by Illustration by: Brett Ryder

A coin-sized device changes color when it senses a toxin.

You can’t wear a gas chromatograph,” says Nicholas Abbott, a chemical engineering professor at the University of Wisconsin-Madison. He’s referring to a 50-pound device used to check air for chemical poisons. These days, when terrorism is a concern to all, there’s considerable demand for a chemical sensor no larger than a badge.

Abbott and his colleagues have produced it. The coin-sized device changes color when it senses a toxin. It consists of a thin gold film laced with microscopic grooves. Each groove contains chemical receptors bonded lightly to liquid crystals. This bond regulates the orientation of the crystals and determines their appearance. When the receptors encounter a “target” chemical — say, a pesticide — they break their bond with the crystals and grab hold of the target molecules. Thus released, the crystals switch orientation and change color.

When the dangerous compound is no longer present, the crystals reattach to the receptors and the sensor returns to normal.

Currently, the sensors detect hazardous gases, such as industrial pollutants and chemical warfare agents. Pinned to clothing, they could signal exposure to harmful chemicals; packed with meat, they could monitor freshness. Abbott is also working on sensors that target germs.