Phoenix, AZ--By combining electrochemistry with microscopy, researchers at Arizona State University have improved the detection of trace chemicals such as trinitrotoluene (TNT) and other chemicals important to national security, human health, and the environment.1
Spotting nanogram particles
Tao's team was able to detect and identify particles of TNT, each weighing less than a nanogram, 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," said Nongjian Tao, one of the researchers.
Tao's work involves the application of a hybrid technique called electrochemical imaging microscopy, developed in his lab. 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 noninvasive to the chemical system it explores, and provides a detailed map of the surface under study, revealing the chemicals present at every location.
In addition to spotting TNT particles, his group is using electrochemical imaging microscopy to monitor the activities of living cells, as well as to detect protein biomarkers (which 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.
Seeing electrochemical reactions
Tao notes that surface plasmons on an electrode are 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 change in light reflection (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 layer of protein that blocked the flow of electrochemical current, whereas the grooves allowed current to flow, providing the contrast to reveal the fingerprint 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. The technique could successfully detect the grains of TNT even if they were mixed with other species of particles, including traces of dust, airborne particulate matter, or wax.
1. Xiaonan Shan et al., Science, Vol. 327, p. 1363, March 12, 2010.