How hot was it?

Scientists have created heat-sensing polymers that indicate exposure to high temperatures by changing color under ultraviolet (UV) light. If built into food and medicine packaging, such materials could serve as indicators of spoilage.

At the heart of the sensitive polymers are photoluminescent dyes, explains Chris Weder, a polymer chemist at Case Western Reserve University in Cleveland. Individually, each fluorescent-dye molecule emits a specific visible wavelength--a color--when zapped with UV light. When two or more dye molecules are close enough together, however, they redistribute this incoming energy among themselves and emit a color different from that of any individual molecule.

To create the sensor, Weder's team started with a hot solution of a polymer and a dye. If the liquid is cooled rapidly, the dye molecules remain evenly dispersed throughout the resulting solid.

This solid functions as a temperature sensor because reheating it above a certain threshold loosens the polymer's microstructure and the dye molecules, in Weder's words, "move through the polymer and find each other." As a result, a sample that has been exposed to a temperature above the threshold will under UV light appear a color different from that of the original.

The sensor can also indicate how long an object bearing it--a milk carton, for example--had been above a threshold temperature. A short trip from a delivery truck to a refrigerator results only in small color changes. But if that milk carton had been left in the hot sun all day, the color change would be striking. This gives a much better picture of the "thermal history of the product," says Weder.

The group is working on sensors that won't require a UV light. Weder notes that, as anyone who has ever bought spoiled food knows, "there are many applications where you'd like the end customer to see the color changes."--A.C.