Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Department

Chemistry

First Advisor

Mindy Levine

Abstract

The detection of persistent organic pollutants (POPs) in complex biological and environmental samples is an important research priority due to the known persistence, toxicity, and carcinogenicity of these compounds. Some examples of POPs include polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), pesticides, alcohols and phthalates. Human exposure to these compounds has been well-documented in the scientific literature, and can occur through a variety of mechanisms, such as through an individual’s exposure to contaminated air and soil or through the ingestion of contaminated food or water. Additionally, individuals who have been exposed to POPs contain small amounts of these compounds in their blood, breast milk, and urine.

Currently used methods for the detection of POPs rely heavily on mass spectrometry-based techniques. These methods are known to be highly sensitive; however, they are not ideal for rapid detection applications for several reasons, including: (a) Mass spectrometry-based techniques are time-consuming, due to the significant time spent performing method development and possible need for extraction of compounds from aqueous media prior to analysis. These instruments are also extremely expensive and require significant user expertise. Due to these drawbacks, there is a need for a practical, rapid, sensitive, and selective method for the detection of persistent organic pollutants in complex biological and environmental matrices, especially one that does not rely on mass spectrometry-based techniques.

Previous work in our group focused on addressing the problem of POP detection in complex environments through the use of cyclodextrin-promoted energy transfer for the detection of photophysically active, aromatic toxicants. This system relies on the ability of cyclodextrin to act as a supramolecular scaffold and promote proximity-induced energy transfer from an aromatic energy donor to a high quantum yield fluorophore acceptor. These interactions lead to a bright, turn-on fluorescence signal that is unique for each cyclodextrin-fluorophore-toxicant combination. These fluorescence signals can be used to develop array-based detection systems, where each cyclodextrin-fluorophore-toxicant combination results in a unique signal; by using a variety of different cyclodextrins and/or fluorophores, a unique identifier for each toxicant can be generated. Our group has published a number of articles highlighting the advantages of cyclodextrin-promoted energy transfer for the detection of persistent organic pollutants. We have shown that this method operates successfully in purified buffer systems, that it can form a part of an oil spill remediation strategy using cyclodextrin-promoted toxicant extraction following by fluorescence detection, and that it can be used for the detection of a wide range polar and nonpolar, photophysically active, aromatic toxicants.

While the use of cyclodextrin-promoted energy transfer has been well-documented by our group, it is only effective for the rapid, sensitive, and selective detection of photophysically active, aromatic toxicants. Recent events involve environmental contamination with non-photophysically active toxicants, including the 2014 chemical spill in Elk River, West Virginia and a 2015 tanker truck crash in Bourne, Massachusetts. In response to these recent events, our group has developed a new detection method we have termed cyclodextrin-promoted fluorescence modulation. This method relies on the ability of cyclodextrin to promote proximity-induced non-covalent interactions between a high quantum yield fluorophore and a non-photophysically active toxicant. These interactions, in turn, lead to a proximity-induced change in the fluorescence signal of the high quantum yield fluorophore and are highly combination specific for each cyclodextrin-fluorophore-toxicant. These fluorescence changes can be subjected to array-based statistical analyses to develop extremely high analyte selectivity while maintaining sensitivity. The work reported in this dissertation is divided into two objectives: (1) Development of a cyclodextrin-based detection system for non-photophysically active toxicants; and (2) Application of cyclodextrin-promoted fluorescence modulation for toxicant detection in complex biological and environmental samples.

The first manuscript, “Sensitive and Selective Detection of Alcohols via Fluorescence Modulation,” describes the detection of aliphatic alcohols using cyclodextrin-promoted, proximity-induced fluorescence modulation of a high-quantum yield fluorophore in the aftermath of a January 2014 chemical spill in Elk River, West Virginia. Fluorescence modulation occurred when the analyte was held in close proximity to the fluorophore via non-covalent cyclodextrin analyte-fluorophore interactions. These interactions led to unique modulation responses for each combination investigated. This method was highly sensitive (low micromolar detection limits) and selective (99% differentiation between structurally similar analytes). This work was the first report of this fundamentally new detection approach for alcohols. This manuscript was published in Supramolecular Chemistry in 2016 and was awarded the Sessler Early Career Research Prize in 2018.

