Date of Award

5-2019

Document Type

Thesis

Degree Name

Master of Science (MS)

College/School

College of Science and Mathematics

Department/Program

Chemistry and Biochemistry

Thesis Sponsor/Dissertation Chair/Project Chair

Glen D. O’Neil

Committee Member

Lynn F. Schneemeyer

Committee Member

David Talaga

Abstract

Electrochemical sensors are important for a variety of applications. However, they suffer from physical constraints, as each electrode requires a dedicated wire. Although arrays of 500 electrodes have been reported, achieving high-density electrochemical measurements is still difficult.

In a light-addressable electrode, electrochemical processes occur only when the electrode is illuminated. Light-addressable electrodes could potentially solve many of the drawbacks of traditional electrode arrays, as light is used to spatially confine the redox process to only a small part of the electrode and only one connection is needed. It could also allow for new imaging techniques.

Light-addressable electrodes require a semiconductor layer, often Si, with one or more protective layers. A spontaneous SiOx layer forms when Si comes into contact with oxygen or water, so a protective layer is required to prevent Si from degrading in aqueous media while facilitating the reaction. The semi conductive nature of Si allows the redox reaction to be controlled by light, as Si has a relatively small bandgap and electrons can jump from the valence band to the conducting band when provided with an extra energy input.

We show the fabrication and characterizations of light-addressable electrodes that are simple to produce, inexpensive and scalable. Moreover, the method does not require clean room techniques or a passivating organic self-assembled monolayer. Instead, we protect the underlying Si layer with electrodeposited Au nanoparticles. We fabricate the electrodes using a two-step method and characterize them in catechol, dopamine and ferrocene methanol using three illumination conditions (Overall illumination, local illumination and no illumination). Briefly, we prepare a n-type Si electrode and remove the native SiOx layer by etching in 40% NH4F and then electrodeposit Au nanoparticles for 20 minutes. The electrodes are characterized using SEM-EDS imaging, cyclic voltammetry and chronoamperometry.

When a voltage is applied under no illumination conditions, the electrode does not pass significant current. However, when the whole electrode surface is illuminated, enough carriers are generated to observe a redox event and the peak current increases roughly two orders of magnitude. When only a small part of the electrode is illuminated, the current density is roughly one order of magnitude lower than when overall illumination is used. For a simple redox event, peak current is directly proportional to electrode area.

The electrodes are stable in aqueous solution for 1000 consecutive cycles. Dopamine and catechol calibration curves with excellent linearity were obtained using total illumination, dopamine. These results pave the way for future experiments, such as measuring dopamine release by a cell.

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