Conductive polymer PEDOT:Tos (3,4-ethylenedioxythiophene doped with molecular tosylate) gained a considerable attention in various devices for bioelectronic applications, such as organic transistors and sensors. Many of these devices function upon oxidation/reduction processes in contact with aqueous electrolytes. So far, a theoretical insight into morphological changes, ion injection and water intake during these processes was rather limited. In the present work, we combined experiments and molecular dynamics simulations to study the water intake, swelling and exchange of ions in PEDOT:Tos film during the cyclic voltammetry. We showed that the film underwent significant changes in morphology and mass during the redox processes. We observed both experimentally and in simulations that the film lost its mass during reduction as tosylate and Na was expelled and gained mass during oxidation mainly due to the uptake of anions, i.e. tosylate and Cl. The results were in line with UV-VIS-NIR absorption measurements, and X-ray photoelectron spectroscopy (XPS) measurements, which revealed that during red-ox process a portion of Tos was replaced by Cl-as the counter-ion for PEDOT. Also, the relative mass change between the most oxidized and reduced states was ~10-14% according to both experiment and simulations. We detected an overall material loss of the film during voltammetry cycles indicating that a portion of the material leaving the film during reduction did not return to the film during the consecutive oxidation. Our combined experimental/simulation study unravelled the underlying molecular processes in the PEDOT:Tos film upon the redox process, providing the essential understanding needed to improve and assess the performance of bioelectronic devices.
Electrolyte-gated organic field-effect transistors (EGOFETs) represent a class of organic thin-film transistors suited for sensing and biosensing in aqueous media, often at physiological conditions. The EGOFET device includes electrodes and an organic semiconductor channel in direct contact with an electrolyte. Upon operation, electric double layers are formed along the gate-electrolyte and the channel-electrolyte interfaces, but ions do not penetrate the channel. This mode of operation allows the EGOFET devices to run at low voltages and at a speed corresponding to the rate of forming electric double layers. Currently, there is a lack of a detailed quantitative model of the EGOFETs that can predict device performance based on geometry and material parameters. In the present paper, for the first time, an EGOFET model is proposed utilizing the Nernst-Planck-Poisson equations to describe, on equal footing, both the polymer and the electrolyte regions of the device configuration. The generated calculations exhibit semi-qualitative agreement with experimentally measured output and transfer curves.
Figure 8. a) Transfer curves acquired on the same device (2D-IJ) at V DS = −0.5 V. • Virgin device, • device after ss-DNA immobilization, • device after hybridization. b) I D current versus time (V DS = −0.5 V, V GS = −0.6 V) of a device whose gate electrode was functionalized with ss-DNA. The red arrows represent the moments where injections of different solutions inside the PMMA well occurred: 1) injection of pure PBS; 2) injection of PBS + SH-DNA; injections of complementary DNA strands at 3) 10 µm; 4) 20 µm; 5) 40 µm; 6) 60 µm; and 7) 100 µm. Inset: gate current recorded on the same transistor.
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