ABSTRACT
The hot colloidal synthesis technique is still the most effective method to prepare highly crystalline and monodisperse QDs. The synthesis of QDs requires a nucleation burst event followed by a slower growth on the nucleated nuclei.34 This can be achieved by injecting required chemical precursors into a hot reaction mixture containing surfactants and high-boiling-point solvents.35 With the introduction of the precursors into the hot colloidal reaction mixture, it will raise the precursor concentration above the nucleation threshold and result in a supersaturation of nanocrystals in the solution.36 Most importantly, the consumption of the precursors during the growth of the nanocrystals must not go beyond the rate of the precursor introduced into the mixture, because this will hinder the formation of new nuclei.37 By systematic tailoring the synthesis parameters, such as temperature, precursor concentrations, growth time, and the choice of surfactants, one can manipulate the size and crystallinity of the QDs.38 In general, the concentration of the surfactants is commonly three to four times higher than that of the precursors because high surfactant concentrations will provide better passivation to the QD surface and lead to the formation of monodisperse QDs. The surfactants are attached to the surfaces of
the nanocrystals, creating a hydrophobic organic moieties layer that protects the nanocrystals from forming aggregates in solution.34Ligands that bind more strongly to the surface of the nanocrystals will generate greater steric hindrance that slows the growth of the nanocrystals and allows one to obtain a desirable nanocrystal size upon the addition of appropriate surfactants. An alternative method to narrow the size distribution of the particles involves the use of a binary or even tertiary surfactant mixture in which the surfactants attach tightly to the nanocrystal surface, thereby slowing their growth in the solution.34 When the nanocrystal reaches the desired size, addition of organic solvents such as chloroform or toluene into the hot colloidal mixture will cool the solution and prevent the growth of the nanocrystals. A semipolar solvent such as ethanol, methanol, or butanol is introduced into the colloidal nanocrystal solution to clean and collect the nanocrystals. These alcohol solvents have an unfavorable interaction with the surfactant molecules anchored on the nanocrystal surface and will destabilize the nanocrystals’ dispersion and cause them to agglomerate. Centrifugation of the aggregate suspension allows the surfactant solution and unreacted precursors to be removed, and the precipitate of the nanocrystals can be obtained. The above method is generally used in the production of the “core” of QDs. These “core” nanocrystals must be coated with a biocompatible layer before they can be safely applied in biological applications.28 Methods for coating a semiconductor nanocrystal core with a second semiconductor shell are well reported, and different kinds of core-shell structures have been prepared39 (CdSe/ZnS, CdSe/ZnSe, CdTe/ZnS, CdHgTe/ZnS, and InP/ZnS core-shell nanocrystals, to name a few). There are common guidelines for the preparation of high-quality core-shell nanocrystals:34 (i) the synthesized nanocrystal cores must remain their structure and crystallinity when the second semiconductor material is deposited; (ii) the lattice mismatch between the two semiconductor materials must be minimized so that a uniform epitaxial growth of the shell can occur around the core, resulting in low density of defects in the shell; and (iii) the coating of semiconductor nanoparticle cores must be chosen in a way that they will not break down under the deposition reaction or else this will cause the loss of particles or generate foreign nanocrystal species. Generally, nanocrystal cores are prepared first and then redispersed in a low-boiling-point organic solvent (e.g.,
hexane or chloroform). The colloidal nanocrystal dispersion is then mixed with high-boiling-point surfactants and heated to the desired reaction temperature, and at the same time the inorganic shell precursors are slowly introduced into the reaction mixture. This process will allow the shell materials to heterogeneously nucleate on the nanocrystal cores. It is important to note that as long as the rate of the inorganic shell precursor addition does not go beyond the rate of its deposition onto the cores, the precursor concentration never reaches the threshold for homogeneous nucleation of foreign nanocrsytals.34 11.3 Types of Quantum Dots Available for
We here briefly discuss the types of QDs utilized in biomedical imaging and therapy. Figure 11.1 shows commonly used QDs and specific biological applications where they are suited. The emission band of these QDs can be tuned from the visible to NIR region by manipulating their size, shape, composition, and structure. Each type of QDs has its own advantages and disadvantages, and the discussion below is generated to provide useful guidelines for researchers to engineer QD probes meeting their own specific needs in cancer-relevant applications.
