ORCID Identifier(s)

0000-0002-0674-0232

Graduation Semester and Year

2021

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Choong-Un Kim

Abstract

Ultrasensitive detection of chemicals and biological analytes has been a focus of a lot of studies in the recent years. In this search for the versatile detection technique, Surface Enhanced Raman Spectroscopy (SERS) has proved to be one powerful method. SERS is a powerful vibrational spectroscopy technique that allows for label free ultrasensitive detection. This method utilizes the Localized Surface Plasmon resonance (LSPR) effect of nanoparticles. LSPR effect is profound in noble metals such as silver (Ag) and gold (Au). We have developed a novel SERS substrate using Ag dendrites that can be applied as a ready-to-use device in a variety of applications. The substrate was fabricated via self-deposition of silver nanodendrites on copper wires of 80 microns inside different channels of glass or silver coated PDMS. The deposition was done by the galvanic replacement reaction between silver nitrate and copper. This reaction was previously thought as the simple high-school chemistry reaction, i.e. formation of Ag dendrites can be explained by a simple reaction mechanism as follows :. a reduction reaction, Ag+ + e- ? Ag, takes place only on a metal surface, first on Cu and then on newly formed Ag. The electron comes from the oxidation reaction Cu ? Cu2+ + 2e-, which takes place at Cu surface, where Cu2+ is released into the solution, and electron transport inside the metal-to-metal surface to reduce Ag+. The reduction rate is higher on the Ag (1 1 1) surface, leading to preferential growth in the [1 1 1] direction. However, a series of experiments revealed that this reaction mechanism is much more complex than a simple two step galvanic replacement reaction: metal Ag could form through reduction of Ag+ by intermediate nitrite ions inside the solution on a surface away from Cu. It was also found that Ag dendrites developed through a particle-mediated growth process. This new reaction mechanism can be utilized to generate completely freestanding, pure, and clean single-crystal Ag dendrites at room temperature within a few minutes. After a series of studies on morphology and reaction conditions, we developed a robust 3-D SERS substrate using Ag dendrites, which has extremely high sensitivity and excellent reproducibility. More importantly, such device can detect molecules in-liquid without sampling and drying. The sensitivity of the substrate was verified with rhodamine 6G (R6G) with the lowest concentration of about 1 x 10^-14 M. We also experimented with other common SERS molecules like Methylene Blue and 4-Methoxybenzoic Acid and were able to detect them as well at very low concentration. For better understanding of the mechanism behind such high sensitivity, we conducted FDTD simulations on representative 2D/3D models of dendrites according to the SEM images. These simulations revealed a high electric field enhancement of 10^6 – 10^7 from narrow gaps between branches in the dendrite structure. Aiming for practical application of the device, we developed a method of calibrating these devices using low concentrations of R6G. After calibration, we were able to reliably detect low concentrations of around 0.5 ppm of Thiabendazole(TBZ), which is a commonly used pesticide has been labelled as a potential carcinogen. These 3-D SERS substrates were proved as a precise and fast in-situ Raman detection method for flowing liquid media.

Keywords

Raman, Surface enhanced raman scattering SERS, Silver dendrites, Nanodendrites, SERS application

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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