April 4, 2012
World’s Most Sensitive Nanomechanical Sensor Revealed
A team of physicists in Barcelona, Spain have for the first time fabricated and tested the world´s most sensitive Nanomechanical sensor, capable of detecting changes in mass of 1.7 yoctograms -- roughly the mass of a single proton.
Team leader Professor Adrian Bachtold, of Catalan Institute of Technology´s Quantum NanoElectronics Group, and colleagues used the sensor, which is similar to a guitar string vibrating at very high frequencies (2 GHz), to compare the resonating frequency of the nanotube before and after additional mass was bound to its surface. Using this technique allowed the team to quantify the added mass.Weighing single protons is no simple task. A scale must be accurate enough to measure a yoctogram, which is just one septillionth (0.000000000000000000000001) of a gram. Until now, the smallest mass measured was 100 yoctograms, or about a tenth of a zeptogram.
Bachtold´s work, published today in Nature Nanotechnology, used low temperatures in making the measurements, as it was deemed best for measuring frequency. Although the equipment was placed in a vacuum to minimize interference from other atoms, Bachtold removed any stray atoms by temporarily turning up the heat on the nanotube to disrupt any bonds to atoms. After which, the sensor was able to weigh an atom of xenon to the nearest yoctogram.
Bachtold´s nanotube, typical of nanomechanical resonators, was driven with an alternating electric current. When extra mass was added to the tube, the resonant frequency changed correspondingly -- adsorption of atoms and molecules on the tube´s surface decreased the resonant frequency.
“The yoctogram mass sensitivity achieved by the Catalan team is certainly spectacular - the challenge ahead will be to routinely manufacture nanotube sensors at low cost,” Rachel McKendry, a nanoscientist at University College London, told New Scientist.
The team was able to measure the binding energy of the xenon to the tube by determining how much the resonant frequency changed as the atoms departed from the surface. After comparing their results with what is known about xenon binding to graphite surfaces, they found that binding energy depends in part on the diameter of the nanotube.
A nanotube -- a single sheet of graphite rolled into cylinders -- has great potential for measuring very small masses, allowing for extremely high precision in mass spectroscopy. But it´s not only the mass; the precise location of the adsorbed particle also alters the resonant frequency. This means that, if we can get an impurity to a specific location on the nanotube, its mass may be measured directly, which has exciting implications for future research.
Bachtold and colleagues said they plan to do future work to create a single “trapping site” on the nanotube, which should enable improved mass measurements by reducing fluctuations in the nanotube´s resonant frequency.
They also hope their research can lead to diagnosis of health conditions by identifying proton-scale differences in molecular mass that are markers for disease.
It could also have major implications in fields such as mass spectrometry, magnetometry, nanometrology and surface science.