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Nanotubes Improve Fluorescence Imaging

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STANFORD, Calif., June 1, 2011 — An improved imaging method using fluorescent carbon nanotubes allows researchers to see centimeters deep into a mouse with far more clarity than conventional dyes provide.

For a creature the size of a mouse, a few centimeters makes a great difference. 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. With the fluorescent dyes currently used to image the interior of laboratory mice, however, the view becomes very murky several millimeters under the skin.

“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 Hongjie Dai, a professor of chemistry at Stanford University. “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.” Dai is co-author of a paper describing the research published online this month in Proceedings of the National Academy of Sciences.

An enhanced color image of fluorescence from single-walled carbon nanotubes (right) shows internal organs of a mouse next to a reference illustration (left). In the fluorescent image, the pancreas (thin green strip on the left side of the mouse) is sandwiched between a kidney (yellow) and the spleen (pink). In the reference image, the kidneys are orange-brown, the spleen is pumpkin-colored and the pancreas is barely visible as a tiny red triangle between the other two organs. (Images reproduced with permission from PNAS)

Researchers at the university injected the single-walled carbon nanotubes into a mouse and watched as the tubes entered the internal organs via 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-IR wavelengths records the images. By attaching the nanotubes to a medication, 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-IR spectrum than most dyes. Biological tissues — whether mouse or human — naturally fluoresce at wavelengths below 900 nm, 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 1000 and 1400 nm. At those wavelengths, there is barely any natural tissue fluorescence, so background noise is minimal.

The nanotubes’ usefulness is further boosted because tissue scatters less light in the longer-wavelength region of the near-IR, reducing image smearing as light moves or travels through the body, another advantage over fluorophores emitting below 900 nm.

“The nanotubes fluoresce naturally, but they emit in a very oddball region,” 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 chiralities, diameters and other properties, Dai and his team can fine-tune the wavelength at which the nanotubes fluoresce.

Dai and graduate students Sarah Sherlock and Kevin Welsher observed the fluorescent nanotubes passing through the lungs and kidneys within seconds after injection. The spleen and liver lit up a few seconds later.

The group also did some “postproduction” work on digital video footage of the circulating nanotubes to further enhance the image quality using principal component analysis.

“In the raw imaging, the spleen, pancreas and kidney might appear as one generalized signal,” Sherlock said. “But this process picks up the subtleties in signal variation and resolves what at first appears to be one signal into the distinct organs.”

“You can really see things that are deep inside or blocked by other organs such as the pancreas,” Dai said.

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Jun 2011
AmericasBasic Sciencebiological imagingBiophotonicsCaliforniacamerascarbon nanotubesdrug deliveryfluorescent dyesHongjie DaiimagingindustrialKevin Welshernear-IR imagingProceedings of the National Academy of SciencesResearch & TechnologySarah SherlockStanford Universitytissue imagingwhole-animal imaging

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