Rice University scientists have discovered a way to use standard optical microscopes and video cameras to film individual carbon nanotubes, tiny cylinders of carbon no wider than a strand of DNA. 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, co-director of Rice’s Carbon Nanotechnology Laboratory.
Pasquali said the analogy with stringed instruments doesn’t completely fit with the nanoscale world. Unlike the guitar string, for example, the carbon nanotube is plucked randomly in many places at the same time. Also, it cannot resonate like the guitar string because the nanotube has too little mass, and its vibrations die quickly because it’s surrounded by viscous liquid.
So it’s like a guitar string plucked being played by a typical kid at Guitar Center, except underwater.
Carbon nanotubes are hollow, hair-like strands of pure carbon that are 100 times stronger than steel but weigh only one sixth as much. Nanotubes are one nanometer, or one billionth of a meter, wide. Human hair, by comparison, is about 80,000 nanometers wide.
To isolate individual tubes, Pasquali and doctoral student Rajat Duggal put clumps of tubes into a mixture of water and a soap-like surfactant called sodium dodecyl sulphate, or SDS. In order to see individual nanotubes with a standard optical microscope, like those found in most biological laboratories, Pasquali and Duggal added a red fluorescent dye that’s often 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 – to my delight – I found bright dancing nanotubes.”Pasquali and Duggal 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.
“Our method doesn’t provide the sensitivity or precision you get with the infrared, single-nanotube imaging methods developed last year by Rice chemist Bruce Weisman and doctoral student Dmitri Tsyboulski, but the equipment we need is less expensive,” Pasquali said. “It’s akin to the difference between playing a Stadivarius and playing a common violin.”
The video is available at the Rice site.
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