Scientists develop tiny
sensor that detects the smallest traces of poison gases
BY MARK SHWARTZ
Deadly gas leaks are an
everyday hazard of modern life.
Unfortunately,
conventional gas alarms often fail to detect the presence
of toxic fumes until it's too late.
But a team of Stanford
scientists has developed a gas detector that is a
thousand times more sensitive than any commercially
available device operating at room temperature.
Hongjie Dai, assistant
professor of chemistry, and Kyeongjae (KJ) Cho, assistant
professor of mechanical engineering, describe their
findings in the Jan. 28 issue of the journal Science.
The experimental sensor,
built in Dai's chemistry lab, uses carbon nanotubes only
one nanometer thick. A nanometer is one-billionth of a
meter -- about 50,000 times smaller than the width of a
human hair.
Researchers placed two
miniature metal pads at opposite ends of a nanotube,
creating a semiconductor capable of detecting tiny
changes in an electrical current when only a handful of
gas molecules are present.
This carbon
nanotube (in blue), just one nanometer in diameter, has
molecules of poisonous nitrogen dioxide gas (in violet,
at bottom) bonded at the bottom. Nanosensors can detect
poisonous gas at room temperature even if only a few gas
molecules are present in the atmosphere. A nanometer is
about 50,000 times smaller than the width of a human
hair.
Illustration
by Shu Peng
Scientists tested the
device using two common forms of noxious gas: ammonia and
nitrogen dioxide.
Ammonia (NH3), a
well-known ingredient in household cleaners, is widely
used in the manufacture of fertilizers and plastics.
Inhaling too much ammonia gas can be fatal. Nitrogen
dioxide (NO2) is a by-product of kerosene heaters, car
exhaust and tobacco smoke. The Environmental Protection
Agency warns that prolonged exposure to NO2 gas may cause
lung damage and increase respiratory infections in
children.
The experimental nanotube
sensor was able to instantly detect ammonia and nitrogen
dioxide molecules at levels of just 20 parts per million
(20 ppm), making the Stanford device a thousand times
more sensitive than conventional gas detectors.
Dai points out that, in
addition to its microscopic size, a nanotube sensor also
has the advantage of being able to operate at room
temperature, unlike commercial gas detectors which have
to heat up to 900o F (500o C) to function.
"This is really a new
kind of material for sensors," says Dai, adding that
the nanotube prototype soon may be tested for commercial
use. He also notes that his research team is working on a
device that will pick up traces of carbon monoxide and
other gases.
Cho believes that the
sensor will have other applications for detecting
biochemical weapons, land mines, air pollution and even
organic molecules in space.
Cho and Dai are part of
the Stanford Nanotechnology Research Group within the
Laboratory for Advanced Materials. The Science
article is co-authored by chemistry graduate students
Jing Kong and Nathan Franklin; postdoctoral fellow
Chongwu Zhou; mechanical engineering graduate student Shu
Peng; and materials science and engineering undergraduate
student Michael Chapline. SR
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