Designing Polymeric Mixed Conductors and Their Application to Electrochemical‐Transistor‐Based Biosensors

Abstract

of biomedical use, the prerequisite of mixed conductors is extended to biocompatibility and long-term stability in vivo. Recently, remarkable advances have been made in materials research, and considerable efforts have been invested to satisfy these demands. [32-35] Despite a good number of review articles on organic bioelectronics and OECTs, only a few reviews have focused on the correlation between the molecular structures of mixed conductors and the performance/stability of resultant OECTs as biosensors. In this article, we review the relationship among intramolecular/intermolecular structures, properties, and stability, and provide the suggested ideal structures for high-performance stable OECT-based biosensors. The first part covers mixed conduction by comparing OECTs and electrolyte-gated transistors. Subsequently, we discuss the structure of the most well-known mixed conductor, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) and the chemical aspects of other ion-permeable conjugated polymers. Next, we address the recent biomedical applications of OECTs to determine the prerequisites for their high device performance and stability in terms of biochemical sensing and bioelectric signal recording in vivo. The last part depicts the structural design criteria investigated in recent publications and, finally, the suggested perspective on the promising chemical structures and the outlook are provided. 2. Organic Electrochemical Transistors Generally, an OECT employs an electrolyte solution/gel for channel current modulation using gate biases. Seemingly, it operates similar to an electrolyte-gated organic field-effect transistor (EGOFET). However, the actual working principle is different; therefore, understanding their differences provides a clear concept of OECTs. In this section, the differences between these two devices are addressed, and the relevant material characteristics that can aid OECT operation are suggested. Finally, the advantages of OECT over EGOFET are discussed in light of the aforementioned features. In EGOFET, a bias applied to the polarizable gate electrode induces the migration of ions in the electrolyte, and the ions form an electrical double layer at both the gate-electrolyte and electrolyte-channel interfaces (Scheme 1, left) because the active layer is ion-impermeable. [36-39] Due to the electrical Organic electrochemical transistors that employ polymeric mixed conductors as their active channels are one of the most prominent biosensor platforms because of their signal amplification capability, low fabrication cost, mechanical flexibility, and various properties tunable through molecular design. For application to biomedical devices, polymeric mixed conductors should fulfill several requirements, such as excellent conductivities of both holes/electrons and ions, long-term operation stability, and decent biocompatibility. However, trade-offs may exist, for instance, one between ionic conduction and overall device stability. In this report, the fundamental un...

Molecular Design Strategies toward Improvement of Charge Injection and Ionic Conduction in Organic Mixed Ionic–Electronic Conductors for Organic Electrochemical Transistors

“…Electron transport takes place in (semi)crystalline regions of the polymer, featuring π–π stacking, while ionic transport occurs in the amorphous regions subject to swelling upon hydration. Reproduced with permission from ref ( 87 ). Copyright 2020 Wiley.…”

Stretchable Redox‐Active Semiconducting Polymers for High‐Performance Organic Electrochemical Transistors

“…More recently, substantial progress has been made in the development of nondoped, redox-active semiconducting polymers (RASPs), [26,27] which not only enable OECTs to function in enhancement mode, but also provide largely increased [µC*]. [25,[28][29][30] Compared to the semiconducting polymers designed for organic field-effect transistors (OFETs), for which effective strategies have been reported for imparting stretchability, RASPs are based on different design principles for enabling efficient ion transport and redox doping in aqueousstable voltage windows.…”

Fused Bithiophene Imide Dimer‐Based n‐Type Polymers for High‐Performance Organic Electrochemical Transistors

“…[13][14][15][16] Up to now,p -type (hole-transporting) polymer semiconductors have been intensively investigated as the channel materials in OECTs with the product (mC*) of charge carrier mobility (m) and volumetric capacitance (C*), the key Figure of merit determining the performance of OECTs,upto522 Fcm À1 V À1 s À1 achieved. [17][18][19][20][21] However,the development of n-type (electron-transporting) polymers lags greatly behind with the mC*v alues typically in the range of only 0.1-1 Fcm À1 V À1 s À1 (Figure 1). [10,[22][23][24][25] Them ismatch between n-type and p-type performance restricts the development of low-power OECT-based complementary circuits for on-site amplification in bioelectronic applications.…”