Researchers from the MIT Media Lab report in Nature that they have weighed single cells and large molecules using a fluidics-based microcantilever device (left). Previously, others weighed a single viral particle, with a mass of 9.5 femtograms (1 femtogram = 10-15 grams), and a cluster of xenon atoms with a mass of 7×10-21 grams. These techniques involved measuring the changes in resonance of the microcantilevers. But weighing particles suspended in a solution posed a problem, because the viscosity of the fluid decreases this resonance, making the measurements far less sensitive.
To overcome this, Scott Manalis and his colleagues filled the resonator with fluid, and placed it inside a vacuum-sealed silicon microcantilever. The device can be used to weigh tiny particles in two ways. One of them, which is suitable for weighing biological molecules, involves running a solution of the molecules through the fluid-filled channels inside the device; individual molecules are adsorbed onto the inner surface of the channels, which are coated with receptors for the molecule being weighed, causing a change in the resonance frequency. The other mode involves the detection of particles as they move through the channels. This mode is well suited to weighing particles in a dilute solution; the MIT group used it to determine the distribution of mass of two types of bacterial cell, weighing 115 and 150 femtograms, to a resolution of 1 femtogram.
These technologies will eventually find important applications in medical diagnostics. With further advances, the ability to detect or weigh single molecules and cells will, for example, enable the design of inexpensive, portable devices that can sense pathogens such as viral particles, bacteria or toxins in blood samples. The channels in the device are too small for large cells to flow through them, but in the future, devices with wider channels could be designed. Such a device could be used to measure the mass and numbers of CD4 cells, the immune cells which are counted to monitor the progression of AIDS.
Burg, T. P., et al. (2007). Weighing of biomolecules, single cells and single nanoparticles in fluid. Nature 446: 1066-1069. [Abstract]