Essay: Delivery of QDs for biological applications into biological bodies for cell imaging

The delivery of QDs For biological applications into biological bodies for cell imaging is a necessary requirement. However, most water-soluble QDs have the negative charges on their surface, which allows QDs to disperse in aqueous solution by the electrostatic repulsion (19, 20) also makes those QDs hardly bind on the surfaces of living cells. The cell uptake efficiency for these QDs is low (21, 22). Different methods have been used for enhanced the intracellular delivery of QD by surface modifications with various functional molecules, such as linking with cationic peptides and encapsulating with liposome’s (23, 24, 25, 26). Liposome’s have outstanding features to act as the biological delivery vehicle, because they are biocompatible, biodegradable and less toxic (27). Liposome’s already became the most clinically established nanometer-scale systems for various drug and gene deliveries (28, 29). Liposomes could form self assembled vesicles, consisting of a lipid bilayer surrounding an aqueous cavity, so that both hydrophobic agents and hydrophilic compounds can be carried. The hydrophobic core/shell QDs were encapsulated by liposomes for the intracellular delivery as reported in several studies, but few works of liposome encapsulation on hydrophilic QDs could be found (30). As the delivery vehicle, the loading content of the hydrophilic compounds by liposome vesicles should be higher than that of hydrophobic ones, because the water phase compounds are contained in the central cavity whose volume is much bigger than that of the lipid bilayer where the organic phase compounds are embedded in. The high loading efficiency for QDs is the important index to fulfill the effective intracellular delivery, and thus the loading of hydrophilic QDs with liposome vesicles is worth investigating further. Liposomes are spherical vesicles consisting of phospholipids bilayers surrounding an aqueous cavity. Because each liposome can contain up to several million fluorescent dye molecules, thereby providing greatly enhanced signals, liposomes have been successfully used as reporter particles in bioassays. Moreover, the biomimetic lipid bilayers of liposomes provide high biocompatibility, thereby enhancing the effectiveness of fluorescent nanoparticles for biological detection in vitro and in vivo.
In this study, we encapsulated green emitter and red emitter QDs into novel liposome preparation (DPPC/CHO/DOAB liposome’s and DSPC/CHO/DOPE/DOAB liposomes). Endosomal entrapment is one of problems that QDs encounter upon entry into a cell. This could be the reason for the lack of significant effect on cell killing by QDs. For an efficient photosensitization, QDs must reach and interact with vital parts of the cell, and the first barrier to this action is their persistence in endosomes and lysosomes. This study was further focused on investigating photostability of QDs encapsulated in lipid vesicles in comparison to intact QDs with the goal to understand how to overcome fluorescence quenching of QDs.