To date, most commercial QDs are synthesized through the traditional organometallic method and contain toxic elements, such as cadmium, lead, mercury, arsenic, etc. The overall goal of this thesis study is to develop an aqueous synthesis method to produce nontoxic quantum dots with strong emission and good stability, suitable for biomedical imaging applications.

Firstly, an aqueous, simple, environmentally friendly synthesis method was developed. With cadmium sulfide (CdS) QDs as an example system, various processing parameters and capping molecules were examined to improve the synthesis and optimize the PL properties. The obtained water soluble QDs exhibited ultra small size (~5 nm), strong PL and good stability. Thereafter, using the aqueous method, the zinc sulfide (ZnS) QDs were synthesized with different capping molecules, i.e., 3-mercaptopropionic acid (MPA) and 3-(mercaptopropyl)trimethoxysilane (MPS). Especially, via a newly developed capping molecule replacement method, the present ZnS QDs exhibited bright blue emission with a quantum yield of 75% and more than 60 days lifetime in the ambient conditions. Two cytotoxicity tests with human endothelial cells verified the nontoxicity of the ZnS QDs by cell counting with Trypan blue staining and fluorescence assay with Alamar Blue. Taking advantage of the versatile surface chemistry, several strategies were explored to conjugate the water soluble QDs with biomolecules, i.e., antibody and streptavidin. Accordingly, the imaging of Salmonella t. cells and biotinylated microbeads has been successfully demonstrated. In addition, polyethylenimine (PEI)-QDs complex was formed and delivered into PC12 neuronal cells for intracellular imaging with uniform distribution. The water soluble QDs were also embedded in electrospun polymer fibers as fluorescent nanocomposite. In summary, the ease of aqueous processing and the excellent PL properties of the nontoxic water soluble ZnS QDs provide great potential for various in vivo application.

Synthesis of QDs

Quantum dots can be produced through the vapor-phase deposition, evaporation, sputtering and epitaxial growth on certain substrates,[11-14] which is the so-called Stranski-Krastanov growth. The lattice-mismatch strain-driven, spontaneously formed, coherent island based quantum dots are widely studied and utilized in devices such as transistors, light emitters, and detectors. The main limitations of this method are the cost of fabrication and the lack of control over positioning of individual dots.

In this work, we mainly focus on the quantum dots synthesized via solution chemistry routes, which have become most popular given their usefulness for biological and biomedical applications in vivo and in vitro. Besides the preparation of QDs with reverse micelle,[15-18] there are several synthetic methods used to form colloidal nanocrystals.

Traditionally, many quantum dots of II-VI and III-V compounds are formed in a hot mixture of the tri-n-octylphosphine (TOP) and tri-n-octylphosphine oxide (TOPO) with the experimental setup as shown in Fig. 2.2.[19] The mixture functions as both a solvent and as the stabilizing ligand. Take the synthesis of CdSe QDs as an example.[20] First, certain amount of TOPO is purged with nitrogen or argon gas flow and heated in a reaction flask to 200oC ~ 300oC. Stock solutions of dimethylcadmium (Me2Cd) and tri-n-octylphosphine selenide (TOPSe) are then added to the flask and mixed with vigorous stirring, where the CdSe nanocrystals nucleate quickly and grow with time. Aliquots of the reaction solution are removed from the flask at regular time intervals to obtain the nanocrystals with different sizes. Isolation and purification of the nanocrystals are required to remove the byproducts of the reaction and the excess solvent. To narrow the size distribution, the large crystallites are precipitated by adding methanol and then re-dispersed in 1-butanol. After this size-selective precipitation is repeated for a few times, the final product is dispersed in organic solvent, having good size distribution with the standard deviation less than 5%. However, this procedure is detrimental to the environment and human health, because the raw materials, especially the organometallic precursors, are extremely toxic, expensive, unstable, explosive, and/or pyrophoric. Although a relatively ‘greener’ approach was developed later using CdO as the precursor,21 it has some inevitable disadvantages. The hydrophobic surface of organic ligand coated QDs are not suitable for biological applications. High temperature reaction is still required. The complexity of the organic synthesis causes the price of the commercial QDs prohibitively high.

To make QDs water-soluble and biocompatible, the QDs surface was modified with silica coating.22 The 3-(mercaptopropyl) trimethoxysilane (MPS) was adsorbed on the surface of nanocrystals and displaced the TOPO molecule, making the QDs stable in water and ready for bioconjugation. But this process is tedious, involving multiple steps to change the solvent from organic to aqueous. On the other hand, the synthesis of QDs directly in water was reported with various systems. The CdTe nanoparticles were formed via the reaction between Cd2+ and NaHTe, and started to crystallize and show visible photoluminescence after reflux at 96oC for hours.23 The QDs capped with 2-mercaptoethanol yielded the undesirable broadband emission at large wavelengths. The CdSe nanocrystals were also synthesized in water phase.24 The chemical etching in a solution of 3-amino-1-proponal/H2O at 80oC was required to achieve the band-edge photoluminescence. Moreover, the CdS-mercaptoacetic (CdS/M) clusters were prepared using mercaptoacetic acid as the stabilizing agent in the aqueous solution.25 The CdS/M cluster was found to be constructed of small nanocrystals, but it was difficult to obtain individually dispersed QD.