PhD defence of Minh Tran – Fluctuation and Stability in Potentiometric Sensors
Abstract
This thesis concerns concepts in noise analysis, thermoelectric devices, and electrochemical sensors, presenting findings across these interconnected fields. In part one, the research advances the understanding of thermoelectric materials by extending the fluctuation-dissipation theorem to reveal the relationship between temperature and voltage fluctuations. This theoretical framework is validated through experiments with an ultra-fast thermoelectric micro-device, demonstrating an enhanced voltage fluctuation spectral density beyond the famed Johnson-Nyquist noise of Ohmic materials. The practical implications for temperature measurement by thermoelectric response are discussed.
In part two, 1/f noise in graphene field-effect transistors is studied, as this is the primary source of noise that limits the resolution of graphene field-effect transistor-based chemical sensors. Experiments demonstrating a facile method to reduce noise by scaling the active area of the sensors are presented, resulting in the lowest room temperature 1/f noise in a graphene transistor reported to date. This advancement leads to the development of a sulfate ion sensor with unprecedented resolution of 1.2 × 10^-3 log molar concentration units, showcasing the potential of graphene-based sensors in high-resolution chemical sensing applications.
Finally, in the third part, attention is turned to the reference electrode and its stability. All potentiometric sensors working in a liquid environment require an element sensitive to the analyte and a reference electrode. The study introduces a novel solid reservoir reference electrode designed to provide stable electrochemical measurements and overcome the high manufacturing cost, short shelf life, and wet storage requirements of traditional liquid-filled electrodes. The solid reservoir reference electrode features a construction analogous to the liquid-filled Ag/AgCl electrode, with an Ag/AgCl layer, a solid KCl reservoir, and a porous polydimethylsiloxane membrane, allowing for planar processing. The new electrode's performance is evaluated in aqueous and non-aqueous solvents like acetonitrile. The electrode's open circuit potential drift was less than 0.37 mV over 17 hours in deionized water and less than 3 mV over 8 hours in acetonitrile. In applications relating to electrocardiogram and electroencephalogram measurements, the solid reservoir reference electrode demonstrates a 50% improvement in signal-to-noise ratio.