Modification of indium - Tin oxide electrodes with thiophene copolymer thin films: Optimizing electron transfer to solution probe molecules

We describe the modification of indium-tin oxide (ITO) electrodes via the chemisorption and electropolymerization of 6-{2,3-dihydrothieno[3,4-b]-1.4- dioxyn-2-yl methoxy}hexanoic acid (EDOTCA) and the electrochemical co-polymerization of 3,4-ethylenedioxythiophene (EDOT) and EDOTCA to form ultrathin films that optimize electron-transfer rates to solution probe molecules. ITO electrodes were first activated using brief exposure to strong haloacids, to remove the top ̃8 nm of the electrode surface, followed by immediate immersion into a 50:50 EDOT/EDOTCA comonomer solution. Potential step electrodeposition for brief deposition times was used to grow copolymer films of thickness 10-100 nm. The composition of these copolymer films was characterized by solution depletion studies of the monomers and atomic force microscopy (AFM), X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy (reflection-absorption infrared spectroscopy (RAIRS)) of the product films. The spectroscopic data suggest that the composition of the copolymer approaches 80% EDOTCA when electropolymerization occurs from concentrated (10 mM) solutions. AFM characterization shows that electrodeposited poly(EDOT)/poly(EDOTCA) (PEDOT/PEDOTCA) films are quite smooth, with texturing on the nanometer scale. RAIRS studies indicate that these films consist of a combination of EDOTCA units with noninteracting -COOH groups and adjacent hydrogen-bonded -COOH groups. The EDOTCA-containing polymer chains appear to grow as columnar clusters from specific regions, oriented nearly vertically to the substrate plane. As they grow, these columnar clusters overlap to form a nearly continuous redox active polymer film. ITO activation and formation of these copolymer films enhances the electroactive fraction of the electrode surface relative to a nonactivated, unmodified "blocked" ITO electrode. Outer-sphere solution redox probes (dimethylferrocene) give standard rate coefficients, k_s ≥ 0.4 cm·^-1, at 10 nm thick copolymer films of PEDOT/PEDOTCA, which is 3 orders of magnitude greater than that on the unmodified ITO surface and approaches the values for k _s seen on clean gold surfaces.