Electrochemical quartz crystal microbalance study on growth and property of the polymer deposit at gold electrodes during oxidation of dopamine in aqueous solutions

Polymer growth at Au electrodes during cyclic voltammetric oxidation of dopamine (DA) in aqueous solutions was investigated as functions of DA concentration, solution pH and potential-sweep rate with the use of electrochemical quartz crystal microbalance technique. When the DA concentration was 2 x 10(-4) M or above and the solution pH was 3.86 or above, the intramolecular cyclization of the first-step oxidation product of DA occurred significantly and further isomerization and oxidation of the cyclization product led to polymer growth at an An electrode. An ECECEE mechanism for DA oxidation and subsequent polymerization of the oxidation product (5,6-indolequinone) at favorable pH is suggested. The poly(indole)-like polymerization pathway was acceptably supported by comparative minigrid-electrode experiments via Fourier transform infrared characterization of poly(indole) and the polymer from DA oxidation. The quasi first-order rate constant of the intramolecular cyclization was estimated at several pH values by cyclic voltammety. It was also found that the intramolecular cyclization and subsequent polymer deposition at the electrode can be notably inhibited by using various high-concentration supporting electrolytes, with inhibition sequences as ClO4-> NO3-> SO42-> gluconate > F-> citrate > CH3COO- for anions and NH4+> Na+> Li+> K+> Cs+> Rb+ for cations. In addition, an An electrode modified with the polymer from DA oxidation exhibited attractive cationic permselectivity, namely, effectively blocking the electrochemical reactions of anionic ferrocyanide and ascorbic acid (AA) while well retaining the electrochemical activities of hexaammineruthenium (III) and DA as cationic species. A 500-Hz polymer film could effectively block the redox current of AA up to 2.0 mM, The semi-derivative voltammetric peak height for DA oxidation was linear with DA concentration up to 1.3 x 10(-5) M, with sensitivities of 0.0766 and 0.119 mu A s-(1/2)/mu M, as well as lower detection limits of 4 x 10(-7) and 2 x 10(-7) M (S/N = 3) in a phosphate buffer solution without AA and with 1.0 mM coexisting AA, respectively. (c) 2005 Elsevier B.V. All rights reserved.