Studies On Quantum Dots Synthesized In Aqueous Solution And Applications In Analytical Science

Quantum dots, also called semiconductor nanocrystals (NCs). In the past two decades QDs have gained increasing attention due to their interesting optical properties comparing to organic dyes. Such as broad excitation spectrum which makes it possible to excite multiple colures of QDs simultaneously with a single excitation wavelength. With size-tunable fluorescence emission, QDs can be generated for any specific wavelength. QDs have narrow and symmetric emission spectrum compared to organic fluorophores; In addition, QDs have a long fluorescence lifetime that was 10 to 100 times longer than organic dyes. At the same time QDs exhibit high photobleaching resistance and excellent photostability, a common problem for organic dyes, thus making them useful for continuous monitoring of fluorescence.Although the surface modification technology and preparation technology of QDs have gained great improvements in the past several years, but their use in biomedicine and analytical science is still in the first stage, many problems need to be solved as quickly as possible. The preparation and surface chemistry theory of QDs is still in researching; quantum yields of QDs prepared directly in water solution are still lower. The intrinsic toxicity of cadmium based QDs has cast a doubtful future for their use in biology field and it's urgent to develop new nontoxic or low toxicity QDs. It's very significative to study QDs preparation technology and their application in correlative fields. In this dissertation we synthesized TGA capped CdS QDs and ZnSe QDs in water solution, based on which the following studies were done. This dissertation consists of three chapters.In chapter one the conception, the phylogeny, the characteristic, the luminescence principle and optical property of Quantum Dots were summarized. And simply introduced surface modification and bioconjugation technologies of Quantum Dots. Last especially discussed Quantum Dots synthesis methods and their applications in analytical chemistry, based on that the research conceives were put forward.In chapter two water-soluble luminescent TGA-capped CdS QDs was synthesized in one step, the optimal synthesize conditions is pH 7.0, Cd²⁺:S^2-: TGA=1:0.75:10 and the reaction time is 8h. The received CdS QDs was characterized by TEM, XRD, UV-Vis and fluorescence spectroscopy. Use this kind of CdS QDs as fluorescence probes, Ag⁺, Zn²⁺,Cd²⁺ enhanced the photoluminescence(PL) of CdS QDs, however,Cu²⁺ and Hg²⁺ quenched the PL. Based on this, a novel method was developed for metal ions determination. We studied the response mechanisms of these five kinds of metal ions with CdS QDs simultaneously. Cu²⁺ ions were absorbed on the surface of CdS QDs, and capture electrons from excited valance band, which resulted in the fluorescence quenching. Hg²⁺ ions were absorbed on the surface of CdS QDs and formed HgS partical. The band-gap of HgS was narrow than CdS, resulted in fluorescence quenching of CdS QDs. Ag⁺ ions, Zn²⁺ ions and Cd²⁺ ions were absorbed on the surface of the CdS QDs and created more radiative centers enhanced the PL of CdS QDs. We studied the effects of the difference on the surface state of CdS QDs on metal ions sensing, the result is that response level become larger with the increase of the proportion of sulfide on the surface state of CdS QDs for Ag⁺ ions and Zn²⁺ ions; But the trend of Cd²⁺ is opposite; and for Cu²⁺ ions and Hg²⁺ ions, the response level showed nearly unchanged with the proportion change of sulfide on the surface state of CdS QDs.In chapter three ZnSe nanocrystals were prepared in aqueous solution by the reaction between Zn²⁺ and NaHSe in the prescence of TGA. The optimal synthesize conditions were studied. The nanocrystals were characterized by TEM, XRD, UV-Vis and fluorescence spectroscopy. The fresh prepared ZnSe QDs appeared nearly no PL without photo-irradiation. We choose UV-irradiation to improve the quantum yields of ZnSe QDs prepared in aqueous. The different influence factors such as light sources, the concentration of ZnSe QDs, pH value, the concentration of free Zn²⁺ ions and TGA were studied. The optimal photo-irradiation conditions were UV light, lower ZnSe QDs concentration, high alkaline media and higher concentration of Zn²⁺ ions and TGA existence in the photo-irradiation system. The PL efficiency of ZnSe QDs dramatically increased after the UV-irradiation and showed a very strong band edge emission. It's totally due to the reaction of Zn²⁺ and sulfide ions on the ZnSe particle surface and formed a structure shell of ZnS. Sulfide ions in the reaction were produced from photodecomposition of TGA molecular attached on the surface of the ZnSe QDs. Because ZnS has broader band-gap than ZnSe, the formation ZnS shell on the ZnSe will effectively passivate the ZnSe surface and give rise to a great increment in PL intensity. We also found that pH value play a very important role in the photo- induced PL enhancing process of ZnSe QDs. At the range of pH 4 to 12, the PL efficiency increased along with the increase of the pH value, and the photostability of ZnSe QDs became better at the same time, this is because that in an alkaline solution, the decomposition of TGA became much faster when it was irradiated with UV light therefore abundant sulfur atoms provided for the ZnS layer formation. But PL efficiency decreased slightly when the solution pH was over 12, probably because overly rapid formation of the ZnS layer created defects. This kind of post-preparation method provides new idea for the preparation of high quantum yield QDs.