Tiny polymeric containers stuffed with red and green quantum dots promise to provide continuous light in biomedical imaging. Engineers at Ohio State University, led by Jessica Winter and Gang Ruan have invented a kind of nanoparticle that shines in different colors because of their contents. Somewhat unusually, the particles glow red, green or yellow, depending on the state of the quantum dots contained within. Winter, an assistant professor of chemical and biomolecular engineering and biomedical engineering, and Ruan describe their patent-pending technology in the online edition of Nano Letters. Researchers routinely tag molecules with fluorescent materials in order to see them under the microscope. Unlike the more common fluorescent molecules, quantum dots shine very brightly, and could illuminate chemical reactions especially well, allowing researchers to see the inner workings of living cells. The new nanoparticles, or micelles, could be a useful addition to the arsenal of biomedical engineers trying to find the roots of diseases, Ruan said. This series of photos shows a novel nanoparticle changing from red to green to yellow over the course of 2 min. (Image: G. Ruan, Ohio State University) "We can tailor these particles to tag particular molecules, and use the colors to track processes that we wouldn't otherwise be able to," he added. "Also, this work could be groundbreaking for the field of nanotechnology as a whole, because it solves two seemingly irreconcilable problems with using quantum dots." Due to quantum mechanical effects, quantum dots "twinkle" — they blink on and off at random moments. When many dots come together, however, their random blinking is less noticeable. So, large clusters of quantum dots appear to glow with a steady light. Blinking has been a problem for researchers, because it breaks up the trajectory of a moving particle or tagged molecule that they are trying to follow. Yet, blinking is also beneficial, because when dots come together and the blinking disappears, researchers know for certain that tagged molecules have aggregated. "Blinking is good and bad," Ruan said. "But one day we realized that we could use the 'good' and avoid the 'bad' at the same time, by grouping a few quantum dots of different colors together inside a micelle." Whereas micelles are useful for laboratory experiments, they are easily found in household detergents — soap forms micelles that capture oils in water, for example. Ruan created micelles using polymers, with different combinations of red and green quantum dots inside them. In tests, he confirmed that the micelles appeared to glow steadily. Those stuffed with only red quantum dots glowed red, and those stuffed with green glowed green. But those he stuffed with red and green dots alternated from red to green to yellow. The color change happens when one or another dot blinks inside the micelle. When a red dot blinks off and the green blinks on, the micelle glows green. When the green blinks off and the red blinks on, the micelle glows red. If both are lit up, the micelle glows yellow because of the additive effect. Nobody can control when color changes happen inside individual micelles. But because the particles glow continuously, researchers can use them to track tagged molecules continuously. They can also monitor color changes to detect when molecules come together. Winter and Ruan said that the particles could also be used in fluid mechanics research — specifically, microfluidics. Researchers developing tiny medical devices with fluid separation channels could use quantum dots to follow the fluid's path. The same research team also is developing magnetic particles to enhance medical imaging of cancer, and it may be possible to combine magnetism with the quantum dot technology for different kinds of imaging. But before the particles would be safe to use in the body, they would have to be made of biocompatible materials. Carbon-based nanomaterials are a possible option. In the meantime, Winter and Ruan are going to continue developing the color-changing quantum dot particles for studies of cells and molecules under the microscope. They also are going to explore what happens when quantum dots of another color — for instance, blue — are added to the mix. For more information, visit: www.osu.edu