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Posted: Jul 7th, 2009
Novel electrostatic coupling method to make quantum dot conjugates
(Nanowerk Spotlight) Research with semiconductor quantum dots (QDs) is increasingly showing their huge potential for in vitro and in vivo cellular imaging. For instance, we have previously reported about how researchers have used QDs for in vivo imaging of embryonic stem cells in mice, a novel technique that has opened up the possibility of using QDs for fast and accurate imaging applications in stem cell therapy ("Quantum dot imaging could benefit embryonic stem cell therapy") or how live cell imaging can be conducted with biodegradable quantum dot nanocomposites.
The usefulness of quantum dots comes from their peak emission frequency's extreme sensitivity – quantum mechanical in nature – to both the dot's size and composition. QDs have been touted as possible replacements for organic dyes in the imaging of biological systems, due to their excellent fluorescent properties, good chemical stability, broad excitation ranges and high photobleaching thresholds.
In order for quantum dots to be useful as as nanoemitters for biological imaging, they need to be linked with a specific sensor molecule that exclusively targets a biomolecule of interest. These conjugations are usually made using small linker molecules although this often demands multistep procedures and may suffer from QD colloidal instabilities during the coupling reactions.
Demonstrating hyaluronic acid-QD conjugates (HA-QDs), researchers in Korea have now solved this problem by simple electrostatic conjugation which offers stable and size-tunable conjugates.
Photo of lymphatic vessels visualized by quantum dot conjugate imaging (Image: Dr. Sungjee Kim, Pohang University of Science & Technology)
"Our conjugation method is simple, size-tunable, and robust" Sungjee Kim tells Nanowerk. "These HA-QDs show cancer specificity and can label lymphatic endothelial cells (LECs). We have also investigated comprehensive cytotoxicity studies of our HA-QDs – they show remarkably low toxicity."
Kim is an assistant professor in the Department of Chemistry at Pohang University of Science & Technology. Together with an interdisciplinary team of scientists from his group, Hanyang University, and Seoul National University, he managed to use quantum dots to visualize in vivo, for the first time, a lymphatic vessel.
The team used hyaluronic acid – a biodegradable natural polysaccharide – instead of antibodies to target LYVE-1, a lymphatic vessel endothelial receptor, which enabled fluorescence staining of LECs.
Kim points out that hyaluronic acid can be used in large quantities because of the cost effectiveness and biocompatibility. "We also demonstrated a simple electrostatic conjugation scheme to conjugate QDs to HA" he says. "Potentially, HA-QDs can be used for cancer targeting and for visualization of lymphatic vessels in vivo for long-term applications."
The scientists say that HA-QDs can be a useful reporter probe that monitors tumor progressions, such as metastasis trafficking,
or that screens anticancer drug efficacies. For example, they can be potentially used to real-time monitor the formation and development of lymphatic vessels around tumor mass such as lymphangiogenesis.
"We have shown that our HA-QDs can be also used to visualize lymphatic vessels by fluorescence staining since they can be delivered into lymphatic endothelial cells through receptors such as LYVE-1," Kim explains. " As a matter of fact, we have been able to fluorescence stain lymphatic vessels of mice up to for a few days. However, being an exogenous imaging contrast agent, the HA-QDs have the inherent limitation of signal dilution due to cell division. To overcome this limitation, multiple injections of HA-QDs may be necessary."
Generally, because HA-QDs can target cancer cells, they can be used for cancer diagnosis and could help in studying the mechanism behind developing tumors. Using multiplexed QD imaging, tumor growth and angiogenesis can be visualized real-time in vivo using animal models without sacrificing the animals.