The idea of using mechanical resonators as mass sensors is an old one, and one that may be explained to a first-year physics undergrad. The (angular) frequency of a mass on a simple Hooke's Law spring is (k/m)0.5, where k is the spring constant. Change the mass, and the resonant frequency changes. With the development of micromachining techniques, there has been a great deal of interest in using tiny, high frequency resonators (e.g., doubly clamped Si beams) as mass sensors. One can have a metal wire along the resonator, and in the presence of a dc magnetic field perpendicular to the wire, an ac current may be used to apply a driving force to the structure. This is the same principle used to move the filament back and forth in those cheesy old flicker light bulbs. By measuring the induced voltage along the wire as it moves through the static magnetic field, the resonator's motion may be detected. Michael Roukes' group at Cal Tech been enthusiastic about the possibility of achieving sensitivities high enough to resolve a single atomic mass unit (1.66 x 10-27 kg).
There are many situations where one would love to have great mass detection capabilities in a liquid environment (e.g., to detect the binding of some cancer marker). The problem is, if you immerse a mechanical resonator in a liquid, viscous damping completely kills your sensitivity by damping the resonance. An old acquaintance of mine from graduate school, Scott Manalis at MIT, has come up with a solution to this problem. Don't put the resonator inside liquid; rather, put liquid inside the resonator. His group has been making mechanical resonators with micro (and now nano)fluidic flow channels inside them. In their latest work, they report a sensitivity of 30 attograms. I think this is very elegant, and a tour de force fabrication exercise.