A plastic film that changes color under stress could provide more detailed information in product safety testing and other areas where pressure and strain are measured. A lab at the University of California, Riverside, used a self-assembly method to string together gold nanoparticles embedded in a polymer film. The system takes advantage of the plasmonic shift that results when nanoparticle chains are pulled apart. “When linked together, the gold nanoparticles originally appear blue,” said Yadong Yin, an associate professor of chemistry whose lab led the research. “But they gradually change to red with increasing pressure as the nanoparticles start disassembling. This easily and visually helps us figure out how much pressure has been applied.” The color change persists even after the stress is removed, prompting researchers to dub the material a “colorimetric stress memory sensor.” Digital images (top) and schematic illustration (bottom) showing the color change of the sensor film after experiencing different amounts of pressure. Courtesy of UC Riverside. The Yin team’s film differs from commercially available pressure sensor films, which indicate pressure by changing the intensity of just one color, the researchers said. It can potentially be used for revealing pressure distribution over even very complex surfaces, such as automobile crash test dummies. “The many electronic stress sensors commercially available are bulky and not suitable for certain applications,” Yin said. “For example, it is difficult to tell the stress distribution over a particular area if the contact surfaces are not flat and uniform. Our sensor films can be painted on the contact surfaces so that the color variance in different areas clearly shows the stress distribution over the contact surface.” The research was funded by a grant from the National Science Foundation. The university is now seeking to patent the discovery. Study results appear in Nano Letters (doi: 10.1021/nl500144k). For more information visit www.ucr.edu.