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From NY Times:
Say you've got a few atoms of gold or another element and you want to weigh them (or, as a scientist would put it, determine their mass). There's no scale in the world sensitive enough to do the job, but you could use a mass spectrometer. That involves stripping electrons off the atoms and sending the resulting ions through a magnetic field.
Physicists at the University of California, Berkeley, have come up with what may well be a better way. They have developed a nanomechanical sensor — a cantilevered carbon nanotube that sways like a diving board. And just as a diving board is affected by the weight of the diver, the nanotube's vibrations change when gold or other atoms are stuck to it. By measuring the changes, researchers can calculate the mass of a single atom.
The key to the sensor's sensitivity is its extremely small size, said Kenny Jensen, a doctoral student who describes the device in a paper in Nature Nanotechnology. (His co-authors are Alex Zettl, director of the university's Center of Integrated Nanomechanical Systems, and Kwanpyo Kim.) The nanotube is only about a billionth of a meter in diameter and 200 billionths of a meter long. "It's the ideal material," Mr. Jensen said.
A nanoscale diving board sounds simple, but there were many hurdles to overcome, not the least being how to measure the vibrational frequency. The researchers ended up measuring it electrically — sending a radio signal to the nanotube and listening for the vibrations, as they put it in the paper.
Dr. Zettl said sensors like this had been the subject of much research. "The holy grail has been can you get down to the molecular or even atomic level, and, in your wildest dreams, can you do it at room temperature," he said. His lab's device accomplishes this and, once it is fully developed, may be particularly useful for measuring large molecules like proteins, which don't fare well in mass spectrometry.
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