Studies On Semiconductor Nanoparticles Synthesized In Aqueous Solution As Luminescent Probes For Biological Labeling

Semiconductor nanoparticles, also known as quantum dots (QDs) haveattracted considerable interest in biological and medical community because oftheir unique size-dependent optical and electronic properties. As luminescentprobes, QDs present considerable advantages over conventional organic dyemolecules. However, luminescent QDs are commonly prepared by organometallicprocedure and not compatible with aqueous assay conditions. Consequently, it isnecessary to substitute the surface organic ligands with hydrophilic cappingagents for biological applications. But for these methods, the conditions ofpreparation of QDs were relatively critical, and the process was complicated. Incontrast, the synthesis method of QDs in an aqueous medium using thiols asstabilizing agents is of simplicity, low toxicity and low cost. Most important isthat the stability of QDs synthesized by aqueous approach is better than that of thewater-soluble QDs transformed from organic solvent. This thesis attempted to 174apply the QDs synthesized in aqueous solution as luminescent probes forbiological labeling. In Chapter 1, the unique optical properties of QDs are introduced. Thesynthesis methods of QDs are summarized. The methods on determining the sizeof the QDs are reviewed. The development and the trends of the QDs in biologicaland medical applications are specially introduced. In Chapter 2, CdSe nanoparticles capped with CdS were synthesized inaqueous solution using 2-mercaptoethanol as stabilizer. The CdS capping with ahigher bandgap than that of the core crystallite has successfully eliminated thesurface traps. The X-ray powder diffraction, transmission electron microscopy andX-ray photoelectron spectroscopy were used to analyze the compositenanocrystals and determine their average size, size distribution, shape, internalstructure and elemental composition. However, these QDs have low quantumyield, poor reproducibility and are not satisfied with the demand of biologicalluminescent probes. Therefore, mercaptopropyl acid-stabilized CdTe nanoparticles(3 nm) with high fluorescence quantum yield (20% ~ 30%) were synthesized inaqueous solution. These QDs can be coupled with biomolecules by the outercarboxyl group. In Chapter 3, mercaptopropyl acid-stabilized CdTe nanoparticles areconjugated with amino groups of the biomolecules, trypsin and bovine serumalbumin, through covalent amide bonding. The absorption spectrum ofCdTe-biomolecule solution was flatter than that of CdTe nanoparticles over thewavelength range of 400 ~ 600 nm. While the emission spectrum ofCdTe-biomolecule solution showed a blue-shift of the emission peak, whereas theintrinsic emission band width was unchanged. It was proved that the change of the 175fluorescence and absorption spectra of CdTe-trypsin was the result of theconjugation between CdTe nanoparticles and trypsin, but not of the oxidationcaused by O2. Furthermore, heating could accelerate the rate of conjugationbetween CdTe nanoparticles and trypsin. However, the conjugation reaction needa relatively long time (26 h). In order to shorten the time, another familiar protein,bovine serum albumin (BSA) was selected. The conjugation between QD andBSA could be finished within 1.5 h and was confirmed by gel electrophoresis,high-performance liquid chromatography and fluorescence micrographs. In Chapter 4, CdTe nanoparticles were effectively bound to positivelycharged protein, papain and avidin, via electrostatic interaction at a certain pHvalue. Compared with the free QDs, the absorption spectrum of QDs-proteinbioconjugates was flatter and the emission peak of the bioconjugates underwent ared shift, but the intrinsic spectral width was unchanged. The redshift of the QDsafter conjugation was dependent on the size of the QDs: the bigger of the particlesize, the more prominent of the redshift. Especially, the biological...