DOI

10.17077/etd.ojia939u

Document Type

Dissertation

Date of Degree

Spring 2017

Access Restrictions

Access restricted until 07/13/2019

Degree Name

PhD (Doctor of Philosophy)

Degree In

Chemistry

First Advisor

Haes, Amanda J.

First Committee Member

Cheatum, Christopher M.

Second Committee Member

Forbes, Tori Z.

Third Committee Member

MacGillivray, Leonard R.

Fourth Committee Member

Tivanski, Alexei V.

Abstract

This dissertation seeks to accurately and sensitively detect and estimate changes in molecule orientation of model and uranium species in complex samples. Currently available methods for detecting these molecules lack sensitivity, specificity, and/or require days to weeks to report trace chemical information. To overcome these limitations, normal Raman and surface-enhanced Raman scattering (SERS) are employed to gain molecular speciation. For instance, changes in uranium speciation depends on the pH and the ions present in solution. These ions form coordination complexes with uranyl, which influence the symmetric uranyl stretch that is Raman-active and can be used for the near real-time identification and relative abundance of uranium speciation in environmental samples. To accomplish this task, a strategy to extract uranyl speciation from Raman spectroscopy was developed. Important analysis methods were assessed using speciation modeling and protocols reported that minimize human subjectivity in spectral analysis. To improve detection limits of normal Raman spectroscopy, nanomaterials are employed for SERS. The adsorption of small aromatic molecules to gold coated silver nanoparticles encapsulated by internally etched silica membranes balances limitations of nanoparticle instability and orientation-dependent vibrational modes orientation relative to the plasmon resonance (electric field). Additionally, adsorption is monitored using localized surface plasmon resonance (LSPR) spectroscopy, SERS, and isotherm modeling. These combined approaches indicate that slight variations in molecular functional groups influence the free energies of adsorption of the target molecules. This provides an understanding of molecule-dependent SERS signals for sensitive, selective, and near real-time detection of small molecules in dynamic conditions. These findings support that nanomaterial surface chemistry greatly impacts molecular detection. As a result, gold nanostars functionalized with carboxyl groups are applied for uranyl detection. The distance dependent SERS response for uranyl is revealed by increasing the carbon chain length from 3-11 in the self-assembled monolayer. The shortest alkanethiol facilitated sensitive uranyl detection down to 120 nM. Finally, SERS detection is combined with electrospun amidoximated polyacrylonitrile (AO-PAN) mats to provide robust and reproducible detection of uranyl in complex matrices. AO-PAN mats are employed to initially extract and isolate uranyl while functionalized gold nanostars facilitate direct SERS detection. Characterization of AO-PAN mats uranyl uptake is examined by SEM, FT-IR and Raman spectroscopy. SERS measurements on the AO-PAN mats are obtained from matrices containing calcium and carbonate ions and synthetic urine with minimized matrix effects. Consequently, selective and sensitive detection of uranyl in environmental samples can be achieved thus broadening the scope of SERS for practical use.

Public Abstract

This dissertation focuses on accurately and sensitively detecting model molecues and uranium from biological and environmental samples. Uranium is a biological, chemical, and radiological toxin but detection is limited because the chemical and physical properties of uranium compounds depend on solution pH and ion composition. State of the art detection of uranium offers trace detectability but requires weeks to report signals and reveals no information regarding the exact uranium chemical compound identification. To overcome these limitations, uranium detection using Raman spectroscopy is used. Herein, a method is developed to interpret Raman spectra so that the chemical information and relative abundance of uranium compounds can be easily reported. Additionally, nanomaterials are employed to improve detection. The chemical and physical properties of nanomaterials increase the Raman scattering signal after nanomaterials selectively interact with incident light. First, gold coated silver nanoparticles are encapsulated with internally etched silica shells to probe target molecules without interference from inter-nanoparticle interactions. The interactions between molecules and the metal surfaces impact the molecular signal in a functional group and analyte concentration dependent manner. Second, this knowledge is used to improve the detectability of uranium. Surface-modified gold nanostars are used to increase the interactions between uranium and gold surfaces and to increase uranyl detectability. Finally, the use of a polymer mats for uranium uptake and nanomaterial deposition provides selective and sensitive uranyl detection from complex matrices.

Pages

xvii, 196 pages

Bibliography

Includes bibliographical references (pages 179-193).

Comments

Chapter 5 reproduced in part with permission from Lu, G.; Johns, A. J.; Neupane, B.; Phan, H. T.; Cwiertny, D. M.; Forbes, T. Z.; Haes, A. J. “Matrix-Independent Surface-Enhanced Raman Scattering Detection of Uranyl Using Electrospun Amidoximated Polyacrylonitrile Mats and Gold Nanostars.” Analytical Chemistry 2018 90 (11), 6766-6772. DOI: 10.1021/acs.analchem.8b00655. Copyright 2018 American Chemical Society.

Copyright

Copyright © 2017 En Tzu Lu

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