Rice videos twanging nanotubes

Rice University scientists have found a simple way to observe carbon nanotubes under standard optical microscopes and have filmed them vibrating harmonically under bombardment by water molecules.

"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.

Nanotubes tend to clump together. 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. When the nanotube clumps were broken apart with ultrasonic sound waves, the SDS surrounded the individual nanotubes and held them apart.

In order to see individual nanotubes with a standard optical microscope, Pasquali and Duggal added a common red fluorescent dye 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.

Duggal said scientists have used electron microscopes to observe the undamped vibrations of nanotubes in vacuum, but his and Pasquali's technique gives scientists the ability to see how nanotubes behave in liquids in real time.

Pasquali and Duggal videotaped 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.

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