Nanotube chemical sensor gains speed

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 researchers.

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).