Fluorescent Nanotubes Illuminate the Inner Workings
of Laboratory Mice
Developing drugs to combat or cure human disease often involves a phase of testing with mice, so being able to peer clearly into a living mouse's innards has real value. But with the fluorescent dyes currently used to image the interior of laboratory mice, the view becomes murky a few millimeters under the skin. Now, however, a team of investigators from Stanford University has developed an improved imaging method using fluorescent carbon nanotubes that create color images centimeters beneath the skin with far more clarity than conventional dyes provide. For a creature the size of a mouse, a few centimeters makes a great difference.
"We have already used similar carbon nanotubes to deliver drugs to treat cancer in laboratory testing in mice, but you would like to know where your delivery went, right?" said Stanford University's Hongjie Dai, a member of the National Cancer Institute's Alliance for Nanotechnology in Cancer. "With the fluorescent nanotubes, we can do drug delivery and imaging simultaneously – in real time – to evaluate the accuracy of a drug in hitting its target." Dr. Dai and his collaborators published their findings in the Proceedings of the National Academy of Sciences.
Dr. Dai's team injects the single-walled carbon nanotubes into a mouse and then watches as the tubes are delivered to internal organs by the bloodstream. The nanotubes fluoresce brightly in response to the light of a laser directed at the mouse, while a camera attuned to the nanotubes' near-infrared wavelengths records the images. By attaching the nanotubes to an anticancer agent, researchers can see how the drug is progressing through the mouse's body.
The key to the nanotubes' usefulness is that they shine in a different portion of the near-infrared spectrum than most dyes. Biological tissues – whether mouse or human – naturally fluoresce at wavelengths below 900 nanometers, which is in the same range as the available biocompatible organic fluorescent dyes. That results in undesirable background fluorescence, which muddles the images when dyes are used. But the nanotubes used by Dai's group fluoresce at wavelengths between 1,000 and 1,400 nanometers. There is barely any natural tissue fluorescence at those wavelengths, so background "noise" is minimal.
The nanotubes' usefulness is further boosted because tissue scatters less light in the longer wavelength region of the near-infrared, reducing image smearing as light moves or travels through the body. "The nanotubes fluoresce naturally, but they emit in a very oddball region," Dr. Dai said. "There are not many things – living or inert – that emit in this region, which is why it has not been explored very much for biological imaging." By selecting single-walled carbon nanotubes with different diameters and other properties, Dr. Dai and his team can fine-tune the wavelength at which the nanotubes fluoresce.
The nanotubes can be seen immediately upon injection into the bloodstream of mice. In fact, the Stanford team was able to see the fluorescent nanotubes passing through the lungs and kidneys within seconds of injection. The spleen and liver lit up a few seconds later. "You can really see things that are deep inside or blocked by other organs such as the pancreas," Dr. Dai said.
There are other imaging methods that can produce deep tissue images, such as magnetic resonance imaging (MRI) and computer tomography (CT) scans, but fluorescence imaging is widely used in research and requires simpler machinery. Dr. Dai said that the fluorescent nanotubes are not capable of reaching the depth of CT or MRI scans, but represent a step forward in broadening the potential uses of fluorescence as an imaging system beyond the surface and near-surface. "I did not imagine [carbon nanotubes] could really be used in animals to get deep images like these," he said. "When you look at images like this, you get a sense that the body almost has some transparency to it."
This work, which is detailed in a paper titled "Deep-tissue anatomical imaging of mice using carbon nanotube fluorophores in the second near-infrared window," was supported in part by the NCI Alliance for Nanotechnology in Cancer, a comprehensive initiative designed to accelerate the application of nanotechnology to the prevention, diagnosis, and treatment of cancer. An abstract of this paper is available at the journal's website.