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).
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