylpyrrole)/poly(styrenesulfonate) (PMP + PSS −), was synthesized by anodic polymerization of N-methylpyrrole onto a platinum electrode in the presence of poly(styrenesulfonate). When this polymer film on platinum was reduced cathodically to PMP 0PSS − in an aqueous solution of dopamine hydrobromide or dimethyldopamine hydrobromide, the cationic, protonated amines were incorporated into the film. Release of these compounds from the film could be accomplished by reoxidation in aqueous electrolyte solution. The total amount released and the release rate were studied using a thin layer cell with UV spectroscopic detection of the organic compound in the solution. Pulsed release was possible. At open circuit the film potential drifted, leading to changes in the amount of ion bound in the film. Under potentiostatic control, it was shown that the film reached equilibrium with the solution at various potentials over the range −0.4 to 0.4 V (SCE). Nemst plots of the equilibrium data for protonated dimethyldopamine (1H +) (0.5 or 1 m M) incorporation from aq. 0.01 M KCl into films of 0.5, 1.0, and 2.0 μm thickness had slopes of 250 mV. The films were half oxidized at about 0.1 V, but a reliable E o' could not be determined. Use of 50% sulfonated PSS − or Nation® in place of 100% sulfonated PSS − gave similar film properties, and similar Nemst plots. Films from polyvinylsulfonate or PAMPS did not bind appreciable amounts of 1H +. Potential step experiments showed that both uptake and release of 1H + were diffusion controlled. The apparent diffusion coefficient ( d app), measured spectrophotometrically, was invariant with the redox state of the film. D app was found to be independent of the magnitude of the potential step, the direction of the potential step, and the presence of supporting electrolyte (0.01 M KCl). Small variations in D app were observed when the solution concentration of 1H +, the film thickness, and the polyanion structure (PSS − vs. Nafion) were varied. Binding and release of BNA + methyl viologen, Ru(bpy) 3 2+, and protonated procaine gave similar results to those for 1H +.