Zeptogram Mass Detection---Weighing Molecules

Michael Roukes and his Caltech colleagues produce some of the finest nanoscopic electromechanical systems (NEMS) devices in the world. His latest achievement is performing mass measurements with nearly zeptogram (zg) sensitivity, that is, with an uncertainty of only a few times 10^-21 grams. At this level you can start to weigh molecules one at a time. In experiments, the presence of xenon accretions of only about 30 atoms (7 zg, or about 4 kilodaltons, or the same as for a small protein) have been detected in real time.

Minuscule masses are measured through their effect on an oscillating doubly clamped silicon carbide beam, which serves as the frequency-determining element in a tuned circuit. So, in practice, the beam would be set to vibrating at a rate of more than 100 MHz and then would be exposed to a faint puff of biomolecules. Each molecule would strike the beam, where its presence (and its mass) would show up as a changed resonant frequency.

After a short sampling time, the molecule would be removed and another brought in. Through this kind of miniaturization and automation, the NEMS approach to mass spectroscopy could change the way bioengineering approaches its task, especially in the search for cancer and its causes. The Roukes (roukes@caltech.edu, 626-395-2916) group reported its findings at last week’s meeting of the American Physical Society (APS) in Los Angeles.