A tiny springboard constructed from a carbon nanotube can weigh individual atoms as they fall onto its surface. The device could replace high-resolution mass spectrometers, which tend to destroy the samples they weigh.
Resonators, materials that naturally oscillate strongly at certain frequencies, help to enhance the sound of many musical instruments.
But physicists also take advantage of resonators to calculate tiny masses. When extra mass lands on the surface of a resonator, it alters the frequency of the resonator, which gives physicists a means to calculate the extra mass.
Existing mass sensors of this kind, however, are constructed from relatively dense materials, such as quartz.
When atoms, which generally have a mass under a zeptogram (a trillionth of a billionth of a gram) land on the quartz, they are too small to make any impression on its vibration frequency. To weigh individual atoms, physicists need a resonator of a much lower density.
Kenneth Jensen, Kwanpyo Kim and Alex Zettl at the University of California in Berkeley have discovered that carbon nanotubes are perfect for the task.
Because nanotubes are hollow, they have a mass four orders of magnitude lower than specially built micromachined resonators. That brings their mass into the attogram range (a billionth of a billionth of a gram), and means they respond to single atoms.
Jensen’s team took a multi-walled carbon nanotube roughly 250 nanometres in length and attached one end to an electrode while leaving the other end free, forming a tiny springboard.
They placed the device in an ultra-high-vacuum chamber and broadcast a radio signal to the nanotube, causing it to resonate. Next they added a few gold atoms to the vacuum chamber. As the atoms landed on the nanotube “springboard”, they altered its mass enough to change the vibration frequency.
The researchers calculated that the change was due to just 51 gold atoms, demonstrating that the device is sensitive enough to work on the atomic scale.
Further analysis suggested that the nanotube could respond to a change in mass as small as 0.13 zeptograms – more then sensitive enough to register individual gold atoms.
But measuring the exact change of mass is complicated, says Zettl. Just as a springboard bends further as the diver approaches the far end, so the carbon nanotube’s response to extra mass depends on where the atom lands along the nanotube’s length.
“The analysis would be much simpler if all the atoms landed on just one part of the nanotube,” Zettl says. “But we did not force this to happen.” Instead, they used a statistical analysis of the nanotube’s response to the gold. “Knowing that all gold atoms have the same mass, and assuming that they arrive randomly in time and land randomly along the nanotube lets us do the statistical analysis,” he says.
Zettl thinks that useable sensor platforms like this are an “exciting breakthrough” for the nanoelectromechanical systems field. “This device is more than a novelty,” he says.
Journal reference: Nature Nanotechnology (DOI: 10.1038/nnano.2008.200)