Chemical and biological sensors based on organic electrochemical transistors

Abstract

Organic thin film transistors (OTFTs) have been explored for sensing applications for several decades due to their many advantages like easy fabrication, low cost, flexibility, and biocompatibility. Among these OTFTs, organic electrochemical transistors (OECTs) have attracted a great deal of interest in recent years since the devices can operate stably in aqueous environment with relatively low working voltages and are suitable for applications in chemical and biological sensing. In this thesis, ion-sensitive properties of OECTs based on poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) have been systematically studied. The transfer curves (IDS vs. VG) of the OECTs shifted to lower gate voltage horizontally with the increase of the concentration of cations, including H⁺, K⁺, Na⁺, Ca²⁺, and Al³⁺, in the electrolyte. It was found that the gate electrode played an important role on the ion-sensitive properties of the OECT. For the devices with Ag/AgCl gate electrode, Nernstian relationships between the shift of the gate voltage and the concentrations of the cations were obtained. For the devices with Pt and Au gate electrodes, the ion sensitivities were higher than that given by the Nernst equation, which could be attributed to the interface between the metal gate electrode and the electrolyte. Moreover, OECTs based on PEDOT:PSS were integrated into flexible microfluidic systems. It was found that the device performance was stable under different bending status of the device. Then a novel label-free DNA sensor was developed, in which single-stranded DNA probes were immobilized on the surface of Au gate electrode. These devices successfully detected complementary DNA targets at concentrations as low as 1 nM. The detection limit was also extended to 10 pM by pulse-enhanced hybridization process of DNA. The sensing mechanism of this type of DNA sensor was discussed. Besides label-free DNA sensors, OECTs based on PEDOT:PSS were also exploited as cell-based biosensors for the first time. The devices showed stable performance and excellent biocompatibility in culture medium. Human esophageal squamous epithelial cancer cell lines (KYSE30) and fibroblast cell lines (HFF1) were successfully grown on the surface of PEDOT:PSS film in the devices. Then the devices were used for in-vitro monitoring cell activities when the living cells were treated by trypsin and an anti-cancer drug, retinoic acid. It was found that the devices were sensitive to the change of surface charge and morphology of adherent cells. The sensing mechanism of this type of cell-based biosensors was discussed. Finally, OECT arrays in micro dimensions were successfully fabricated using photolithography. The fabrication process was mainly divided into three steps, i.e. fabrication of gold electrodes, fabrication of PEDOT:PSS films, and fabrication of PEG mirowells. Moreover, three different fabrication methods were discussed. Compared with macro dimensional OECTs, micro dimensional OECTs showed better electrical performance. The micro devices were more stable in aqueous solution and showed faster response time. Micro dimensional OECT arrays are expected to have applications in point-of-care and real-time analysis in diagnosis.