Novel Avenues Toward Controlling the Photophysical Properties of Ultra-Small Silicon Quantum Dots
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.