August 27, 2001

Blue Is for Biohazard

A novel sensor uses liquid crystals to detect environmental contamination.

By Alan Leo

A chemical sensor that changes color in the presence of toxins may one day save lives on the battlefield and in hazardous industries, report researchers in the August 17 issue of Science magazine.

"It's a direct visual indication, and it requires no electric power," Nicholas Abbott, the study's lead investigator, told technologyreview.com. "It's sufficiently simple that it might find use in personal monitoring."

Get in the Groove

Abbott's device consists of three components: a bottom layer of gold patterned with nanoscale grooves, receptor molecules embedded in that layer, and a top layer of liquid crystals attached to the receptors by a loose hydrogen bond. This bond controls the liquid crystals' orientation, which determines their appearance to the human eye.

When the sensor encounters a target compound (a toxic gas, for example), that compound forms a strong bond with the receptor molecules, breaking the liquid crystals' weak hydrogen bonds. The suddenly free liquid crystals orient themselves along the grooves in the gold layer, which changes the sensor's appearance. When the target compound is no longer present, the liquid crystals reattach to the receptors and the sensor's appearance returns to normal.

In their experiment, Abbott and his colleagues used carboxylic acid groups for receptors. The acid groups formed a weak bond with the liquid crystal layer, but a strong bond with the poisonous target compound, hexylamine.

This approach, called competitive binding, helps the device avoid false positives. The sensor distinguished the target compound from alcohol, hexanes, water vapor and other chemicals. Some experimental detectors, Abbott says, have an especially hard time distinguishing their targets from water, a shortcoming that limits them to the laboratory.

"The Abbott system is very exciting work for detection of vapor phase (gas) toxins," says Jerome Workman Jr., a senior research fellow at Kimberly-Clark and an expert in analytic devices.

Fast, Cheap and Under Control

Today, the most common field instrument for detecting environmental toxins is the portable gas chromatograph, which costs thousands of dollars and weighs upwards of 50 pounds, although devices that detect only one or two compounds may weigh as little as ten or fifteen pounds, says Eric Anderson of Buck Scientific, a manufacturer of analytical instruments.

Abbott says his detector is not only cheaper to manufacture, but more portable and easier to use: a person could wear the device as a color-changing badge, similar to those that measure radiation exposure. The badge could feature receptors for many different compounds, arranged in an array or in patterns. "The pattern that develops could be a letter or a word, so it could be understood by a non-technical person," Abbott says.

Although the device currently detects only gaseous compounds, Abbott hopes to develop a version that works with fluids. He is also working to develop liquid-crystal detectors for complicated biological targets, including viruses and bacteria, and reports some success already detecting protein binding.

"This is a real frontier issue, detecting environmental contaminants of all types," says Bruce Rittman, professor of environmental engineering at Northwestern University. "Getting rapid information is the weak link now; some of this takes a really long time. If you can do this, it's a fantastic benefit."