Date of Award

Summer 8-24-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular Science and Nanotechnology

First Advisor

Prabhu Arumugam

Abstract

Electrochemical sensors have emerged as a promising solution for monitoring neurotransmitters and heavy metals, which could ensure good health and safety in the environment. This dissertation focuses on advancement in the field of electrochemical microsensors by improving the electrode properties and sensor metrics such as rapid response time, multiplexing, high sensitivity, high selectivity, and accuracy for the real-time monitoring of neurotransmitters (glutamate and GABA) and heavy metals (cadmium and lead). In this dissertation, a novel silicon (Si) multifunctional biosensor probe with four platinum ultramicroelectrodes (UMEs) and an on-demand in situ calibration (ODIC) microfluidic channel was optimized for sub-second simultaneous real-time detection of GLU and GABA. The Si probe features four surface-functionalized platinum UMEs for detecting GLU and GABA, a sentinel site, and integrated microfluidics for in-situ calibration. Optimal enzyme concentrations, size-exclusion phenylenediamine layer, and micro-spotting conditions were systematically investigated so that the Si probe could achieve high sensitivity and selectivity. Baseline recordings (n=18) in live rats demonstrated a useful probe life of at least 11 days with GLU and GABA concentrations changing at the levels of 100's and 1000's of μM and with expected periodic bursts or fluctuations during walking, teeth grinding, and other activities and with a clear difference in the peak amplitude of the sensor fluctuations between rest (low) and activity (higher), or when the rat was surprised (a reaction with no movement). Significantly, the probe could improve methods for large-scale monitoring of neurochemical activity and network function in disease and injury in live rodent brains. Additionally, this dissertation also addresses the pressing need for environmental sensors capable of large-scale, on-site detection of a wide array of heavy metals with highly accurate sensor metrics. Here, we present a novel approach using electrochemically polished (ECP) carbon screen-printed electrodes (cSPEs) for high-sensitivity detection of cadmium (Cd) and lead (Pb). By applying two key techniques, electrochemical impedance spectroscopy, and cyclic voltammetry, a detailed investigation was done on the impact of the electrochemical potential scan range, scan rate, and the number of cycles on electrode response and its ability to detect Cd and Pb. Additionally, scanning electron microscopy and Raman spectroscopy techniques were used to support the outcomes further. Our findings revealed a 41±1.2% increase in voltammogram currents and a 51±1.6% decrease in potential separations (n = 3), indicating a significantly improved active electrode area and kinetics. The impedance model elucidates the microstructural and electrochemical property changes in the ECP-treated electrodes, showing an 88±2% (n=3) decrease in the charge transfer resistance, leading to enhanced electrode electrical conductivity. A bismuth-reduced graphene oxide nanocomposite modified, ECP-treated electrode demonstrated very low limits of detection, 0.27 ppb, and 0.5 ppb cadmium and lead, respectively. Additionally, the electrocatalyst modified ECP-treated electrodes, resulting in sub-ppb detection limits in spiked real water samples. Our study underscores the potential of optimally ECP-activated electrodes as a foundation for designing ultrasensitive heavy metal sensors for a wide range of real-world heavy metal-contaminated waters.

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