Porous photonic crystal microsensor particles on the ends of optical fibers can detect organic pollutants. Photo: Brian King, UCSD Chemistry and Biochemistry. |
A new kind
of sensor could warn emergency workers when carbon filters in the respirators
they wear to avoid inhaling toxic fumes have become dangerously saturated.
In a
recent issue of Advanced Materials, a team of researchers from the Univ. of California,
San Diego and
Tyco Electronics describe how they made the carbon nanostructures and
demonstrate their potential use as microsensors for volatile organic compounds.
First
responders protect themselves from such vapors, whose composition is often
unknown, by breathing through a canister filled with activated charcoal—a gas
mask.
Airborne
toxins stick to the carbon in the filter, trapping the dangerous materials.
As the
filters become saturated, chemicals will begin to pass through. The respirator
can then do more harm than good by providing an illusion of safety. But there
is no easy way to determine when the filter is spent. Current safety protocols
base the timing of filter changes on how long the user has worn the mask.
“The new
sensors would provide a more accurate reading of how much material the carbon
in the filters has actually absorbed,” said team leader Michael Sailor,
professor of chemistry and biochemistry and bioengineering at UC San Diego. “Because these carbon nanofibers have the same chemical properties as the
activated charcoal used in respirators, they have a similar ability to
Repeating bands of greater density give this bundle of carbon nanofiber photonic crystals a characteristic color. When the porous fibers absorb chemicals, they change color, making the material a sensitive optical sensor for (volatile organic compounds). Image: Timothy Kelly, UCSD Chemistry and Biochemistry. |
absorb
organic pollutants.”
Sailor’s
team assembled the nanofibers into repeating structures called photonic
crystals that reflect specific wavelengths of light.
The
sensors are an iridescent color too, rather than black like ordinary carbon.
That color changes when the fibers absorb toxins—a visible indication of their
capacity for absorbing additional chemicals.
The agency
that certifies respirators in the U.S., the National Institute of
Occupational Safety and Health, has long sought such a sensor but the design
requirements for a tiny, sensitive, inexpensive device that requires little
power, have proved difficult to meet.
The materials that the team fabricated are very
thin—less than half the width of a human hair. Sailor’s group has previously
placed similar photonic sensors on the tips of optical fibers less than a
millimeter across and shown that they can be inserted into respirator
cartridges. And the crystals are sensitive enough to detect chemicals such as
toluene at concentrations as low as one part per million.
