Vibrating springboards have long been the darlings of nanomechanics wanting to measure the mass of small things.
Their thinking goes like this: a springboard vibrates at a specific resonant frequency that depends on its stiffness and mass. So you can work out the mass of anything that becomes stuck to the springboard by measuring any change in its resonant frequency.
Various groups have used this idea to detect all kinds of organic and inorganic molecules using springboards carved out of silicon.
But improving the sensitivity even further means reducing the mass of these springboards. The question is how.
The answer is provided today by Alex Zettl and his team at the University of California, Berkeley, who have created a springboard out of a single carbon nanotube. And their machine is one helluvan elegant device.
For starters, they exploit nanotubes’ unusual ability to act as radio transmitters to determine how fast it is vibrating. They zap the nanotube with radio waves and listen out for the radio signals it emits in return. This signal tells them how fast the nanotube is vibrating.
And because nanotubes are four orders of magnitude lighter than silicon cantilevers, they are four orders of magnitude more sensitive to mass.
All that adds up to device that is able to measure the mass of individual gold atoms as they settle on to its surface. “The sensitivity of our device is 0.40 Au atoms/√Hz. This is the lowest mass noise ever recorded for a nanomechanical resonator,” say Zettl and buddies.
And if that doesn’t impress you, how about this: their measurements were made at room temperature rather than in cryogenic conditions.
The team points out that their device works as a unique kind of mass spectrometer: it is compact, does not require powerful magnets and can easily be built into a chip. Expect to see more of them.
Ref: arxiv.org/abs/0809.2126: An Atomic-Resolution Nanomechanical Mass Sensor