Solution Processed Electrolyte-Gated Thin Film Transistors and their Sensing applications

The thin film transistor (TFT) is one of the most important fundamental building blocks in modern electronic devices. Examples of TFT applications are in integrated circuits (ICs), amplifiers, addressing of flat panel displays, and also as chemical sensors, e.g. as ion- selective field effect transistors (ISFETs). Although the most common semiconductor materials used in thin film transistors (TFTs) is silicon (Si), the versatility of TFTs allows other semiconductors to be used instead of Si-based materials. Recent research effort has been directed towards alternative TFT semiconductors, for example solution-processable semiconductors that enable new manufacturing options. Since the 1990s, soluble semiconducting polymers have been intensely researched for TFT applications. The possibility to engineer their transport (HOMO/LUMO) levels via chemical synthesis, their low temperature processing from solution, and their mechanical flexibility offers the potential for economical device production, as well as large-area and flexible applications.
In the last view years, the interest as promising materials for the next generation of TFTs has shifted towards oxide based semiconductors such as ZnO due to their high performance and relative stability to ambient conditions. The potential of ZnO was boosted by the discovery that it can derived by pyrolysis of an organic precursor that is soluble in polar organic solvents (e.g. alcohols, ketones) thus enabling solution processing of ZnO TFTs and other devices.
In addition, it has been demonstrated that both organic thin films and ZnO films can be gated by a field effect with very low threshold using deionized (DI) water as an electrolytic gate medium, leading to the ‘water- gated thin film transistors’ (WGTFTs). This discovery marks the beginning of a new method for the sensing of waterborne analytes, which differs from the classic ISFET in one significant point: Here, the aqueous sample under test is an active portion of the transducer. A number of works have since been undertaken using this discovery as a novel sensor concept for detecting waterborne analytes. Such sensors rely in the integration of analyte- specific sensitisers into the TFT architecture. For the selective sensing of ions, one of the most prominent families of water- insoluble ion sensitisers (‘ionophores’) are the calixarenes, a family of organic macrocycles. Calixarenes can be designed to selectively to complex specific cations, anions and neutral molecules.