High-Performance Field-Effect Transistor-Type Sensors Based on Nanoscopically Engineered Organic Semiconductors

Abstract

Sensors based on organic field-effect transistor (OFET) platforms show great promise for use in chemical and biological sensors due to their prominent advantages, including high sensitivity, light-weight, low-cost, simple platforms, and flexible applications. Functional properties of active organic semiconductor layers can be tailored by material design or/and surface functionalization to enhance selectivity. To date, a large number of sensors for chemical and biogenic substances have used high-cost immobilization methods and high-end technologies. OFET-based sensors are particularly attractive for applications in simple, cost-effective, high-performance electronics. Furthermore, the sensitivity, selectivity, response time, stability, reproducibility, and limit of detection of sensors can be optimized by choosing or engineering more suitable fabrication techniques and materials for the active layers. Such on-demand, structure-engineered, and surface-engineered organic semiconducting layers are highly desirable for the practical uses of OFETs. In my thesis, commendable molecular engineering, process engineering and interface engineering are highlighted to demonstrate the feasibility of high-performance nanoscopically engineered organic-transistor-based sensors. Here, I begin with an introduction to OFET and organic sensors, with an emphasis on the organic semiconductor engineering strategies in chapter 1. In detail, in chapter 1, typical properties of organic semiconductors, a discussion of OFET operation, and a working principles of this OFET-type sensors are introduced. Chapter 2 presents molecular engineering strategies to enable the fabrication of n-channel-dominant ambipolar OFETs. The electrical charge transport through fluorine-substituted semiconducting materials is investigated. These investigations are easily applied to demonstrate complementary inverters with a reasonable performance. In chapter 3, I focus on the device design and fabrication of high mobility OFETs made by using organic–organic heterointerface. Pentacene is used as an active layer above, and m-bis(triphenylsilyl)benzene is used as the bottom layer. Sequential evaporation process without breaking vacuum of these materials results in high-quality organic semiconductor thin films with far fewer grain boundaries. In addition, the pentacene film exhibits myriad nanometre-sized pores in the organic layers. This surprising structure, the pore-rich structure improves the sensitivity of organic-transistor-based chemical sensors. This approach demonstrates a conceptually novel methodology for the fabrication of “structurally engineered” organic semiconducting thin films and our work has a significant impact in the fields of materials science as well as organic electronics. Furthermore, organic semiconductor engineering strategies to improve sensitivity and selectivity for biogenic substances by direct semiconductor surface functionalization and to enhance sensitivity and selectivity towards psychostimulants by modification with specific selective sensing layer are given in chapter 4 and 5, respectively. In chapter 4, highly sensitive organic-transistor-based sensors that can selectively detect a neurotransmitter acetylcholine without enzyme immobilization are fabricated using organic thin films functionalized with a synthetic receptor, a cucurbit[6]uril (CB[6]) derivative. The liquid-phase sensing experiments are successfully performed by using organic semiconductor layer with high operational stability in water. The findings provide a low-cost, simple, and feasible method for the fabrication of high-performance water-stable sensors for biogenic substances. In addition, the results obtained herein describe the first demonstration of acetylcholine sensing without any enzymatic reactions using the synthetic receptor-functionalized OFET-platform. In chapter 5, the direct detection of amphetamine-type-stimulants (ATS) is suggested for the illicit and designer drugs sensing OFET platforms. Their novel sensing system and sensing mechanism are studied using other CB homologues, a cucurbit[7]uril (CB[7]) derivative decorated OFET-based sensors. By synergistic combination of a highly selective synthetic host molecule and a highly sensitive OFET device, the first ATS sensors with specific synthetic receptor-engineered OFET-platform are demonstrated flexible polymer substrates. These sensors in physiological buffer system and even in urine samples show highly sensitive sensing behaviors.