Sensors that contain active elements that
are around the same size as the molecules to be detected are potentially
portable, low-power and quick. Such sensors could be deployed as networks
of security and environmental monitoring devices.
Researchers from the Naval Research Laboratory have made single-walled
carbon nanotube chemical sensors that transmit information by measuring
the charge in the nanotubes' capacitance, or ability to store electric
charge. Carbon nanotubes are rolled-up sheets of carbon atoms that have
useful electrical properties and can be narrower than a nanometer, or
one millionth of a millimeter.
The researchers built a prototype that detects a chemical that
simulates the nerve gas sarin within a few seconds. Arrays of the sensors
could eventually be used to detect a range of chemical vapors including
volatile organic compounds and semivolatile nerve agents, according to
The sensors consist of a layer of nanotubes that are surrounded
by single-molecule layers of substances that selectively bind to a specific
biological molecule or chemical. When a target chemical comes in contact
with the single layer of molecules coating a nanotube, it is adsorbed,
or bonded to the surface. Electricity applied to the nanotubes produces
a strong electric field at the surface of the tubes. This causes adsorbed
molecules to become electrically polarized, or aligned, which increases
the nanotubes' capacitance.
Most carbon nanotubes chemical sensor prototypes measure changes
in the nanotubes' electrical resistance. Capacitance signals are less
noisy, making for sensors that are faster and more sensitive, according
to the researchers.
The researchers' prototype detects the sarin simulant in concentrations
as low as 50 parts per billion within four seconds. Their previous sensor
took over a minute to detect the gas.
The capacitance-based detector could be used practically in one
to three years, according to the researchers. The work appeared in the
March 24, 2005 issue of Science (Chemical Detection with a Single-Walled
Carbon Nanotube Capacitor).
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