Mechanistic Understanding and Optimization of Printed Floating Gate Transistors for Chemical Sensing Applications

Abstract

Monitoring of human environments, food and health for toxin, carcinogen, allergen and pathogen detection motivates the development of chemical and biosensing platforms that can be deployed in portable field applications. Transistors are suitable transducers for such devices due to their direct electronic response, compact size, and multiplexing capabilities. Electrolyte-gated transistors (EGTs) can provide additional advantages including low voltage operation and the use of fast and simple fabrication methods such as printing. The Floating Gate EGT (FGT) is a sensing derivative of the EGT that utilizes a floating gate to physically separate yet still electronically couple the active sensing area with the transistor. Previous work has shown that FGTs can provide fast and reliable detection of DNA, ricin, and gluten. The aim of this thesis is to investigate fundamental operating mechanisms of the device, improve its sensing capabilities and characterize its design space. The first study, detailed in chapter 3, implemented well-established acid-terminated self-assembled monolayer (SAM) chemistry on the sensing area to characterize the role of interfacial charge in generating device responses. The shifts observed are further compared with Grahame’s equation, derived from Guoy-Chapman double layer theory, and is found to match closely with the experimentally observed shifts. This represents the first quantification of the charge response of floating gate transistor sensors. Chapter 4 focuses on the detection of capacitance, an important physical quantity for the detection of charge-neutral targets, which has proved to be a challenge for transistor-based sensing devices. In this study, alkylthiol chains of increasing lengths are used to alter the capacitance of the sensing surface. A simple amplification circuit called an inverter is used to amplify the change in output when the capacitance is perturbed. The FGT platform was found to respond to the capacitive change in a manner distinguishable from the charge-based sensing. This represents the first demonstration of quasi-static capacitance detection in the FGT platform as an alternative to charge detection, a critical issue in transistor-based sensing for neutral targets or in high electrolyte concentrations. In chapter 5, a theoretical model is derived for the device response and it is utilized to predict the performance and sensitivity of floating gate devices using well-known transistor current equations. The derivation yields 5 parameters, which are combinations of physically understood variables that can effectively tune the response of the device. To validate the model experimentally, SAMs are utilized to generate capacitive and charge-based signals, and the area of the sensing surface is systematically reduced. The model is found to match experimental performance and sensitivities well for higher sensing area capacitances (>1 nF). The model predictions are further extended across large ranges of the relevant parameters to provide general design rules for sensing using thin film organic electronic devices that can be utilized regardless of materials choice. The overall contribution of this project is to understand quantitatively the mechanisms behind transistor-based detection, specifically charge and capacitance, and provide guidelines for device sizing and materials choice, in order to make transistor-based sensors more accessible and move closer to the overarching goal of a rapid, portable, general purpose sensor for chemical and biosensing in distributed field applications.