Nanotubes threaten to replace the mass spectrometer

While the device is currently nowhere near as accurate as a conventional mass spectrometer, it has the potential to become an integral part of future lab-on-a-chip analysis devices and enable direct mass measurement of molecules present.

The nanomechanical mass sensor was made from a single double-walled carbon nanotube cantilever attached to an electrode and was described in a paper appearing in the current edition of Nature Nanotechnology​ by Professor Alex Zettl's group at the University of California, Berkeley.

Mechanical resonators have become widely used to detect small quantities of materials absorbed on their surface by measuring changes in either the cantilever's deflection or oscillation frequency.

Such cantilever-based 'inertial balances' are finding use in MEMS (micro-electro-mechanical systems) to accurately determine force, position and masses of various objects, including detecting the presence of specific types of cells such as the CD4 cells used to follow the progression of AIDS.

The device was made by attaching a single, double-walled carbon nanotube to a negatively charged electrode, with the other end free to vibrate 'like the tip of a diving board' past a positively charged electrode.

Electrons can flow from the tip of the nanotube to the positive electrode and the size of the current depends on the frequency at which the nanotube is vibrating.

Most 'conventional' cantilever-based mass measuring devices have been made from silicon and are much heavier and thicker than the carbon nanotube which measures about 2nm in diameter by 250 nm in length.

The minute size of the nanotube makes the cantilever much more sensitive to the very small changes in the mass of the cantilever.

To test the sensitivity of the device, the researchers introduced a small number of gold atoms into a vacuum chamber containing the device knowing that some of them would stick to the nanotube.

Of course, exactly where each gold atom would stick is a random process, leading to irregular changes in the resonating frequency. However, using statistical analysis methods the team calculated the mass of a single gold atom to weigh 0.29 zg (zeptograms, or 10-24 kg) +/- 0.05 zg.

This compares favourably to the accepted mass of a gold atom of 0.327 zg, as measured by a conventional mass spectrometer.

The team speculates that the device could be used to study large and fragile (bio) molecules that would fragment inside a mass spectrometer.

Last October, the group used a similar carbon nanotube to build the world's smallest radio - a hundred billion times smaller than the first commercial radio sets.

The small size of the device is in part due to the carbon nanotube acting as an all-in-one antenna, tuner, amplifier and demodulator, which are all separate components in a standard radio.