To make the device sensitive to smaller masses, the researchers had to shrink the size of the cantilever, which behaves much like a diving board, Olcum says. When a diver bounces at the end of a diving board, it vibrates with a very large amplitude and low frequency. When the diver plunges into the water, the board begins to vibrate much faster because the total mass of the board has dropped considerably.
To measure smaller masses, a smaller "diving board" is required. "If you're measuring nanoparticles with a large cantilever, it's like having a huge diving board with a tiny fly on it. When the fly jumps off, you don't notice any difference. That's why we had to make very tiny diving boards," Olcum says.
In a previous study, researchers in Manalis' lab built a 50-micron cantilever about one-tenth the size of the cantilever used for measuring cells. That system, known as a suspended nanochannel resonator (SNR), was able to weigh particles as light as 77 attograms at a rate of a particle or two per second.
The cantilever in the new version of the SNR device is 22.5 microns long, and the channel that runs across it is 1 micron wide and 400 nanometers deep. This miniaturization makes the system more sensitive because it increases the cantilever's vibration frequency. At higher frequencies, the cantilever is more responsive to smaller changes in mass.
The researchers got another boost in resolution by switching the source for the cantilever's vibration from an electrostatic to a piezoelectric excitation, which produces a larger amplitude and, in turn, decreases the impact of spurious vibrations that interfere with the signal they are trying to measure.
|Contact: Sarah McDonnell|
Massachusetts Institute of Technology