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Coating Changes Color under Pressure

Thursday, May 15, 2014

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New nanoparticle research has produced a polymer that can reveal pressure points by changing color.

"We have developed a high-resolution pressure sensor that indicates pressure by varying its color—a sensor that all of us can use with just our eyes," said Yadong Yin, an associate professor of chemistry at the University of California, Riverside, whose lab led the research.

The sensor film can be painted on contact surfaces to show the stress distribution over the surface, Yin says.

The new technology has potential for safety applications, among others. For example, it could be painted on crash-test dummies to indicate areas of impact and stress.

University of California, Riverside
Yin Lab, UC Riverside

Digital images (top) and schematic illustrations (bottom) show the color change of the sensor film after experiencing different amounts of pressure.

The Yin Research Group focuses on the synthesis, self-assembly and functionalization of nanostructured materials. The lab makes nanoparticles and then organizes them together in ways that will produce new properties based on particle interactions.

Aiding Yin in the research was Xiaogang Han, a former postdoctoral researcher in his lab; and Yiding Liu, who recently won the graduate student silver award at the Materials Research Society in San Francisco.

The research was funded by a grant from the National Science Foundation.

Particles Under Pressure

The researchers used the self-assembly method to string together gold nanoparticles, which were then embedded into a polymer film.

The film became deformed when pressed, causing the gold nanoparticle strings to stretch. As the strings stretched, the distance between the gold nanoparticles increased.

"This increased separation alters the way the nanoparticles interact with light," Yin explained.

Yin said that the gold nanoparticles originally appeared blue when linked together, but gradually changed to red as they start disassembling with the increased pressure.

"This easily and visually helps us figure out how much pressure has been applied," Yin said.

The sensor is a solid plastic film that deforms like conventional plastics under stress. The new color remains after the stress is removed.

Yadong Yin
L. Duka, UC Riverside

"We have developed a high-resolution pressure sensor that indicates pressure by varying its color—a sensor that all of us can use with just our eyes," said Dr. Yadong Yin, of UC Riverside, who led the research.

"This is why we are calling it a 'colorimetric stress memory sensor,'" Yin said, adding that silver and copper could also work in place of gold.

Mosaic of Colors

"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.

Commercially available sensor films indicate pressure by changing the intensity of just one color. For example, they may change from a pale red to darker red.

"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," Yin explained.

The new film technology produces a "mosaic" of colors that can be easily distinguished and offers higher contrast and resolution.

Reversible Effect

The researchers speculated that the disassembly process would reverse the color change.

"We found to our surprise that mechanical force could achieve this disassembly," Yin said.

"Considerable effort has been made by researchers to study nanoparticle self-assembly. Indeed, gold nanoparticles have conventionally been used as sensors based on the self-assembly process.

"What is novel about our work is that it shows that the disassembly process can also find great applications if the assembly is designed to be reversible."

A paper on the research results, "Colorimetric Stress Memory Sensor Based on Disassembly of Gold Nanoparticle Chains," was recently published in Nano Letters, an American Chemical Society publication.

The UCR Office of Technology Commercialization has filed a patent on the technology reported in the research paper.


Tagged categories: Coating chemistry; Color; Health and safety; Nanotechnology; Research

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