Electrochemical synthesis and characterization of poly(3,4-ethylenedioxythiophene) (PEDOT) and PEDOT derivatives
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Date
2023
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University of Delaware
Abstract
Electrochemically deposited thin films of poly(3,4-ethylenedioxythiophene) (PEDOT) and similar conjugated polymers have become widely used in a variety of applications including chemical sensors and photovoltaic devices. A combination of factors including the dual ionic and electronic conductivity that arises from doping with ions and conjugation of the carbon backbone respectively along with chemical stability are responsible for the scientific and commercial interest in PEDOT. Functionalized versions of the EDOT monomers (EDOT+) and corresponding PEDOT polymers (PEDOT+) and copolymers make it possible to precisely and systematically modify the properties of these materials. These modified monomers and polymers can in principle also be electrochemically deposited onto substrates, however the detailed polymerization and deposition conditions are sensitive to these changes in chemistry. ☐ Maleimide-functionalized 3,4-ethylenedioxythiophenes (EDOT+’s) were modified with biologically relevant groups of cholesterol, adamantane, and cysteine to form functional EDOT+ monomers. Investigation of EDOT+ polymerization lead to a better understanding of the electrochemical polymerization process through solvent control and current flow. The PEDOT+ polymers themselves differed from the unmodified PEDOT films based on the functionality of the group attached and how well suited that made the EDOT+ for electrical deposition. Optimization of EDOT+ properties was further investigated by performing layered deposition of different EDOT+ compositions. The biocompatibility of the monomers was examined using cell viability testing. ☐ Impedance spectroscopy is a powerful technique for characterizing conjugated polymers of interest for bioelectronic applications. This technique enables detailed understanding of the electrical systems and equivalent circuit models involved. By examining the charge transport processes of the whole system, the critical component limiting charge flow become apparent and allows for future, system specific device design. Systematically changing the conductivity of the electrolytic solution (which is a proxy for the many different biological systems in the human body) has drastic effects on the impedance spectra. The influence of electrodes, solution, and PEDOT coatings with changing electrolytic conductivity was directly correlated to equivalent circuit models. A more comprehensive understanding of transport behavior was detailed along with a solution resistance independent metric of low frequency transition (fL) and high frequency transition (fH) to characterize PEDOT deposition. ☐ Liquid-solid phase transition properties of PEDOT systems were examined using both a commercial PEDOT:PSS suspension and a PEDOT:PSS DMSO hydrogel. Combined impedance spectroscopy and microscopy allowed for direct correlation of the electrical circuit element systems to known, visible transitions. DMSO was found to slow down the transitions due to the change in the PEDOT structure that traps water to form the hydrogel and causes lowered impedances. Drying of PEDOT suspensions showed a similar change in the equivalent circuit model as seen when depositing PEDOT which can be described by a simple circuit of a resistor (R) followed by a capacitor (C) and constant phase element (CPE) in parallel (R(CPEC)). ☐ A novel method for the synthesis of PEDOT+ through the use of deep eutectic solvents (DES) has been explored. This method allowed us to use low temperatures, and high concentration monomer-solvent-polymer mixtures. Traditional EDOT+ solutions require low concentrations due to low monomer solubility which limits the final polymer chain length and resulting polymer properties. By taking advantage of hydrogen bonding solvents composed of a hydrogen bond donor EDOT+ (EDOT-acid and EDOT-urea) and hydrogen bond acceptor (choline chloride), EDOT+ DES were synthesized and characterized using a variety of thermal, optical, electrical, and spectroscopic techniques.
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Keywords
Electrochemistry, Impedance spectroscopy, Polymer characterization, PEDOT, Chemical sensors