Caltech-led team creates device to weigh molecules

Scientists have created a tiny measuring scale 300 times smaller than the width of a human hair that can weigh a single molecule at a time. The device may one day help doctors diagnose disease and illuminate the complex inner machinery of cells, its makers say.

An international team led by Caltech researchers built the device to measure the mass of large molecules that are difficult to analyze through conventional mass spectrometry methods. The scale features a long, bridge-like structure that vibrates at a specific frequency. When molecules are fired at the bridge in succession, they alter the frequency according to their weight.

"Think of it as a violin or a guitar string," senior author Michael Roukes said of the bridge-like resonator. "If you put a little blob of solder on it, the weight would make the frequency change, ever so slightly.... That's what we're measuring."

Roukes, an experimental physicist, explained that electrostatic forces are used to "strum" the resonator as molecules are flung at it. The resulting dips in the frequency are recorded in hertz, or vibrations per second, on a computer.

The scientists described their invention in last Sunday's issue of the journal Nature Nanotechnology and showed it could accurately weigh immunoglobulin M — an antibody produced by immune cells in the blood — and 5-nanometer gold particles. (A nanometer is one-billionth of a meter in length.)

The device, called a nanomechanical system (NEMS) resonator, fills a gap in what Roukes calls a no man's land in molecular measurement. The accuracy of conventional mass spectrometry begins to fade with larger, albeit still tiny, objects. The NEMS resonator can weigh those heftier items, which include proteins, air pollution particles and viruses with masses more than 500 kilodaltons — equivalent to half a million hydrogen atoms.

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