Photostability Of Water-soluble CdTe Quantum Dots And Its Application

With the development of biomedical application and the microscope technology, the study of new type fluorescence probes become more reasonable and have bright future.Water-soluble semiconductor fluorescent quantum dots(QDs) are considered as a very important bio-labeling material owing to their outstanding optical properties such as size-tunable emission,broad absorption and narrow symmetric emission,high photo-stability,large 2-photon absorption cross section,and etc.This thesis focused on the photostability of thiol-capped CdTe QDs.We studied the effects of the photostability of QDs,conculed the theory of photobleaching process.Base on the theory,we developed two application:1) link QD and phthalocyanines to improve the singlet oxygen quantum yield for photodynamic therapy;2) using surface-treatment to improve photostability of QDs.The main results are listed as follows:1.The photostability of thiol-capped CdTe quantum dots(QDs) in Euglena gracilis (EG 277) and human embryonic kidney(HEK 293) cells was studied.The photobleaching for the cellular QDs is dependent both on the irradiation power density and the QD local concentration.The photostability of cellular QDs is better than that of chlorophyll and of green fluorescence protein(GFP) and is much better than that of FITC when the local concentration of QDs is not too low. The photobleaching of cellular QDs was remarkably reduced in the nitrogen treated EG 277 cells,indicating that photobleaching in living cells mainly results from photo-oxidation.The effect of photo-oxidation on QD photobleaching was further confirmed by comparing the situations in oxygen treated and nitrogen treated QD aqueous solutions.The photobleaching rate is related to the irradiation power density and the local density of QDs.The higher irradiation power density and oxygen abundance and lower QD concentration will result in a higher photobleaching rate.2.The process and mechanism of photochemical instability of thiol-capped CdTe quantum dots(QDs) in aqueous solution were experimentally studied.After laser irradiation,the corresponding Raman bands of the Cd-S bond decreased obviously, indicating bond breaking and thiol detachment from the QD surfaces.Meanwhile, a photoinduced aggregation of QDs occurred with the hydrodynamic diameter increased to hundreds of nanometers from an initial 20 nm,as detected with dynamic light scattering measurements.The bleaching of the photoluminescence of QDs under laser irradiation could be attributed to the enhanced nonradiative transfer in excited QDs caused by increased surface defects due to the losing of thiol ligands.Singlet oxygen(1O2) was involved in the photooxidation of QDs,as revealed by the inhibiting effects of 1O2 quenchers of histidine or sodium azide (NaN3) on the photobleaching of QDs.The linear relationship in Stern-Volmer measurements between the terminal product and the concentration of NaN3 demonstrated that 1O2 was the main pathway of the photobleaching in QD solutions.3.By means of the direct detection of near-infrared 1270 nm,we found that the water-soluble thiol-capped CdTe QDs can photoproduce 102 in deuterated water with a low quantum yield(QY) of 1%.When sulfonated aluminum phthalocyanines(AISPc's) were connected to these QDs,forming water-soluble QD-Pc composites,the 1O2 QY of the composites increased to 15%under the excitation of 532 nm.QD-Pc composites can fully utilize visible region light excitation to effectively produce 1O2,which may facilitate the applications of QD-Pc composites in broad areas.4..The surface treatment of thiol-capped CdTe quantum dots(QDs) was carried out with a small amount of sodium borohydride(NaBH4) in aqueous solution at room temperature.The treatment effectively enhanced the photostability of QDs and increased the photoluminescence(PL) efficiency by a factor of two as against the original ones.By measuring the PL trajectories of single QDs with a total internal reflection fluorescence microscope,the photostable lifetimes were determined to be 15.2±5.9 s for surface-treated QDs,and 5.8±1.9 s for the original ones, respectively.