The second manuscript, “Selective Detection of Non-Aromatic Pesticides via Cyclodextrin-Promoted Fluorescence Modulation,” describes the application of cyclodextrin-promoted fluorescence modulation to another class of analytes, aliphatic organochlorine pesticides. These analytes were detectable through quantifying proximity-induced changes in the fluorescence emission spectrum of a high quantum yield fluorophore upon introduction of the analyte. This work resulted in detection limits as low as 5.2 μM and 100% selectivity in array-based identification. This manuscript was published in New Journal of Chemistry in 2016.

The third manuscript, “Detection of Organochlorine Pesticides in Contaminated Marine Environments via Cyclodextrin-Promoted Fluorescence Modulation,” describes the rapid, sensitive, and selective detection of four previously investigated organochlorine pesticides in a variety of water sources across the state of Rhode Island using cyclodextrin-promoted fluorescence modulation. The research reported in this manuscript builds on previously reported work on the detection of organochlorine pesticides in purified buffer systems (Manuscript 2) and introduces complex matrix effects associated with marine environments. This work reported high sensitivity (low micromolar detection limits), high sensitivity (100% differentiation between structurally similar analytes), and general applicability (for different water samples with varying salinity and pH as well as for different water temperatures). This manuscript was published in ACS Omega in 2017.

The fourth manuscript, “Detection of Benzene and Alkylated Benzene Derivatives in Fuel Contaminated Environments,” describes the detection of fuel components such as benzene, toluene, and xylenes using cyclodextrin-promoted fluorescence modulation. This method was applied to the detection of fuel components in purified buffer systems as well as in contaminated aqueous environments following a March 2015 tanker truck crash in the Bourne Rotary in Bourne, Massachusetts. This detection scheme was highly sensitive and selective, and showed well-separated signals for structural isomers of xylene (ortho, meta, and para) which is hard to do without specialized mass spectrometry-based techniques. This manuscript was published in Clean – Soil, Air, Water in 2016.

The fifth manuscript, “Efficient Detection of Phthalate Esters in Human Saliva via Fluorescence Spectroscopy,” describes the application of cyclodextrin-promoted fluorescence modulation to another class of compounds (phthalate esters) and to a complex, biological fluid (human saliva). Saliva contains enzymes that can break down γ-cyclodextrin, so α-cyclodextrin and β-cyclodextrin were used as supramolecular hosts for this work. This work was highly selective and was the first report from our group on the use of array-based analysis to differentiate between various concentrations of a single analyte. This manuscript was published in Analytical Letters in 2018.

The sixth manuscript, “Detection of Bisphenol A and Derivatives in Human Urine via Cyclodextrin-Promoted Fluorescence Modulation,” describes the detection of Bisphenol A (BPA) and structurally-similar BPA derivatives in purified buffer and human urine using cyclodextrin-promoted fluorescence modulation. Bisphenols have been used as plasticizers and have been found in the biological fluids of people who have come into contact with these compounds. These compounds are well-documented endocrine disruptors; therefore, their detection is of high priority. This work uses only γ-cyclodextrin to promote fluorescence modulation of three high quantum yield fluorophores: BODIPY-Me, BODIPY-Ph, and Coumarin 6. Unique changes in fluorescence were seen for each cyclodextrin-analyte-fluorophore combination investigated. These results translated to 100% differentiation between structurally similar bisphenol compounds in both purified buffer and human urine environments. This manuscript was published in Analytical Methods in 2018.

The work included in this doctoral dissertation provides crucial information, demonstrating the scope of cyclodextrin-promoted fluorescence modulation. This information will be used in the future development of solid-state devices for the rapid, sensitive, and selective detection of persistent organic pollutants in a wide range of biological matrices and for a variety of environmental applications.

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