Nano Lett. doi:10.1021/nl101107u (2010)

A mechanical cantilever has a natural resonance frequency that decreases when mass is added to it. This phenomenon has been exploited to make nanomechanical devices that can measure the mass of samples with atomic resolution when working in vacuum. However, the performance of these mass sensors deteriorates when they are operated in liquid environments because the vibrations of the cantilever are damped by the liquid. The ability to measure samples in liquid is essential in many biological applications.

In 2002 Thomas Burg and Scott Manalis showed that this problem could be overcome by confining the liquid to channels inside the cantilever; four years later they went on to use this approach to weigh single nanoparticles, cells and proteins with a mass resolution of better than one femtogram. Now, by reducing the size of the cantilever and the channel inside it, they have improved the performance of their system further to achieve a mass resolution of 27 attograms.

They also exploited the centrifugal force caused by the vibrations to trap particles at the free end of the cantilever. This reduces the error caused by uncertainty in the position of the particle and also improves resolution by allowing measurements to be averaged over longer periods of time. The work was carried out at the Massachusetts Institute of Technology, the Max Planck Institute for Biophysical Chemistry, and Innovative Micro Technology.