A new mass sensor to weight atoms with an unprecedented resolution

The CIN2 (Research Center for Nanoscience and Nanotechnology), is a joint centre belonging to the Spanish National Council for Scientific Research (CSIC) and the Nanotechnology Catalonian Institute (ICN).

The device is based on a carbon nanotube of 1 nanometer diameter which is clamped at both ends to two electrodes. It works as an electromechanical resonator characterized by a mechanical resonance frequency as if it was a string on a guitar. When atoms are directed towards the nanotube, they hit and stick to its surface. This increases the nanotube mass, thereby reducing its resonance frequency: this slowing of the vibration is used to quantify the mass of the atoms.

At room temperature, the nanotube resonator has a resolution of 25 zeptograms (zg) but cooling the nanotube down to 5 Kelvin (268.15 degrees C below zero) the resolution improves to 1.4 zeptograms. A zeptogram equals 10 -21 grams or, which is the same, a thousandth part of one millionth of one millionth of one millionth of a gram.

A sensor of this resolution would allow the detection of tiny amounts of mass such as the mass of proteins or other molecules with atomic resolution. Also, it could be used to monitor nuclear reactions in individual atoms, or biological molecules in chemical reactions.

The researchers tested the device by measuring the mass of evaporated chromium atoms, and the performance, as explained in an article published in the journal Nanoletters, is exceptional. The other members of the team are Benjamin Lassagne and Daniel Garcia, both of CIN2, and Albert Aguasca, from the Universitat Politècnica de Catalunya.

A remaining challenge

One of the challenges of nanotechnology and nanomechanics is having a mass spectrometer working at subatomic level. The maximum resolution had been achieved with some silicon resonators (with a resolution of about 7 to zeptograms temperature of 4.2 Kelvin). Now, the work of Bachtold and co-workers has substantially increased that resolution through the use of carbon nanotubes.

The mass of a nanotube is very low, barely a few atograms (which is a millionth of one millionth of a microgram, or 10 -18 g), so that any tiny amount of added mass will be detected. In addition, the nanotubes are mechanically ultrarigid, which makes them excellent candidates to be used as mechanical resonators.

Now, the team of Bachtold is improving the measurement set up and hopes to achieve in the near future the resolution of 0.001 zg, the mass of one nucleus. The researchers will then place proteins on the nanotube and monitor the change of the mass during chemical reactions (when a hydrogen atom is released from the protein, for instance).

Nanotechnology has been advancing rapidly in the few last years. Even so, there remain many challenges ahead, and one of them is a mass spectrometer to allow work at that level, with small biological molecules or atoms.

The development of the CIN2 team has coincided in time with others of similar characteristics, both from the U.S.A. One, at the Technical University of California (Caltech) and the other at the University of California (Berkeley). Both groups have developed mass sensors based on carbon nanotubes, with minor differences between the methods used. The fact was recently highlighted in the journal Nature Nanotechnology.