Chemical sensors gain true portability

In a step toward warning badges for military personnel and spoilage indicators for supermarket items, researchers have created novel films that change color on contact with selected chemicals, such as those found in nerve gas or rotting fish.

A film coated with liquid crystals appears black (left) until molecules resembling those in a nerve gas displace the liquid crystal molecules (right). Science

Scientists have already devised a variety of sensors for specific chemicals (SN: 2/19/00, p. 125), but such technology generally requires lasers, spectrometers, power sources, and other large, complicated devices to read out its messages. The new sensors, however, are simpler–flat films that require no electronic parts and that visibly change in color or shading to warn of troublesome molecules in the air.

“My hope is that simplicity will make it useful,” says Nicholas L. Abbott of the University of Wisconsin-Madison. He and Rahul R. Shah, formerly of the University of Wisconsin and now at the 3M Corporation in St. Paul, Minn., describe their new sensor strategy in the Aug. 17 Science.

To make 1-centimeter-square sensors, Abbott and Shah first create textured gold films with ridges and valleys just nanometers high. They layer each film with chemical groups, such as carboxylic acid groups. These, in turn, bond to liquid crystals akin to those in watches and computer displays.

The chemical group chosen also selectively binds a type of airborne molecule, even when that molecule is present in low concentrations. The incoming molecules supplant the liquid crystals by attaching more strongly, says Abbott. Displaced liquid crystals reorient on the corrugation, and a telling change in hue results.

The sensors work even in environments rife with water vapor, acetone, alcohol, and other contaminants, Abbott says. Some of the films also revert to their original appearance when removed from the source of their target molecule.

Similar devices might eventually be built with bases of polymers rather than gold, making sensor production even easier and less expensive, Abbott suggests. In one scenario, a disposable sensor stuck on a package of chicken breasts might generate the word dispose when the meat begins showing chemical signs of spoilage.

Such sensors might also someday identify substances ranging from dangerous molds in the home to subtle signs of disease on a person’s skin or breath, comments Ralph Nuzzo, a chemist at the University of Illinois at Urbana-Champaign. The sensors’ advantage is their ultimate simplicity, he comments.

“You don’t want to be dragging around all the lasers in the universe,” he says.

“You can’t wear a spectrometer,” concurs Orlin D. Velev, a chemical engineer at North Carolina State University in Raleigh. By offering the potential of inexpensive, portable chemical sensors, the new technology “fills a niche that has been left empty,” he says.

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