Fluorescence Properties of Quantum Dots and Their Utilization in Bioimaging

Quantum dots (QDs), especially colloidal semiconductor QDs, possess properties including high quantum yields, narrow fluorescence spectra, broad absorption and excellent photostability, making them extremely powerful in bioimaging. Moreover, high surface-to-volume ratio further renders QDs ideal for functionalization to obtain good biocompatibility and sensitivity to the environment. In this thesis, we studied the fluorescence properties of QDs and attempted multiple ways to boost applications of QDs in bioimaging field. By time-correlated single photon counting (TCSPC) measurement and detailed analysis of fluorescence decay spectra, we quantitatively interpreted the fluorescence mechanism of colloidal semiconductor QDs. Most importantly, it revealed that the broad fluorescence spectra of CdSe QDs intrinsically originated from the Fermi golden rule. To enhance QD fluorescence, we used a porous alumina membrane, which has been widely used for biomacromolecule filtration, as a photonic crystal structure to modulate QD fluorescence, resulting in a distinguished modulation on QD fluorescence spectrum. This may be utilized as an effective means to identify QDs embedded in the porous alumina membrane. We studied the acid dissociation of 3-mercaptopropionic acid (MPA) coated QDs and found that the electrophoretic mobility of 3-MPA coated CdSe QDs was very low when the pH value of the QD solution was below 7.0 or with addition of Ca ions. meanwhile, the intensity of the QD fluorescence was reduced when with the conditions mentioned above. The electrophoretic mobility and fluorescence intensity of Ca-treated QDs was restored when an approximate amount of Ca chelators, ethylene glycol tetraacetic acid (EGTA), was added to the QD solution. Further analyses and first-principles study showed that the acid dissociated QD3MPA strongly interacted with Ca, forming a charge neutral QD-3MPA-Ca in the absence of EGTA. The devising strategies in this paper could also be applicable for other studies. Blinking phenomena of both CdSe-CdS/ZnS core-shell QDs and 3C-SiC nanocrystals (NCs) were studied. For CdSe QDs, we meticulously studied their blinking phenomena by various surface modifications, including QDs with different shell thickness, Ca ion and glycerol treatments. It was clearly shown that off-state distributions obeyed the inverse power law distribution, while the on-state distribution was much more complicated and closely correlated with QD surface conditions. By studying the optical properties under the treatment with glycerol, we unraveled the fluorescence mechanism of 3C-SiC NCs. The fluorescence intensity of 3C-SiC NCs increased with glycerol treatment while the fluorescence decay remained unchanged, indicating that the electron transition pathway of the fluorescence was not associated with the energy relaxation pathway via surface states. To examine QD effect on ciliated cells, we conducted a 70-day long experiment on the bioelectric and morphological response of human airway epithelial Calu-3