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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor

Charpentier, Paul A.

Abstract

Quantum dots (QDs) have attracted an increasing attention in the last decade over many conventional organic dyes. This is due to their unique optical properties including broad absorption spectra, high photostability, and size-tunable photoluminescence (PL). However, some toxicity concerns associated with traditional quantum dots have hindered their wide applicability. Interestingly, silicon quantum dots (SQDs) have been shown to be more advantageous than most of QDs thanks to their excellent biocompatibility and biodegradability, low cytotoxicity, and versatile surface functionalization capability. Thus, SQDs are promising candidates for various biological and biomedical applications such as bioimaging, biosensing, and photodynamic therapy. Unfortunately, only a few studies in literature investigated factors that impact the optical properties of SQDs. In this thesis, we studied the impact of functionalization of ultra-small SQDs (< 2 nm diameter) with different aromatic fluorophores and/or spacers as a means to control their optical properties. We first functionalized the SQDs with phenanthrene, pyrene, and perylene fluorophores through a conjugated spacer which led to an efficient energy transfer from the fluorophores to the SQDs core. As a result, the photoluminescence of the SQDs was red- or blue-shifted and its emission quantum efficiency (QE) was moderately enhanced depending on the fluorophore type. Furthermore, we investigated the impact of different spacers, e.g. N-propylurea and propylamine spacers, on controlling the optical properties of SQDs in which perylene dye was utilized as the capping agent. The nature of spacer played a vital role influencing the interaction of the aromatic dye with the electronic wave function of SQDs. Energy transfer was proven to be the predominant process when propylurea spacer was utilized, while propylamine spacer was found to facilitate electron transfer process. Finally, SQDs were functionalized with different fluorescein and rhodamine derivatives using different spacers that vary in length, chemical nature, and attachment position with the dye. This led to an efficient energy and/or electron transfer in all dyad systems leading to an enhanced QE and photostability for at least one year. To demonstrate the potential application of the functionalized SQDs for bioimaging applications, they were examined for fluorescent imaging of HeLa, HEK293, and U2OS cells.

Summary for Lay Audience

Cancer is a complex group of individual diseases which differ in causes and effects. The Canadian Cancer Society sadly estimates that nearly 1 in 2 Canadians is expected to get cancer in their lifetime. All types of cancers start in our cells. Unfortunately, there is a lack of tools that can enable the early detection of very limited quantities of cancer inside the body and thus they are left without treatment for long time. As a result, cancer cells would spread into nearby tissues or organs. In this thesis, we have made a new suite of silicon nanocrystals (so called silicon quantum dots) and investigated their potential applications as biomarkers for the bioimaging of cancerous cells. We have also studied their photostability and cytotoxicity. Interestingly, the silicon quantum dots (SQDs) reported in this thesis showed promising results when used for in vitro fluorescent imaging of cervical cancerous HeLa cells and human bone cancerous U2OS cells. These SQDs were of low toxicity and exhibited a high photostability for at least one year. We are now aiming to further develop the properties of these SQDs using synthetic methods to allow for their use as photodynamic therapy agents for the apoptosis of cancer cells.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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