Beyond 'nano' to 'atto' with a scale sensitive enough to weigh a virus
5 May 2004
News
Cornell University researchers already have been able to detect the mass of a single cell using submicroscopic devices. Now they are zeroing in on viruses. And the scale of their work is becoming so indescribably small that they have moved beyond the prefixes ‘nano’ ‘pico’ and ‘femto’ to ‘atto.’ And just in sight is ‘zepto.’
Members of the Cornell research group report they have used tiny oscillating cantilevers to detect masses as small as 6 attograms by noting the change an added mass produces in the frequency of vibration. Their submicroscopic devices, whose size is measured in nanometers, are called 'nanoelectromechanical' systems, or NEMS. But the masses they measure are now down to attograms. The mass of a small virus, for example, is around 10 attograms. An attogram is one-thousandth of a femtogram, which is one-thousandth of a picogram, which is one-thousandth of a nanogram, which is a billionth of a gram.
Picture shows a gold dot (±50 nm in diameter) fused to the end of a 4 µm long cantilevered oscillator. A one-molecule-thick layer of a sulphur-containing chemical deposited on the gold adds a mass of about 6 attograms – which is more than enough to measure
The work is an extension of earlier experiments that detected masses in the femtogram range, including a single E. coli bacterium, which recorded a mass of about 665 femtograms. For the latest experiments, the sensitivity of the measurement was increased by reducing the size of the NEMS cantilevers and enclosing them in a vacuum. Eventually, the researchers say, the technology could be used to detect and identify microorganisms and biological molecules.
The researchers manufactured the tiny cantilevers out of silicon and silicon nitride. The cantilevers can be set into motion by an applied electric field, or by hitting them with a laser. The frequency of vibration can be measured by shining a laser light on the device and observing changing reflection of the light. This frequency of vibration is, among other things, a function of mass. The tiny cantilevers vibrate at radio frequencies (1 to 15 MHz), and because they are so small to begin with, adding just a tiny bit more mass makes a measurable change in frequency.
For more information see The Craighead Research Group, www.hgc.cornell.edu
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