Electrochemical deposition and characterization of carboxylic acid functionalized PEDOT copolymers
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Conjugated polymer films are of considerable current interest for functionalizing the surfaces of a wide variety of devices including implantable biomedical electronics. Toward these ends, copolymer films of 3,4-ethylenedioxythiophene (EDOT) with a carboxylic acid functional EDOT (EDOTacid) were electrochemically deposited and characterized as a systematic function of the EDOTacid content (0, 25, 50, 75, and 100%). Chemical surface characterization of the films confirmed the presence of both EDOT and EDOTacid units. Toluidene blue assays showed that the surface concentration of the carboxylic acid groups increased to a maximum of 2.75 nmoles/mm2, and the contact angle measurements confirmed the increased hydrophilicity of the films with increasing EDOTacid content (decreasing from 52.6 to 32.5 degrees). Cyclic voltammetry showed that the films had comparable charge storage capacities regardless of their composition. The morphology of the films varied depending on the monomer feed ratio. The addition of EDOTacid induced a transition from a nodular, porous surface to a more dense, pleated surface structure. These methods provide a facile means for synthesizing electrically active carboxylic acid functional poly(3,4-ethylenedioxythiophene) copolymer films with tunable hydrophilicity and surface morphologies.
I. INTRODUCTION
A wide variety of biomedical devices involve the longterm integration of metallic or semiconducting components with living tissue. Examples include pacemakers,1 deep brain stimulators,2 cochlear implants,3 retinal prosthetics,4 and microfabricated cortical electrodes.5 Conjugated polymers are of interest for these devices because they transmit charge by both electron and ionic transport.6 They also provide an intermediate level of mechanical compliance between the rigid inorganic device substrate and the soft biological tissue. However, to optimize the performance of these devices, there remains an unmet need for developing functionalized conducting polymers that can be modified with biomolecules to improve the interface between the hard electrode and soft tissue.7 Conjugated polymers are also of interest for a wide variety of devices, including electrochromic displays, photovoltaics, and chemical sensors.8 A materials system that has been of particular scientific commercial interest is substituted polythiophenes, with particular focus on poly(3,4-ethylenedioxythiophene) (PEDOT). PEDOT is of particular interest because of its low oxidation potentials, high conductivity, superior environmental stability, and good chemical and electrochemical properties.7,9–12 Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2014.314 J. Mater. Res., Vol. 29, No. 23, Dec 14, 2014
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However, the EDOT side groups are not chemically reactive, making it difficult to create materials with more specific functionality for particular applications. To generate polymer films with more tailorable surface chemistries, it is
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