关键词: 共价有机框架, 有机电化学晶体管, 离子浓度检测, 沟道材料, 纳米多孔材料

Abstract: While the past decade has witnessed remarkable advances in the field of organic bioelectronics, organic electrochemical transistors (OECTs) have been regarded as one of the most promising device platforms for this purpose. An OECT is a type of transistor where the source-to-drain current is electrochemically modulated by applying biases on the gate electrode. In comparison with other organic electronic devices, OECTs have several advantages, including simple device fabrication, strechability, relatively low operation voltages, and decent on-off current ratios. Accordingly, many researchers have developed various types of OECT-based bioe-lectronics with the capability of sensing DNAs, hormones, metabolites, and neurotransmitters, or of monitoring cells, tissues, or brain activities. 
To understand the mechanism of OECT device operation, the mixed transport of holes/electrons and ions through an organic channel should be considered simultaneously. When an electrical bias is applied to the gate electrode, the conductivity of the organic layer is controlled by driving small cations (or anions) from the electrolyte medium to the channel layer, so as to dedope (or dope) the constituent organic conductor, resulting in the efficient modulation of source-to-drain current. In this regard, OECTs employ the whole volume of organic film as an effective channel, unlike typical organic field-effect transistors (OFETs) with the interface between semiconducting and dielectric layers functions as a major channel. From the perspective of engineering the channel microstructure, in-plane π–π stacking among the conjugated moieties, as well as well-organized out-of-plane ordering, is highly desired to facilitate both intrachain and interchain transport of charge carriers along the channel direction. Meanwhile, porosity with micro/nanoscopic voids and molecular-scale dispersion of ion-conductive moieties (e.g., polyelectrolytes or ion-conductive side chains) should be uniformly distributed throughout the organic layer to enable facile permeation of small ions into the channel layer (e.g., conjugated molecules or polymers), leading to an effective control over charge carrier density. Among a variety of soft electronic materials, poly(3,4-ethyle-nedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) has been one of the most frequently used channel materials for OECTs and related bioelectronic devices, owing to its high electrical conductivity, good optical transparency, and decent biocompatibility. However, the poor stability of PEDOT:PSS in water and the occurrence of chemical cross-linking after modification make the PEDOT:PSS film dense and interfere with interchain charge transfer, resulting in a significant decrease in electron and ion mobility. Therefore, it is urgent to to develop a channel material with high stability and electron and ion mobility.               
Covalent organic frameworks (COFs) are widely used in gas adsorption/separation, energy storage and conversion, catalysis and other fields due to their excellent stability, conjugated structure, and adjustable functionality. At the same time, COF is also a promising channel material for OECT. In this study, surface-initiated Schiff base-mediated aldol condensation reaction was used to successfully grow COF films in situ on silicon wafers, and its structure was characterized by XRD, FTIR, SEM, AFM, TEM, etc. The effect of COF films in OECT application was tested through device assembly, and about 100 times the switch ratio, a low threshold voltage of 0.4 V, and a field-effect mobility of 0.53 cm2/(V·s) were obtained. The results of this study have expanded the application of covalent organic framework films in the field of electronic devices.

Key words: covalent organic framework, organic electrochemical transistor, ion concentration detection, channel material, nanoporous material