Reporting in the January/February issue of Bioconjugate Chemistry, Byron Ballou and colleagues at Carnegie Mellon, in collaboration with the Quantum Dot Corporation, found that the company's quantum dots (Qdot® Particles) coated with an amphiphilic polyacrylic acid polymer are stable in vivo. A member of the team, Lauren Ernst, also found that modifying the surface molecules by adding a second polymer coat prolongs the time quantum dots circulate in the body.
"Because uncoated quantum dots are too fragile for most biological studies in vivo, the coating is the most critical step," explained Ballou, research scientist at the Molecular Biosensor and Imaging Center (MBIC) at Carnegie Mellon's Mellon College of Science. "The new coatings allowed us to observe quantum dots much longer than previously demonstrated. We had concerns that the coats might dissolve or be digested away, so we were pleased with the long persistence of fluorescence, as well as the large increase in circulating time caused by increasing the thickness of the outer polymer coat." Both these features enabled the quantum dots to deposit effectively within tissues, Ballou noted.
First commercialized by the Quantum Dot Corporation in 2002, Qdot Particles are nanosized crystalline particles composed of a few hundred to a few thousand atoms of a semiconductor material (typically cadmium selenide). Quantum dots emit light in a variety of colors, depending on size. Unlike traditional fluorescent markers used in research and medicine, quantum dots are very bright, do not bleach under intense illumination and, when properly coated by a proprietary method invented by scientists at Quantum Dot Corporation, keep their fluorescence for long periods of time.
Marcel Bruchez, principal scientist at the Quantum Dot Corporation, collaborated on the project. Other collaborators at MBIC include Christopher Lagerholm and Alan Waggoner.
"Our findings are a promising step toward using quantum dots for non-invasive imaging in humans to monitor and treat diseases such as cancer," said Ballou. "Using our modified quantum dots, we were able to non-invasively image structures in living mice by fluorescence, then prove that the quantum dots were present by electron microscopy. No other fluorescent label lets you verify its exact location on scales from the whole animal to molecular dimensions."
Scientists also could modify the surface of these long-lived quantum dots by attaching both biological and non-biological molecules, thereby altering the properties of quantum dots to accomplish a specific goal, noted Ballou. By attaching molecules to the surface of quantum dots, one could target tumors to image them more effectively and allow surgeons to remove cancers with greater accuracy, according to Ballou. "Before these applications can happen, quantum dots must first be modified so that they remain in circulation long enough, and we must ensure that quantum dots cause no toxicity in healthy cells," he said.
To increase the time quantum dots remained circulating in live animals, the researchers coated their surfaces with one of three polyethylene glycol amine (PEG) molecules of varying lengths. They then monitored the circulating lifetimes of each quantum dot variant using non-invasive imaging techniques. While the paper reports initial success at visualizing the quantum dots over four months, the long-term experiment is still continuing, and Ballou has demonstrated that the PEG-coupled Qdot Particles have remained fluorescent for eight months.
In the animals, the modified quantum dots remain localized primarily in the liver, spleen, lymph nodes and bone marrow. These locations have a high concentration of phagocytes, immune cells that remove circulating debris inside the body. This finding strongly suggests that quantum dots were collected by phagocytic cells, according to the scientists. Because quantum dots accumulate in these areas, they could potentially be useful for defining some tumor types and for detecting sentinel lymph nodes, according to Ballou. (A sentinel lymph node is the first lymph node to which a cancer spreads.)
The Carnegie Mellon researchers plan to extend their research by targeting quantum dots to cells other than phagocytes. They also hope to modify quantum dots using biological molecules that could create in situ biosensors to report the presence of specific compounds that signal cellular responses inside the body. For instance, these modified quantum dots could potentially report how a tumor is responding to therapy.
MBIC is recognized worldwide for its work in revolutionizing light microscopy and fluorescent probes for live cell imaging. The center's current focus involves the development of instrumentation and chemistry systems for biological research and medicine.
Founded in 1998, Quantum Dot Corporation (QDC) and its advisors are the world's leading experts in semiconductor nanocrystal (Qdot®) technology and its application in biology. QDC markets and sells Qdot nanocrystal products worldwide, directly and through distributors. QDC has a dominant and extensive patent position covering quantum dot compositions, synthesis methods and methods of use. QDC is the exclusive licensee of quantum dot technology, in the field of biological applications, of pioneering intellectual property licensed from the University of California, MIT, Indiana University and the University of Melbourne. The research was funded by the National Institutes of Health.