A new imaging tool tracks carbon nanotubes inside living cells and throughout the bloodstream – and it could soon hone the particles’ usefulness in biomedical research and clinical medicine. Carbon nanotubes have potential applications in drug delivery and imaging. Two types of nanotubes are created in the manufacturing process: metallic and semiconducting. Until now, however, no technique has visualized both types in living cells and the bloodstream, said Ji-Xin Cheng, associate professor of biomedical engineering and chemistry at Purdue University. A new imaging tool tracks carbon nanotubes in living cells and the bloodstream, work that could aid efforts to perfect the nanostructures’ use in laboratory and medical applications. Here, the imaging system detects both metallic and semiconducting nanotubes, false-colored in red and green, in live hamster cells. Courtesy of Weldon School of Biomedical Engineering, Purdue University. The imaging technique, called transient absorption, uses a pulsing near-IR laser to deposit energy into the nanotubes, which then are probed by a second near-IR laser. The researchers have overcome the previous hurdle of imaging nanotubes in live cells and laboratory mice, Cheng said. Because the new technique works at such a high speed, they can now see what is happening in real time as the nanotubes circulate in the bloodstream, and the technique is label-free. Conventional imaging methods use luminescence, which is limited because it detects the semiconducting nanotubes but not the metallic ones. “Current methods are not sufficient for monitoring nanotubes in living cells,” Cheng said. “The gold standard for imaging nanotubes is near-infrared luminescence. This method is, however, only sensitive to semiconducting nanotubes.” The nanotubes have a diameter of about 1 nm – far too small to be seen with a conventional light microscope. One challenge the scientists encountered while using the transient absorption imaging system for living cells was that they needed to eliminate the interference caused by the background glow of red blood cells, which is brighter than the nanotubes. They overcame this obstacle by directing the signals from red blood cells and nanotubes into two separate channels. Light from the red blood cells was slightly delayed compared with light emitted by the nanotubes, enabling this process. The work was described online in Nature Nanotechnology (doi: 10.1038 /nnano.2011.210). Cheng explained that this is an important step for drug discovery because scientists need to know how long nanotubes remain in blood vessels after being injected. He and his colleagues have taken images of nanotubes in the blood vessels inside the earlobes of mice, as well as in the liver and other organs, and they are using the imaging technique to study other nanomaterials, such as graphene.