Quantum Dots and Gold Nanoparticles
A quantum dot is a microcrystalline semiconductor nanostructure . It is able to restrict movement of conduction band electrons, valence band holes, or excitons in all three spatial directions. It has been applied extensively to track and image the intracellular fate of nanoparticles. A significant advantage of quantum dot over traditional fluorescent techniques is that it does not photobleach significantly, which allows investigators freedom to design experiments . However, quantum dots do possess fluorescent properties (albeit without significant photobleaching), which render them an attractive system for diagnostic and therapeutic tool. In one study quantum dots were observed in endosomal and lysosomal vesicles (Fig. 8.8). This trafficking mimics the pathway traversed by drug-loaded nanoparticles. Quantum dots coupled with transmission electron
Figure 8.8 Different intracellular areas were scanned for S and Cd. Parts A—F represent the areas selected in this figure and parts A0—F0 show the corresponding S/Cd electron spectroscopic images. Images B and B0 show a homogenous distribution of quantum dots within an intracellular vesicle with a specific signal for S/Cd, whereas other images are only related to noisy unspecific S/Cd background. No further analysis on these structural origins was performed, but it is assumed that the structures present in A, D, and F are contaminants of transmission electron microscopy embedding and staining with heavy metals, whereas the structures in C and E possibly represent protein or lipid aggregates. All scale bars equate to 50 nm .
microscopy and confocal microscopy are being explored as powerful tools to investigate intracellular trafficking of nanosized structures.
Apart from endosomal and lysosomal compartments, quantum dots may also be targeted to other organelles such as mitochondria and nucleus . Conjugation of 23 amino acid nuclear and 28 amino acid mitochondrial localization signals leads to localization of quantum dots into nucleus and mitochondria, respectively. Quantum dots are localized in their respective compartments within 30 min of beginning of cellular uptake . Conjugation of signaling peptides to quantum dots provides an idea of mechanisms and pathways responsible for uptake by specific organelles. This process allows us to develop drug-loaded nanoconstructs that can be specifically targeted to particular organelles. Peptide-conjugated quantum dots also allow us to determine the localization kinetics of nanostructures in different targeted organelles (Fig. 8.9).
Nanotechnology in Intracellular Trafficking, Imaging, and Delivery of Therapeutic Agents 183