Quantum Dots: Applications as Bioimaging and Biosensing Agents

This thesis presents the synthesis and functionalization of quantum dots (QDs) for bioimaging and biosensing applications. QDs are very small (≤ 100 nm) inorganic crystalline semiconductors that possess size-tunable physical and optical properties. QDs provide a versatile platform for biosensing probes due to their strong absorption and efficient emission, as well as the many facile strategies for water solubilization and functionalization. Despite many remarkable demonstrations of their use in biosensing, there are concerns for the future application of these dots in clinical trials as most reported systems contain cadmium. Therefore, the development of cadmium free QDs has become topical; however, it should be noted that the toxicity of cadmium materials does not appear to be an issue even in animal studies. In this thesis I present the synthesis of novel non-cadmium near-IR emissive AgInS2/ZnS QDs. They were characterized using analytical techniques such as XPS, XRD, TEM, DLS, fluorescence and UV/Vis spectroscopy. The QDs were water-solubilized and functionalized to construct an oxygen sensing probe by conjugating water-soluble perylene-PEG-amine dye to the surface of the QDs. The response to the low oxygen levels (hypoxia) was investigated using an enzymatic oxygen scavenging system. In vitro studies were also conducted to demonstrate the sensor efficiency within a biological milieu. A biosensor comprising of green emitting CdSe/ZnS QDs and rhodamine B dye linked by a disulfide bond was constructed to detect biological levels of H2S, which is a gasotransmitter that is involved in many biological pathologies. The response mechanism is based on termination of FRET due to the reduction of the disulfide bond by the analyte. The ratiometric response that was observed in HeLa cells is one of the best reported demonstrations of QD-based ratiometric sensing. Two versions of a novel QD-based nano-optodes are reported for the detection of sodium and calcium ions based on a traditional bulk optode response mechanism. Use of QDs improved the sensitivity and the response time significantly, but the selectivity and the reproducibility of the methods were not suitable for further investigations.