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Heat-up Synthesis of Photoluminescence Tunable Silver Indium Sulfide Based Nanocrystals and Their Cellular Imaging Applications
Electrical and Biomedical Engineering
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Semiconductor nanocrystals (NCs) are of great interest in bioimaging and biosensing due to their unique optical properties. Recently, cadmium-free I-III-VI NCs as well as their core/shell structures have been paid great attention due to their low toxicity and excellent optical properties. However, significant efforts are still needed to develop heat-up synthetic strategies to produce high quality I-III-VI NCs with high quantum yields and flexible photoluminescence tunability, especially for less studied Ag-In-S (AIS) NCs. Therefore, the work presented in this thesis focuses on the following two aspects. First, a simple and safe heat-up based synthetic system was first developed to synthesize high quality AIS NCs and AIS/ZnS with high quantum efficiency. The quantum yields (QYs) of the produced AIS NCs and AIS/ZnS NCs are around 13% and 41%, respectively. Their morphologies, compositions and crystal structures were further studied by TEM, EDX and XRD. We also investigated the effect of different indium precursors on NC optical properties. It was found that the photoluminescence (PL) peak-wavelength of AIS NCs prepared from indium acetate is in the range from 596 to 604 nm, and that of AIS NCs using indium chloride is from 641 to 660 nm. AIS and AIS/ZnS NCs prepared from indium acetate present around 15% and 40% QYs, and both AIS and AIS/ZnS NCs prepared from indium chloride present around 31% QYs. The PL decay study reveals that the lifetimes of AIS and AIS/ZnS NCs prepared from indium chloride are 24 times larger than those of AIS and AIS/ZnS NCs using indium acetate. Furthermore, AIS NCs prepared from indium chloride have a slower photobleaching dynamics than AIS NCs prepared from indium acetate, and ZnS shell coating on both types of AIS NCs significantly improves their photostability against UV exposure. To further tune the emission wavelength of AIS NCs, high quality Cu doped AIS and AIS/ZnS NCs have been first synthesized via a surface doping approach. By increasing Cu precursor concentrations in doping, Cu doped AIS NCs exhibit a photoluminescence red-shift from ~ 600 nm to 660 nm with quantum yields from around 30% to 20%. After ZnS coating process, Cu doped AIS/ZnS NCs exhibit photoluminescence peaks from around 570 nm to 610 nm with significantly enhanced quantum yields to 50 ~ 60%. Moreover, we found that Cu doping in AIS can prolong the photoluminescence lifetime of NCs, and the average photoluminescence lifetime for the prepared Cu doped AIS and AIS/ZnS NCs is in the range of 300 ~ 500 ns. Last, to demonstrate the cellular imaging applications of the prepared NCs, AIS/ZnS NCs were encapsulated within poly(lactide-co-glycolide)-poly(ethylene glycol) (PLGA-PEG) micelles through self-assembly. Glioma-specific peptide chlorotoxin (CTX) was covalently coupled to the prepared AIS/ZnS micelles. The cellular uptake/internalization study indicates that the CTX conjugated AIS/ZnS micelles can be specifically and significantly internalized into brain tumor cells U-87 via a CTX-membrane-bound matrix metalloproteinase-2 (MMP-2) interaction, but not into non-glioma cells.