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Color-Changing Array Identifies Odors
CHAMPAIGN, Ill. -- For years scientists have
been attempting to emulate a pretty good chemical detector: the
human nose. Now an array of chemical sensors may provide a
straightforward process for identifying smells that is far more
sensitive.
The array consists of
metalloporphyrins deposited on a silica gel. The metalloporphyrins
are produced by attaching a metal atom, such as tin, cobalt or iron,
to a tetraphenylporphyrinate molecule. The different molecules
respond differently to the presence of chemical analytes.
 
A
chemical array developed at the University of Illinois sees smells.
Odor molecules create different patterns on the array. The array
detects the smell of overripe fruit, left, and rum.
Specifically, the metalloporphyrins change
color in a phenomenon that the researchers call "smell-seeing."
"Smell-seeing is just so simple and elegant," said Kenneth S.
Suslick, a professor of chemistry and leader of the research team at
the University of Illinois. "The same approach could have been done
50 years ago with film and filters, but no one ever thought of it."
To generate the color patterns, the
researchers inserted the array into a stainless steel cell with a
mixture of chemicals and nitrogen gas. They recorded the color
patterns that emerged with an ordinary flatbed scanner and, using
Adobe PhotoShop software, stretched the color scales and subtracted
the baseline image from the images in the presence of various
chemical compounds.
Suslick and graduate
student Neal Rakow found that each chemical creates a unique color
signature. They also found that, because the metalloporphyrins do
not display hysteresis effects, subsequent exposures do not
compromise the repeatability of using the technique with an array.
"The aroma molecules bind to the metal centers, but the binding is
reversible, and they will come off the surface in a minute or so,"
said Suslick.
Moreover, he noted that, while
other processes under development to sense smells are typically
sensitive to humidity, the smell-seeing process is not. And it is 10
to 100 times more sensitive than the human nose for some compounds.
The smell of success
In the first
arrays, the metalloporphyrins reacted in about 30 seconds because
they covered a large surface area. A smaller version, with 500-µm
metalloporphyrin spots on a Teflon substrate, demonstrated much
faster color saturation. Suslick will continue to miniaturize the
detector array and hopes to match it with a CCD. With dedicated
custom software, he expects to produce an odor sensor the size of a
Sacajawea dollar.
Suslick said that dosimeter
badges made with the technology may be useful for monitoring toxin
exposure in the workplace or serving as early warning systems
against poisonous gas. Other applications may include detecting
spoiled food or performing medical diagnoses by the "smell" of
bacteria. Another possible application, said Suslick: "Burnt popcorn
prevention for all of our microwaves!" 
Richard Gaughan
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