Chemical sensing based on water-gated polythiophene thin-film transistors

Compact chemical sensors are in high demand for on-site detection of chemical substances in aqueous media. Polymer thin-film transistors (PTFTs) are among the most promising candidates for such sensors owing to their advantages. Water-gated PTFTs (WG-PTFTs) fabricated by a simple procedure can be operated at an ultralow voltage (<1 V) that avoids undesirable electrochemical reactions. Hence, we have developed WG-PTFT-based sensors using a chemical stimulus-responsive polythiophene derivative (i.e., poly[3-(5-carboxypentyl)thiophene-2,5-diyl] (P3CPT)), for which the changes in the electric double layer capacitance (EDLC) by accumulation and desorption of charged species are a key sensing mechanism. In this Focus Review, we report two examples: (1) the detection of biogenic amines such as histamine through hydrogen bonding and electrostatic interactions between the carboxylate side chain of P3CPT and targets, and (2) the highly sensitive detection of the herbicide glyphosate (GlyP) by competitive coordination binding among P3CPT, Cu2+, and GlyP. More importantly, in comparison to fluorescent chemical sensors fabricated using the same materials, WG-PTFT-based sensors show a lower detection limit. This result can be explained by the synergism of “intra” and “inter” molecular wire effects that originate from the aggregation of P3CPT under a field effect. Therefore, we believe that the sensing strategy developed here based on WG-PTFTs can contribute to improving the quality of life in real-world scenarios. Water-gated polymer thin-film transistors (WG-PTFTs) fabricated by a simple procedure can be operated at an ultralow voltage, whereby the WG-PTFTs with molecular recognition units allow chemical sensing. Upon the addition of analytes, the electric double layer capacitance of the WG-PTFTs can be changed by accumulation and desorption of charged species at the interface of the semiconductor and electrolytes. For example, the WG-PTFTs have successfully detected biogenic amines and a herbicide glyphosate. In short, our proposed WG-PTFTs are among the most promising platforms for sensing applications based on π-conjugated polymer materials.