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Arizona State Univeristy scientist N.J. Tao and his colleagues at the
Biodesign Institute have hit on a new, versatile method to
significantly improve the detection of trace chemicals important in such
areas as national security, human health and the environment.
Tao's team was able to detect and identify tiny particles of the
explosive trinitrotoluene or TNT—each weighing less than a billionth of a
gram—on the ridges and canals of a fingerprint. "We can easily detect
the TNT traces because we combine the strength of optical microscopy,
which gives spatial resolution, with the high sensitivity and
selectivity of electrochemical detection," he said. Results of this
research appear in the March 12 issue of Science.
Tao's work involves the application of a hybrid technique—called
electrochemical imaging microscopy—developed in his lab. "We don't use
electrochemistry alone," said Tao, director of Biodesign's Center for
Bioelectronics and Biosensors and electrical engineering professor in
the Ira A. Fulton Schools of Engineering. "We combine electrochemical
sensing with other techniques, including optical detection."
The technique has several advantages over more conventional methods
of detection, and is a more powerful tool than either optical or
electrochemical sensing alone. It is rapid and non-invasive to the
chemical system it explores, and provides a detailed map of the surface
under study, revealing the chemicals present at every location.
Although Tao's published results highlight the power of
electrochemical imaging microscopy to uncover explosive residues, he
notes that the method can be usefully applied to a full assortment of
detection applications. His group is currently using electrochemical
imaging microscopy to monitor the activities of living cells, as well as
to detect protein biomarkers—early warning beacons that can alert
clinicians to pre-symptomatic signs of disease. This could offer
improved speed and a lower cost for biomarker discovery when compared
with current microarray approaches. Other potential uses include
detection of heavy metal ions in drinking water.
The technique dispenses with the traditional microelectrode used for
chemical sensing. "The key idea here," Tao explains, "is to convert
an optical signal into local electrochemical current." This is
accomplished thanks to a phenomenon known as surface plasmon resonance.
In an electrode—a metal conductor through which electric current is
passed—electrons move freely and oscillate in a wavelike fashion called a
plasmon. Shining light on the surface plasmon causes the electrons to
absorb energy and enter an excited state. Tao notes that the plasmon is
exquisitely sensitive to any changes occurring near the electrode's
surface. If, for example, an electrochemical reaction involving
oxidation or reduction takes place (where electrons are lost or gained,
respectively), the plasmon registers this change as a reflection of
light (electrochemical current can be inferred from the changes in
optical signals detected). The technique allows for the resolution of
trace chemicals down to a small fraction of a micron in diameter.
The TNT experiments were carried out by first depositing a
fingerprint on the surface of an electrode. The raised ridges of the
fingerprint formed a delicate layer of protein that blocked the flow of
electrochemical current, whereas the grooves allowed current to flow,
providing the contrast to reveal the fingerprint in vivid relief when an
electrical potential was applied.
Next, the applied potential was lowered to correspond to the
specific reduction potential of TNT, at which point spots of the
explosive particles appeared, providing both visual and chemical
confirmation. Remarkably, the technique could successfully detect the
grains of TNT, even if they were mixed with other species of particles,
including traces of dust, airborn particulate matter or wax.
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