COVALENT BINDING OF VIOLOGEN TO ELECTRODE SURFACES COATED WITH POLY(ACRYLIC ACID) PREPARED BY ELECTROPOLYMERIZATION OF ACRYLATE IONS .2. EFFECT OF THE IONIZATION STATE OF THE POLYMERIC COATING ON THE FORMAL POTENTIAL OF VIOLOGEN

The contribution of carboxy groups of poly(acrylic acid) (P(AA)) to the electrochemical properties of soluble and linked viologen molecules was studied by cyclic voltammetry. P(AA)-coated gold electrodes showed a pH-dependent selectivity towards anionic (hexacyanoferrate) or cationic (methyl viologen) redox compounds. A formal redox potential of -0.62 V (vs. saturated calomel electrode), independent of pH, was found for aminopropyl methyl viologen (MAV) in solution. However, the redox potential of this viologen covalently linked to P(AA) showed a complex dependence on the pH: it was constant when the pH was lower than 4.2 or higher than 6.5, while there was a shift between these two negative potentials when the pH varied from 4.5 to 6.5. This pH dependence was suppressed after esterification or amidation of the free carboxy group's of P(AA). The redox potential of viologen bound to P(AA) changed with the loading of viologen. Redox potentials of soluble and covalently linked viologen showed a different dependence on NaCl concentration at pH 7.5. The electrochemical behaviour of viologen bound to P(AA) is attributed to a change in the intrinsic oxidizing or reducing strength of the MAV.+/MAV2+ pair as a consequence of its incorporation with P(AA). The possibility of tuning the redox potential of the immobilized viologen in a controlled way is demonstrated and its application to enzyme electrodes discussed.