Date of Award

Summer 8-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Prabhu Arumugam

Abstract

Nanoelectrodes have become widely used in electrochemical sensing in recent decades. When compared to microelectrodes, it has many unique advantages such as high signal to noise ratio, small sample volume requirement, and lower detection limits.

This work reports on the microfabrication and characterization of a gold nanoring electrode (Au NRE) patterned on top of a silicon (Si) micropillar. An NRE of 165  10 nm in width was micropatterned on 4.6  1 µm diameter  17.5  2.5 µm long Si micropillar with an intervening 50 nm thick hafnium oxide insulating layer. Scanning electron microscopy and energy dispersive spectroscopy data confirmed excellent cylindricality of the micropillar with vertical sidewalls and a completely intact layer of Au NRE encompassing the entire micropillar’s perimeter. The electrochemical behavior of the Au NRE was characterized by a steady-state cyclic voltammogram (CV) with extremely high signal-to-noise ratios of 2500 and charging currents as small as 1.5  0.3 pA. A “semicircle spectrum” from the Nyquist plot using electrochemical impedance spectroscopy (EIS) indicates a kinetically-controlled voltammetric current response, which is unique to nanoscale electrodes.

The variations in the exchange current values, which are dependent on the NRE area and standard rate constant, place a greater emphasis on the isotropic gold wet etching process that defined the geometry of the Au NRE. Circuit fitting of the electrochemical impedance spectra suggests that the NRE geometry can be varied from inlaid to nanotrench with depths controllable by the etching process. This resulted in differing steady-state voltammetric currents and interfacial properties in terms of charge transfer resistance, constant phase element and trench resistance values. The applicability of Au NREs to electrochemical sensing is demonstrated by detecting lead, a neurotoxin at 100 ppb levels. By surface-modification with multi-walled carbon nanotubes, dopamine, which is a neurochemical implicated in various brain disorders, is detected at a sensitivity as low as 100 nM with 1000-fold selectivity versus common interferents. The flexibility of the microfabrication approach allows for the creation of multiple NREs of controllable width and nanometer spacing on a single micropillar. In particular, the NRE is micropatterned on three-dimensional microstructures that will be reported here with unique electroanalytical capabilities such as intracellular electrochemistry and highly multiplexed detection for emerging biological applications.

The NREs are also designed and fabricated to conduct redox cycling experiments. Based on the fabrication procedure of NRE, a silicon pillar coated with multiple gold layer has been used to pattern the designed NREs. The amplification of the current signals at the NREs during redox cycling can reach up to 84.7% with a pair of nanodes used for generation and collection.

In addition, ultrananocrystalline diamond (UNCD) microelectrode is designed and fabricated using the single NRE fabrication procedure. The UNCD microelectrodes are patterned in an array format (up to 42 microelectrodes) on a 1.6 cm by 1.6 cm chip. The electrochemical analysis (CV, EIS) has been applied to study the behavior of UNCD microelectrodes. A high signal to noise ratio of 3400 has been obtained from such microelectrodes.

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