Is it safe to eat the leftover cheese in the fridge or put on the eye shadow that has been in your cabinet for several years? A smart label could help. That’s the hope of a team of researchers who have developed a new sensor containing nanostructures that change color when they bind to compounds that indicate spoilage or contamination by bacteria. Whereas currently available sensors use liquid solutions that migrate on channels, the newly developed sensor has all the reagents incorporated in a postage stamp–sized piece of paper. This means it can be directly applied to the samples that have to be tested; for example, it could be added to makeup packaging or dabbed onto leftover food. The researchers, who will present their results today at the 254th National Meeting & Exposition of the American Chemical Society in Washington, D.C., have already used the paper sensor to detect antioxidant compounds in tea and wine, which could be used for authentication purposes. But the sensor, they add, could also be used to identify new medicinal plants or natural sources of antioxidants in remote areas such as the Amazon rainforest.