Alex Tseng1,Tomasz Rebis2,Toshiya Sakata1
The University of Tokyo1,Poznan University of Technology2
Alex Tseng1,Tomasz Rebis2,Toshiya Sakata1
The University of Tokyo1,Poznan University of Technology2
Integrating electrically conductive and redox polymers (RPs) is a promising approach for sustainable development of high-performance electrical and electrochemical materials, not only for energy storage but also for bioelectronics and biosensors. Lignosulfonates (LS) exemplify a particularly green materials selection, being low cost and biorenewable, yet readily providing phenolic groups to access (<i>ortho-</i>)quinone redox (i.e., reversible proton coupled electron transfer, PCET) and sulfonic acids for polyanionic doping of conductive polymers (CPs). When blended with oxidatively stable PEDOT (poly(3,4-ethylenedioxythiophene), remarkable specific capacity (170 F.g<sup>-1</sup>) was reported in acidic conditions (0.1 M HClO<sub>4</sub>) [1] despite significant loading of electrochemically inactive components.<br/>Besides enhancing pseduocapacitance, quinone PCET may be further leveraged to serve as an electrochemical interface between technological devices and biological catalyst cycles [2]. However, in the mild pH conditions amenable to biology, it is well known that <i>ortho-</i>quinones exhibit limited stability towards redox cycling. Several possible explanations been offered up over the years, for instance: microstructural changes inhibiting proton diffusion [1,4], dissolution of the redox polymer [3], hydroxylation to higher order polyphenols [2], phase separation of CP and redox polymer [5], and cross-linking by the formation of ether linkages [6] or phenyl dimerization [7].<br/>These varied conclusions are likely views of the same fundamental property of <i>ortho-</i>quinone RPs—filtered through a range of fabrication and experimental conditions—which is that reversible quinone PCET is a kinetically controlled process. Future biological applications, therefore, require an understanding of how and when alternate electrochemical pathways become activated by the scarcity of protons in physiological conditions. Hence, we investigated the pH-dependent behaviour of PEDOT:LS thin-films via <i>in-situ </i>UV-Vis spectroelectrochemistry, charge transport characterization in an electrochemical transistor, and thermodynamic/kinetic analysis by the Laviron method. Our results paint a surprising picture: PEDOT, typically considered to be a “transparent” conductor serving simply as a wire to enhance electron transfer with quinones, is implicated in the formation of strongly bound complexes that appear to stabilize semiquinone radicals. Accordingly, previously observable PEDOT and quinone states are removed in proportion from measurement on experimental timescales. This kinetic process is mediated by the effective overpotential vs. PCET as a function of pH and polarization, and thus informs the optimal operating conditions for biosensing using PEDOT:LS.<br/><br/><br/>[1] F. N. Ajjan <i>et al.</i>, <i>J. Mater. Chem. A</i>, vol. 4, no. 5, pp. 1838–1847, Jan. 2016.<br/>[2] G. Milczarek, <i>Langmuir</i>, vol. 25, no. 17, pp. 10345–10353, Sep. 2009.<br/>[3] M. O. Bamgbopa <i>et al.</i>, <i>J. Mater. Chem. A</i>, vol. 7, no. 41, pp. 23973–23980, Oct. 2019.<br/>[4] T. Rebis <i>et al.</i>, <i>Electrochimica Acta</i>, vol. 204, pp. 108–117, Jun. 2016.<br/>[5] C. Che <i>et al.</i>, <i>Advanced Sustainable Systems</i>, vol. 3, no. 9, p. 1900039, 2019.<br/>[6] A. René <i>et al.</i>, <i>J. Phys. Chem. C</i>, vol. 116, no. 27, pp. 14454–14460, Jul. 2012.<br/>[7] A. A. Vereshchagin <i>et al.</i>, <i>Nanomaterials</i>, vol. 12, no. 11, p. 1917, Jan. 2022.