Nanotube Videotaped With Standard Microscopes & Cameras

Rice University scientists have used standard optical microscopes and video cameras to film individual carbon nanotubes. The movies show that nanotubes can be "plucked" by individual molecules of water and made to bend like guitar strings. "Nanotubes are fairly stiff, and when they are long enough, the bombardment by the surrounding water molecules makes them bend in harmonic shapes, just like the string of a guitar or a piano," said lead researcher Matteo Pasquali, associate professor of chemical and biomolecular engineering and chemistry, and co-director of Rice's Carbon Nanotechnology Laboratory.

Unlike the guitar string, a carbon nanotube is plucked randomly in many places at the same time. It cannot resonate like a guitar string because the nanotube has too little mass, and its vibrations die quickly because it's surrounded by viscous liquid. Nanotubes also tend to clump together, so to isolate individual tubes, Pasquali and doctoral student Rajat Duggal, now a research engineer at General Electric Co., put clumps of tubes into a mixture of water and a soap-like surfactant called sodium dodecyl sulphate, or SDS. When the nanotube clumps were broken apart with ultrasonic sound waves, the SDS surrounded the individual nanotubes and held them apart, in the same way laundry detergent surrounds and separates dirt particles in the wash.
To see individual nanotubes with a standard optical microscope, the researchers added a common red fluorescent dye that is used to stain cells. The dye, which attached itself to the SDS surrounding each nanotube, glows brightly enough to be seen with the naked eye under a microscope.

"I had been working on fluorescence visualization of DNA, and other students in the lab were working on nanotubes," Duggal recalled. "A colleague was disposing of nanotube suspensions after an experiment, and I asked them to spare me a vial so I could try them with an optical microscope. I thought of decorating the nanotubes with a fluorescent dye that would prefer to be with the SDS rather than the water, and when I looked under the microscope, I found bright dancing nanotubes."

Scientists have used electron microscopes to observe the underdamped vibrations of nanotubes in vacuum, but the Rice technique gives scientists the ability to see how nanotubes behave in liquids in real time.

The researchers videotaped dozens of nanotubes at 30 frames per second. A frame-by-frame analysis of the tapes revealed harmonic bending in several nanotubes that were 3-5 microns long and showed that the measured amplitude of the bending motion is consistent with earlier predictions of Rice materials scientist Boris Yakobson, professor of mechanical engineering and materials science and of chemistry.

The method works with other surfactants and it may be useful for life scientists who want to find out how nanotubes interact with cells, biomolecules, and other biological entities.

To view the video, visit www.rice.edu/media/nanotubevideo.html

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