Researchers break sensitivity record for mass measurement
By Brian Dodson
May 20, 2012
A sprocket of research engineers (yes, apparently that's the collective noun for a group of engineers) at the Catalan Institute of Technology (CIT) has succeeded in breaking the record for sensitivity of mass measurement. By measuring the resonant frequency of a short length of single carbon nanotube, masses as small as a single nucleon (proton or neutron), having a mass of about 1.7 yoctogram (1 yg = 10^-24 grams) were measured, thereby exhibiting a level of sensitivity several orders of magnitude better than previous devices. This new technology enables the detection and identification of individual atoms and molecules and tracing the fate of individual atoms in a chemical reaction.
The mass sensor technology is based on development of nanoelectromechanical systems (NEMS) which oscillate at a very precise frequency through electromechanical interaction of an electronic oscillator and a mechanical resonator made up of a single carbon nanotube. As seen in the figure, the oscillating nanotube is suspended between the drain and source connections of a field-effect transistor (FET).
This introduces a time-variable capacitance in parallel with the FET, which causes the gain of the FET to vary in the same manner. External electronics feed signals which pass through the FET back to the FET input, thereby making an electromechanical oscillator which has the extremely narrow resonance of a light, strong tube. The signal from the oscillator depends on both the rate and magnitude of the mechanical oscillations.
The CIT mass sensor comprises a nanotube resonator 150 nm in length and 2 nm in diameter, which oscillates at about 2 GHz. The mass of the central moving portion of such a nanotube is about 100000 yg. Materials (molecules, atoms, etc.) which bind to the nanotube lower the resonant frequency in proportion to their mass. Being able to detect the difference in mass between the nanotube and the nanotube with an atom of hydrogen (1.66 yg) on it requires detection of a frequency change of about 35 kHz. To do so the NEMS oscillator must have a very narrow and low noise resonance.
The previous record was held by an NEMS oscillator with a very similar structure. What allowed the CIT team to attain one hundred times better sensitivity was finding a way to drastically reduce the noise level of the oscillating nanotube. By sending a "large" current (~8 microamperes) through the nanotube for 5 minutes while maintaining a vacuum of 3x10^-11 Torr (about 2x10^-14 smaller than atmospheric pressure), the noise level plummeted.
Apparently there were some atoms absorbed on the surface whose bonding was sufficiently weak that they could move onto alternate binding sites on the nanotube surface. This produced a variable loading on the oscillator which in turn considerably broadens the resonance of the oscillator, so that detection of very small mass differences becomes quite challenging.
The researchers used their nanobalance to detect single naphthalene molecules and small numbers of xenon atoms. Analyzing this data showed that the NEMS mass sensor has a resolution of 1.7 yoctograms, roughly the mass of a single nucleon. This means that the mass sensor is capable of distinguishing isotopes of the same element. The new NEMS sensor could also tell apart compounds differing by only a few protons. This would form a new probe into real-time chemical reaction dynamics.
“The record mass sensitivity of our device is related to its tiny size, but the nanoworld in general, and nanoresonators in particular, is ultrasensitive to small masses, forces, charges and magnetic moments,” said Dr. Julien Chaste, one of the developers of the new NEMS device. “As well as mass detection, nanoresonators operating at ultralow temperatures might also be very interesting for fundamental studies in quantum physics.”
Clearly, with such apparatus it is only a matter of time before experimenters are able to put the mechanics back into quantum mechanics, possibly leading to fundamental new ideas.