Elucidating the Structure-Function Relationship of Poly(3,4-theylenedioxythiophene) Films to Advance Electrochemical Measurements

Previous work has demonstrated the utility of poly(3,4-theylenedioxythiophene) (PEDOT) electrodes for electrochemical detection of neurochemicals. Although these electrodes have been implemented successfully, there is minimal data linking redox mechanisms, electron-transfer characteristics, and sensor lifetime to the electrode molecular composition. Common polymer electrodes are made from commercially available PEDOT:polystyrenesulfonate (PEDOT:PSS), which is easily processed but has slow electron-transfer kinetics and short electrochemical lifetimes. Here, we describe vapor-phase synthesized PEDOT:tosylate films that have a higher conductance and a much lower apparent capacitance than PEDOT:PSS (99 ± 8 versus 2390 ± 130 μF/cm²). Additionally, we show that the electron-transfer kinetics and electrochemical lifetime are both improved. To investigate the chemical causes of these improvements we used ultraviolet-visible absorbance and X-ray photoelectron spectroscopy (XPS). We discovered that the high density of PEDOT incorporated into PEDOT:tosylate films coupled with the lack of impurities and replacing the polymeric dopant (PSS) leads to both increased conductance and reduced film capacitance. This is most clearly demonstrated through the doping ratio of 3.80 ± 0.10 in vapor-phase synthesized PEDOT:tosylate versus 0.20 ± 0.02 in PEDOT:PSS. Furthermore, the electrochemical lifetime of the films is dependent upon the amount of PEDOT present. XPS data was used to elucidate the mode of failure of these electrodes. This begins to illuminate the mechanism of electron transfer at conducting polymers electrodes. Understanding both the characteristics that improve the quality of conducting polymer electrodes and the mechanism of electron transfer therein is a crucial step in the wider adaptation of these materials in biosensor applications.