Strategically tailoring ethylene glycol side chains with bridged-carbonyl ester in polythiophene-based organic electrochemical transistors for bioelectronics

Abstract

Organic mixed ionic-electronic conductors (OMIECs) hold great promise in the fields of bioelectronics, optoelectronics, and energy storage devices, and are characterized by unique microregions that facilitate electron and ion transport during electrochemical operations in electrolytes. In particular, conjugated polymers (CPs) can serve as interfaces for OMIECs to facilitate reversible reactions during electrochemically-driven ion implantation. In applications involving organic electrochemical transistors (OECTs), this mechanism greatly enhances the amplification of electrical signals, while the ion transport properties vary depending on the design of the polymer side chains. In this study, we strategically designed the side chains in the synthesis of polythiophene (PT)-based CPs and systematically investigated their effects on the electrochemical properties as well as the modulation of the charge entry and exit characteristics of the films. To gain insight into the material properties and stacking states of the poly(ethylene glycol) (PEG) side chains on the CP backbone, X-ray photoelectron spectroscopy (XPS) and theoretical calculations of the CP surface energy were performed. In addition, we evaluated their hydrophilicity and protein-repelling properties by wettability tests and electrochemical dissipative quartz crystal microbalance measurements, respectively. The films of P1 (with branched alkyl-bridged carbonyl ester side chains), P2 (with tetraethylene glycol-bridged carbonyl ester side chains), and P3 (with tetraethylene glycol side chains) have distinctly different ion/electron transport properties. Unlike P1 and P3, P2 reduced film swelling during electrochemical operation in an aqueous NaCl electrolyte and showed a greater ability to repel proteins, especially bovine serum albumin. Subsequently, we integrated these CP films into the comb-like active-layer channels of OECTs. Our results show that the P2 film improves the ion/electron transport properties and adhesion on the electrodes without affecting the carrier mobility (μ) and clearly exhibits fully reversible electrochromic behavior at low operating voltages (<1 V). Furthermore, P2 exhibits superior long-term stability during OECT operation compared to P3. Our findings establish material design principles for bridged carbonyl ester PEG side-chain PTs, which are essential for fine-tuning their suitability for biomedical applications in OECTs.