Credit: © 2007 Nature

Nanomechanical devices are able to weigh samples with extremely small masses when working in vacuum, but their performance deteriorates when they are operated in liquid environments because the motion of the cantilever is damped by the liquid. Four years ago, however, Thomas Burg and Scott Manalis of the Massachusetts Institute of Technology showed that this problem could be overcome by confining the liquid to channels inside the cantilever. Burg, Manalis and co-workers1 have now used this approach to weigh single nanoparticles, cells and proteins with a mass resolution of better than 1 femtogram.

Their devices have a U-shaped channel that runs from the fixed end of the cantilever to the free end and back again. As in a conventional mass sensor, the cantilever oscillates with a natural resonance frequency, which depends on its mass and other factors. In the new devices, the resonance frequency changes when the channel contains molecules or particles that do not have the same mass density as the fluid.

Masses can be determined by measuring how the resonance frequency decreases and then increases as a single nanoparticle passes through the channel. Alternatively, the channel can be coated so that the molecules being detected bind to the surface, which leads to a frequency shift that grows with time.