Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Chemistry

Specialization

Organic Chemistry

Department

Chemistry

First Advisor

Mindy Levine

Abstract

The goal of this graduate research has been to develop novel materials for the detection and removal of small molecule toxicants that have been introduced into natural water sources as a result of human industries. Traditional methods for detection of small molecule toxicants rely on laboratory grade equipment, such as gas chromatography (GC), mass spectroscopy (MS), GC-MS, and high-performance liquid chromatography (HPLC). These traditional methods have high sensitivity; however, they suffer from a lack of portability, a high degree of training needed to use them, expensive instrumentation, and extended times for data processing and analysis. To address this problem, several novel conjugated fluorescent polymers have been developed for the rapid detection of multiple small molecule toxicants.

Fluorescence as a means of detection was chosen due to the high sensitivity, ease of use, and the existence of inexpensive portable instrumentation. This research has focused on conjugated fluorescent polymers for their typically high quantum yields, low toxicity, and their burgeoning use as components of hydrophobic nanoparticles. Conjugated fluorescent polymers can form colloidal nanoparticles in water which provide a large surface area and a loose structure in which small molecules can interact or agglomerate. As the nanoparticle is held together through hydrophobic association, the hydrophobic nature of the toxicants will favor interacting with the nanoparticle, leading to a highly sensitive detection system.

The first manuscript “Turn-on Detection of Pesticides via Reversible Fluorescence Enhancement of Conjugated Polymer Nanoparticles and Thin Films” describes the detection of organochloride pesticides by monitoring the fluorescence modulation of conjugated polymer nanoparticles. This nanoparticle system was able to detect DDT, its metabolites DDD and DDE, and notably its structural isomer o, p -DDT with high degrees of differentiation among the analytes. This system has a limit of detection (LOD) within the literature-reported levels of concern for these analytes, with an LOD of 1.6 ppm for DDT. In addition to high sensitivity, this system was proven to be reversible with the introduction of molecular iodine, increasing the reusability of the detection system. Finally, polymer thin films were made and used for the detection of DDT vapor, showing the robustness of this detection scheme across multiple polymer platforms.

The second manuscript “Novel Fluorescent Fluorene-Containing Conjugated Polymers: Synthesis, Photophysical Properties, and Application for the Detection of Common Bisphenols” describes the synthesis of eight novel polymers, their photophysical properties, and their application for the detection of bisphenol A (BPA), bisphenol F (BPF), and bisphenol S (BPS). The experiment begins with an optimization of the general Suzuki polycondensation typically used to synthesize conjugated fluorescent polymers. Through experimental optimization, the chain length of synthesized polymers was doubled compared to the general Suzuki polycondensation. This optimization is of particular importance as these polymers have few solubilizing side chains leading to the polymers having a low solubility which severely limits the chain length of these polymers during synthesis. Through this optimized Suzuki polycondensation, eight novel polymers were synthesized, five of which had Stokes shifts of over 100 nm. Such large Stokes shifts better separate the excitation signal from the emission signal, allowing the fluorescence emission to be more accurately measured without interference from the excitation signal. Finally, all eight polymers were used as solutions of nanoparticles for the detection of BPA, BPF, and BPS. Using linear discriminant analysis, the changes in fluorescence of the polymers can be used to differentiate 100% among all three analytes, demonstrating the potential of these polymers for use in practical bisphenol detection.

The third manuscript “Effects of Structural Variation in Conjugated Side Chains on the Photophysics of Conjugated Polymers in Nanoparticles” investigates how the structural identity of aromatic side chains affects the photophysics of conjugated fluorescent polymers, in a manner that is highly dependent on the polymer’s state of aggregation. Three novel polymers were synthesized, each having an aromatic side chain attached to the polymer backbone either with an alkene or alkyne linker. Nanoparticles made from these polymers were then swollen using tetrahydrofuran (THF) so that the change of aggregation could be measured by dynamic light scattering (DLS) and the change in photophysical properties could be judged by measuring the fluorescence of the polymer nanoparticles. This study revealed that the aromatic entity on the side chain had a large impact on the fluorescence of the nanoparticle and the linker has a very modest effect on the interaction between the polymer chains.

The fourth manuscript “Hydrophobically coated cyclodextrin metal-organic frameworks for the rapid removal of small molecule toxicants from contaminated aqueous environments” describes four novel metal-organic frameworks (MOFs) and their use for the removal of nonpolar toxicants from water. MOFs have been the focus of a lot of research recently as they are highly porous and versatile materials. In this work a MOF was made using potassium cations and gamma-cyclodextrin, then after fabrication the MOF was covalently functionalized with four different nonpolar entities yielding four novel MOF materials. The highly porous MOF structure and the use of cyclodextrin, which has a cavity suitable for small molecule encapsulation, creates a material with an exceedingly high internal volume optimal for small molecule storage. However, cyclodextrin MOFs (CD-MOFs) degrade into their component parts in water making them ill-suited for use in aqueous environments. In this work the CD-MOFs were covalently functionalized with nonpolar molecules which increased the CD-MOFs bulk hydrophobicity allowing them to remove selected small molecule toxicants from water while retaining their structure. This work is not yet published and the properties and abilities of these novel CD-MOFs are still being elucidated, but so far they have demonstrated a high potential for the removal of various small molecule toxicants from water.

Available for download on Sunday, April 19, 2020

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