Zinc Oxide Electrolyte-Gated Transistors and Electrochemical Platforms for Low-Voltage Biosensing Applications

This thesis presents a comparative investigation of zinc oxide (ZnO) electrolyte-gated field-effect transistors (EGFETs) and commercial screen-printed electrodes (SPEs) for low-voltage biosensing, focusing on ionic detection and virus-like particle (VLP) sensing under physiologically relevant conditions. Solution-processed ZnO-EGFETs exhibited clear ionic sensitivity across varying sodium chloride concentrations, consistent with electric double-layer modulation. The devices also demonstrated partial reusability following rinsing and thermal activation. Antibody functionalisation produced modest shifts in output signal, indicating that further optimisation of the transistor architecture is required for robust biosensing performance. A systematic evaluation of single-, double-, and triple-layer ZnO films revealed that multilayer structures significantly improved device behaviour, including current output, subthreshold swing, and gate-cycling stability. These enhancements correlated with increased surface roughness identified through atomic force microscopy. In contrast, tin-doped ZnO (ZTO) devices underperformed, consistent with their more amorphous film structure, which was less compatible with effective electric double-layer formation. SPEs functionalised with pre-thiolated aptamers targeting respiratory syncytial virus generated strong and specific responses to RSV VLPs, yielding several fold increase in differential pulse voltammetry peak currents and distinguishing target from non-specific controls. Optimisation of redox mediator conditions and blocking strategies also identified bovine serum albumin as the most effective agent for minimising non-specific signals in this work. Collectively, these findings demonstrate that ZnO-EGFETs function most effectively as tunable platforms for engineering electrolyte-gated thin films. In particular, increasing the number of ZnO layers—and the resulting rise in surface roughness—enhanced mobility, operational stability, and cycling robustness under aqueous conditions. By contrast, the SPE-based sensors achieved more reliable VLP discrimination through optimised surface chemistry and biorecognition. Together, these complementary approaches inform the rational design of modular, surface-engineered biosensors for future point-of-care ionic and viral diagnostics.