Synthesis And Shell Modification Of CdSeS Alloyed Quantum Dots And Comparative Research On Their Environmental Stability And Cell Effects

Quantum dots (QDs) have been attracting a great deal of attention in recent years due to their unique photoelectric properties and technical applications, such as biomedical labeling, light-emitting diodes, photocatalysis, solar cells, lasers, and sensors. The resulting commercial potential has led government agencies to identify QDs, as a class of nanoparticles whose health and environmental risks must be quantified. Current literature reveals that the potential toxicity of these nanoparticles depends on multiple physicochemical as well as environmental factors, which would significantly influence their structure and properties. At present, there are so many reports focusing on the toxic effects and mechanism of various QDs with different physicochemical properties through in vivo and in vitro experiments. However, there are relatively few studies on the stabilities of QDs with different structure, and the change mechanism when exposed in various environments. In this study, core-shell-shell QDs were obtained through microwave irradiation to study the stability under different environments. The main contents and frame work is as follows:(1) A facile method was developed for the synthesis of water-dispersed CdSeS alloyed QDs using microwave irradiation. The obtained QDs showed a good photoluminescent quantum yield, high crystallinity, small particle size and homogeneous size distributions, a cubic phase crystal with a quasi core-shell structure, that is, a CdSe-rich core and a thick CdS shell. The mechanism analysis indicated that the formation of CdSeS QDs was a process of low temperature nucleation and high temperature crystal growth with the deposition of CdS shell.(2) The core-shell-shell CdSeS/ZnS and CdSeS/ZnS QDs were obtained through a simple shell modification of CdSeS alloyed QDs. After the modification, both QDs showed obviously improved photoluminescent quantum yield. X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy, as well as that of spectral analysis, confirmed the successfully epitaxial growth of the ZnS shell or CdZnS on the surface of CdSeS QDs, rather than the formation of CdxZn1-xSeyS1-y alloyed structure. As the content of ZnS in the shell was different, CdSeS/ZnS and CdSeS/ZnS QDs present different crystal structure, though there were amazing similarities in the particle size, size distribution and crystallinity between them.(3) Three types of QDs with different structures, that is, naked CdTe QDs, quasi core-shell CdSeS alloyed QDs, and core-shell-shell CdSeS/CdZnS QDs, were choosed to investigate the stability when exposed in various environment factors. The alloy structure and the shell protection endowed the high stability of CdSeS QDs and CdSeS/CdZnS QDs under the light irradiation. CdSeS/CdZnS QDs exhibited the highest stability when exposed in an acid medium, because their biggest band gap among these QDs, while CdTe QDs showed the weakest acid resistance for the smallest band gap. The protective double shell also endowed CdSeS/CdZnS QDs with higher stability when exposed in a20～60℃constant temperature heater. As they were exposed in an oxidative environment, the fluorescent intensity of CdTe QDs quenched markedly and the particle corroded quickly, while CdSeS QDs and CdSeS/CdZnS QDs present strong anti-oxidation properties, which was attributed to the unique gradient alloyed structure of CdSeS QDs and the thick CdS shell. These results showed that the alloyed structure and shell modification would effectively enhance the fluorescent stability when exposed in various environments.(4) The quench of fluorescent strength and the spectral blue shift showed the obvious corrosion of Simulated Body Fluid (SBF) on CdTe QDs and CdSeS QDs, while CdSeS/CdZnS QDs showed the highest stability in SBF. The structure stability was studied through the determination of released Cd2+concentration of QDs in SBF. The thermodynamics analysis proved the Cd2+release was an endothermal, non-spontaneous and entropy production process. High temperature would aggravate the release process. The biggest standard free energy of CdSeS/CdZnS QDs proved the highest thermodynamic stability, while CdTe QDs the worst stability among these three types of QDs. It was found that Cd2+release of QDs in SBF followed pseudo-first order reaction and CdSeS/CdZnS QDs showed the lowest apparent release rate and the highest apparent activation energy, which further proved the highest stability among these three types of QDs. The corrosion of SBF on QDs was related to the incubation temperature, medium and the oxygen concentration in the medium, while the incubation medium showed the biggest impact on the Cd2+release. The influence factors analysis showed that NaCl in the SBF solution caused the ligand dissociation, and the random thermal motion between QDs and the high level of ions damaged the crystal integrity of QDs were the main causes for the Cd2+released of QDs in the SBF solutions, which gave a good explanation to the biotoxicity of Cd-based QDs when used in practical applications.(5) The comparative research on the effects of Hela cells of three kinds of different structured QDs showed that CdSeS/CdZnS core-shell-shell QDs exhibited no obvious impact on the cell viability, morphology structure and surface structure of Hell cells, suggesting that CdSeS/CdZnS QDs were highly biocompatible. The comparative research on the fluorescence stability of the different structured QDs in fixed Hela cells displayed that intracellular CdSeS/CdZnS QDs showed strong acid-resistance, but also anti-photobleaching properties. Based on this, CdSeS/CdZnS core-shell-shell QDs were chosen to label the live Hela cells and the results proved that the as-prepared CdSeS/CdZnS QDs can be used as a promising fluorescence probe with high stability and good biocompatibility.