Thiophene-containing conducting polymers have been used in a range of applications, including supercapacitors, amperometric biosensors, and microfluidic pumping systems.¹  This versatility is based on the substituents of the thiophene and desired chemical properties that can be tailored through various organic syntheses.²  One particular thiophene polymer of significant interest is poly(3,4-ethylenedioxythiophene) (PEDOT) due to its high conductivity, low oxidation potential, reversible electrochemical doping, and stability in aqueous environments.³  Hydrophobic in nature, electropolymerization of PEDOT in non-aqueous media does not require additional reagents or processing.  However if the application requires aqueous media, as is the case for co-electropolymerization with enzymes, then solubilizing reagents, such as beta cyclodextrin (β-CD), and solubilization processes, such as lengthy sonication, are required.  PEDOT also lacks additional functional groups that could be used to easily derivatize its structure and tailor its chemical properties.  A commercially available thiophene derivative that is similar to PEDOT in structure but soluble in aqueous media is thieno[3,4-b]-1,4-dioxin-2-methanol (EDOT-CH₂-OH).  

Electrochemically deposited PEDOT-CH₂-OH in aqueous conditions exhibits characteristics such as increased surface area and charge capacity similar to that of PEDOT films.  Additionally the hydroxyl group allows for further modification by established synthetic protocols.  One such reaction is esterification of the hydroxyl group with succinic anhydride yielding 4-((2,3-dihydrothieno[3,4-b]-1,4-dioxin-2-yl)-methoxy)-4-oxobutanoic acid (EDOT-CH₂-OCO-(CH₂)₂-COOH).⁴  The extended carboxylic acid moiety from the polymer backbone provides a location for covalent attachment of enzymes and other molecules of interest.  Despite the functional advantages of the monomer EDOT-CH₂-OCO-(CH₂)₂-COOH its dissolution in aqueous conditions is pH dependent and a pristine film results in lower charge capacity than PEDOT-CH₂-OH.

Recent developments on the electropolymerization of PEDOT-CH₂-OH and EDOT-CH₂-OCO-(CH₂)₂-COOH in aqueous conditions on gold electrodes will be presented.  In this work variations of pH and molar ratios of the monomers were made to optimize electropolymerization and electrochemical behavior.  It was observed that although an alkaline pH allows for rapid and complete dissolution of EDOT-CH₂-OCO-(CH₂)₂-COOH in aqueous conditions, the resulting solution did not produce electropolymerized films.  It is believed that charge repulsion between the monomers prevents film deposition.  In acidic conditions the dissolution of EDOT-CH₂-OCO-(CH₂)₂-COOH took more time but resulted in stable films. Therefore it was of interest to determine the optimal pH and molar ratio between both monomers that resulted in facile co-electropolymerization and films with the electrochemical behavior of PEDOT-CH₂-OH and chemical functionality of EDOT-CH₂-OCO-(CH₂)₂-COOH.  Films were deposited by cyclic voltammetry using a three electrode electrochemical cell with gold as the working electrodes, a Ag/AgCl in saturated KCl as the reference, and Pt foil as the counter electrode.  

ACKNOWLEDGMENTS

Research was partially supported through the National Science Foundation (CBET-1336853) and the Arkansas Biosciences Institute, the major research component of the Arkansas Tobacco Settlement Proceeds Act of 2000.

REFERENCES

1. (a) Roncali, J., Conjugated poly(thiophenes): synthesis, functionalization, and applications. Chem. Rev. 1992, 92 (4), 711-38; (b) Roncali, J.; Blanchard, P.; Frère, P., 3,4-Ethylenedioxythiophene (EDOT) as a versatile building block for advanced functional π-conjugated systems. Journal of Materials Chemistry 2005, 15 (16), 1589; (c) Hiller, M.; Kranz, C.; Huber, J.; Baeuerle, P.; Schuhmann, W., Amperometric biosensors produced by immobilization of redox enzymes at polythiophene-modified electrode surfaces. Adv. Mater. (Weinheim, Ger.) 1996, 8 (3), 219-22; (d) Nash, C. K.; Fritsch, I., Poly(3,4-ethylenedioxythiophene)-Modified Electrodes for Microfluidics Pumping with Redox-Magnetohydrodynamics (MHD): Improving Compatibility for Broader Applications by Eliminating Addition of Redox Species to Solution. Anal. Chem. (Washington, DC, U. S.) 2015, Ahead of Print.

2. (a) Perepichka, I. F.; Besbes, M.; Levillain, E.; Sallé, M.; Roncali, J., Hydrophilic Oligo(oxyethylene)-Derivatized Poly(3,4-ethylenedioxythiophenes):  Cation-Responsive Optoelectroelectrochemical Properties and Solid-State Chromism. Chemistry of Materials 2002, 14 (1), 449-457; (b) Brisset, H.; Navarro, A.-E.; Moustrou, C.; Perepichka, I. F.; Roncali, J., Electrogenerated conjugated polymers incorporating a ferrocene-derivatized-(3,4-ethylenedioxythiophene). Electrochemistry Communications 2004, 6 (3), 249-253.

3. Groenendaal, L. B.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R., Poly(3,4-ethylenedioxythiophene) and its derivatives: past, present, and future. Adv. Mater. (Weinheim, Ger.) 2000, 12 (7), 481-494.

4. Luo, S.-C.; Ali, E. M.; Tansil, N. C.; Yu, H.-h.; Gao, S.; Kantchev, E. A. B.; Ying, J. Y., Poly(3,4-ethylenedioxythiophene) (PEDOT) nanobiointerfaces: thin, ultrasmooth, and functionalized PEDOT films with in vitro and in vivo biocompatibility. Langmuir : the ACS journal of surfaces and colloids 2008, 24 (15), 8071-8077.