WEIGHING SINGLE BACTERIA AND VIRUS PARTICLES
It has been known for many years that bacteria are on the order of 1000 nanometers in size and 1 picogram in weight. However, in addition to detecting microorganisms via nanotechnology it is now possible to weigh them individually.
The oscillation frequency of a diving board depends on the mass applied. Scaling down, it is possible to construct a cantilever of micrometer dimensions (approximately 6 microns long by 0.5 micron wide with an end platform about 1 micron square). The oscillation frequency can be measured by using a laser and observing the altered light reflection. Addition of single bacterial cells or even virus particles changes the oscillation frequency of the cantilever. The mass of single cells or virus particles has been measured this way in the laboratory of Harold Craighead at Cornell University (Fig. 7.7).
To hold the bacteria or viruses in place, the cantilever is coated with an antibody that recognizes the microorganism to be weighed. A single cell of Escherichia coli was 1430 × 730 nanometers in size and weighed 665 femtograms (665 × 10−15 grams). Viruses (weighing around 1 femtogram) can be detected by reducing the size of the cantilever and enclosing it in a vacuum. By mid-2005, this technique had been refined to weigh a single macromolecule—a double-stranded DNA of approximately 1500 base pairs (roughly the size of a typical coding sequence). Future developments should allow measurements of small proteins and other molecules in the zeptogram range (10−21 grams).