Sensors tiny enough to fit in cell

DNA molecules wrapped around tiny carbon nanotubes get a charge out of particles from toxic substances such as mercury, or rather the exposure causes the DNA's own charge to change.

In response, the DNA subtly rearranges itself, allowing more water to reach the surface of the nanotube, which causes it to glow, in essence, with near infrared light.

University of Illinois researchers can't see this happening. Single-walled carbon nanotubes are thousands of times smaller than the width of a human hair.

But the cylinders, which consist of carbon atoms interlinked in a hexagonal pattern, have novel properties – like emitting light under certain conditions – that make them potentially useful in a wide range of applications.

The UI scientists can capture those light signals, even when the signals are coming from inside a cell.

That makes the nanotubes wrapped in DNA sensors capable of picking up on and signaling the presence of minute amounts of toxics such as mercury.

"We can flip this DNA back and forth and we get a clear signal from the nanotube," said Michael Strano, the UI chemical and biomolecular engineering professor whose lab is producing the devices.

"It does open the door to new technology that should be sensitive to a wide range of molecules," he said.

And the smaller the detector, it appears, the fewer the molecules of a substance needed to ferret it out.

"The subcellular region is one that seems ripe with possibilities," said Strano, who's also a professor at the UI's Beckman Institute.

UI graduate student Daniel Heller and Strano reported on the device last week in the journal Science. Heller was the paper's lead author and did much of the development work.

Strano said the key finding overall is that you can park a biomolecule like DNA on a carbon nanotube and subtle changes in the molecule – as a result of the presence of toxics or other environmental conditions – will then show up on the nanotube.

The UI researchers were able to detect the light emissions through blood and tissue and from within living cells, according to their paper.

Besides being small and emitting light, nanotubes have another advantageous quality. They're hydrophobic, meaning they don't like water. Put DNA in solution with them and they naturally wrap up in the ribbonlike molecules to avoid contact with the watery surroundings.

In addition, nanotubes shrouded in DNA can take advantage of the natural process cells employ to open up a portal allowing DNA molecules, which hold the body's genetic code, to pass through their protective walls into their interiors.

Carbon nanotubes tend to take on qualities of whatever is on their surface, Strano said. So if the covering is biocompatible, the nanotubes generally are, too. That also should render them nontoxic.

The UI researchers used synthetic DNA whose sequence was designed to be good at attaching to carbon nanotubes and which proved to be good for detecting mercury as well.

Strano said those molecules can be reconfigured into different sequences to detect other substances, anthrax for instance.

In that case, the idea is to wrap a half strand of anthrax-compatible DNA around a nanotube. The strand will then join naturally with any anthrax DNA it contacts, which causes the nanotube to fluoresce differently.

"We get a wavelength shift," Strano said.

"You can wrap, really, any DNA sequence around a carbon nanotube," he said.

Strano said the technique also might be used to monitor processes inside cells, like the progress of chemotherapy treatment designed to attack tumor cells.

UI graduate student Esther Jeng and undergraduates Tsun-Kwan Yeung, Brittany Martinez, Anthonie Moll and Joseph Gastala also contributed to the Science paper. The research was funded by the National Science Foundation.

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