I Think I Gained a Few Zeptograms
By Davide Castelvecchi
Here’s the latest feat of the Caltech nanotech wizards: weighing single molecules.
The new invention is good enough to measure the mass of single proteins, with the ridiculous precision of one zeptogram. (There are one billion trillion zeptograms in one gram.) No ordinary scale can do that, so the physicists had to resort to a clever trick: the fact that a vibrating beam will vibrate slower if you attach a weight to it — think of a metronome with or without the weight on its rod. One of the beauties of this method is that it really measures mass, as opposed to weight, meaning that it will give the right response on the moon (where gravity is weaker), or even in orbit.
In a way that’s similar to how Intel makes Pentiums, the Michael Roukes and his team etched tiny bridges on a chip. Drop a molecule onto the beam, and if it sticks, it will slow down the beam’s vibrations just a bit. Astute physicists can then read change in frequency — and estimate the molecule’s mass — by running an electric current through the beam, kind of like the way an electric guitar works.
Just as shorter guitar strings have higher pitches, a very short silicon bridge will vibrate at incredibly high frequencies — up to hundreds of Megahertz. By making the beam shorter — down to one-hundredth of a millimeter — and stiffer, the team increased its vibration frequency, improving sensitivity by a factor 1,000 over a previous model.
To really be able to tell the difference between different molecules of comparable weight, though, the physicists may have to make their gadget another 1,000 times more sensitive, down to a unit of mass that goes by the funny name of yachtogram. That’s roughly the mass of the lightest kind of atom, hydrogen. Such precision could help for example in the early diagnosis of diseases, by detecting the presence of tell-tale proteins in a person’s blood.
Roukes made his announcement at the March Meeting of the American Physical Society in L.A., where I was helping out in the AIP-run press room.