Expanding the Materials Palette for Organic Electrochemical Transistor Channels Via Electropolymerization and Functionalized Pyrrole, Thiophene, and Aniline Monomers
Authors: Milroy Craig , Kim Steve 
Published: August 2024

Electrochemical transistors are electrolyte-gated devices that permit large signal amplification, ionic-to-electronic signal transduction, and controlled variable resistance. Given that their porous channel materials enable volumetric interaction with the adjacent electrolyte, OECT offer increased detection sensitivity compared to other electrolyte-gated devices such as field-effect transistors (FET).
Although these OECT devices have demonstrated utility in a wide range of applications, from neuromorphic computing platforms to bioelectronic devices, they are still in an early stage of development. As a result, a limited range of channel materials have been investigated. Because the magnitude of signal amplification in OECT is directly proportional to the channel transconductance, the vast majority of OECT studies have employed highly-conductive spin-coated formulations of polyethylenedioxythiophene:poly(styrene sulfonate) (PEDOT:PSS) as the active material. While PEDOT-PSS exhibits many desirable qualities for an OECT channel material (e.g., biocompatibility, high transconductance, relatively low mechanical modulus, and responsivity to aqueous ions and bioanalytes), the material has general susceptibility to changes in the ionic strength of the solution it is measuring, which limits its utility for ion-specific detection and in systems for which changes in ionic strength necessarily accompany the other specific changes that the OECT seeks to detect or influence (i.e. neurotransmitter detection). In addition, the material is almost exclusively deposited via spin-coating, which requires a premade solution of pre-determined physical characteristics (concentration, molecular weight, dopant-to-monomer ratio), and therefore limits the ability to for higher-throughput investigation of wide varieties of material configurations.
Although electropolymerization of conducting polymers like polypyrrole, polythiophene, and polyaniline is well-studied, only a few isolated reports1,2 have evaluated the benefits of electropolymerization for OECT channel fabrication and performance (i.e., compared to spin-coating or vapor-phase polymerization). However, in a more general sense, electropolymerization provides several additional advantages for OECT channel fabrication:
(1) reduce the total time and cost associated with device preparation by reducing the total number of required microfabrication steps;
(2) permit higher-throughput methods for fabricating channels with diverse characteristics and dopants;
(3) use dopants that allow operation in either depletion or accumulation mode (versus spin-coated PEDOT-PSS, which operates only in depletion mode);
(4) enable facile control over channel thickness and form-factor during synthesis;
(5) increase control over material properties that influence both electronic and ionic conductivity, such as polymer chain length/stiffness, spacing, and electrostatic affinity;
(6) provide a strategy for using electrochemical synthesis to tune the channel transconductance.
Unfortunately, there have so far been only isolated repots of electropolymerized OECT channels, and no systematic studies. In this report, our research team will demonstrate the effect of electrosynthesis parameters on channel material characteristics and device performance (i.e., total channel capacity, transconductance, ionic/electronic conductivity, and overall electrochemical impedance). We examined the influence of electrodeposition method (galvanostatic, potentiostatic, linear sweep voltammetry), monomer type, the total degree of polymerization, and the dopant used during electrosynthesis (comparing small, labile dopants such as chloride or tosylate, versus larger semi-labile dopants such as dodecyl benzene sulfonate, versus large immobile polymer dopants such as PSS). We also quantify the impact of introducing different functionalities (e.g., amine, carboxy, sulfonic acid) to the pyrrole, aniline, and thiophene monomers used to fabricate the channels). These functionalities provide motifs for addition functionalization (e.g. using carbodiimide chemistry) with enzymes, biorecognition elements, or redox signaling molecules.
Finally, we will present the steps we have taken to implement these devices within organ-on-a-chip systems, and demonstrate the utility of our devices for studying the formation and longitudinal evolution of neural networks within in vitro neuronal colonies.

References

S. Wustoni, T.C. Hidalgo, A. Hama, D. Ohayon, A. Savva, N. Wei, N. Wehbe, S. Inal. In Situ Electrochemical Synthesis of a Conducting Polymer Composite for Multimetabolite Sensing. Adv. Mater. Technol.

2020, 5, 1900943.

L. Zhang, T.L. Andrew. Deposition Dependent Ion Transport in Doped Conjugated Polymer Films: Insights for Creating High-Performance Electrochemical Devices. Adv. Mater. Interfaces
2017, 4, 1700873.