Scientists Made Color-Changing Paint Out of Gold Nanoparticles

Something unexpected happened when scientists at the University of California, Riverside, started stringing together nanoparticles of gold. 

The gold wasn't golden anymore. It changed colors. 

"When we see these gold particles aggregate, we find out they have very, very beautiful blue colors," chemist Yadong Yin told me. That bright blue would dissipate like a sunset—morphing into purple, then red—when scientists warped the strings, breaking apart the nanoparticles.

The finding was one of those happy scientific accidents that turns into something bigger. "So after we found out the reason why they show blue colors and what the structure was, then we started to think what kind of use they could have," Yin said.

What Yin and his colleagues came up with: Sensors made of gold nanoparticles that change colors as you press on them. Think of it as a Hypercolor—those color-changing T-shirts all the cool kids had in the '90s—but for touch instead of heat. But rather than using color-changing sensors for fashion, Yin and his team see them as the perfect tool for making better crash-test dummies. The sensors can be painted on or applied like a bandage, and they convey differences in pressure at certain points of impact.

Here's how it works: Scientists combine gold nanoparticles in liquid form until the solution turns blue. Then they embed that solution into a polymer film, thin and flexible like a Band-Aid. As the film gets stretched—by being pressed or hit—those nanoparticles start separating. And when the particles break apart, they change color again. Press lightly, and you get purple. Press harder, and the sensor will turn red.  

In other words, these sensors don't just tell you that there was an impact; they let you visualize how strong of an impact it was. Conventional sensors aren't that nuanced. "For a harder hit, you will see more red," Yin said. "Someplace which is not really hit may remain as blue. So you can actually see that immediately. You can see the transition."

The technology has other potential applications. Baseball players could coat their bats with sensors to analyze their grips. Sensors could be attached to beds or other furniture—then hooked up to data trackers—to assess sleeping position or posture. University of California, Riverside, has filed a patent on the technology, and even though it's made of gold, its concentration is low enough to make the ideas marketable, Yin told me. 

Then again, silver would be cheaper. It would also offer a broader spectrum of color—silver particles absorb far more light than gold ones—which is why researchers are working on developing silver nanoparticle sensors next. Gold is more commonly used than silver in nanotechnology because "chemically, gold is more stable," Yin said. "But if we use silver, we'll see a color change from blue to yellow. So if you apply pressure it may become first green then red and eventually to yellow."

Yin says he's already received emails from people who have ideas about using the technology in ways he hadn't imagined. A cellist suggested using nanoparticle sensors to teach students how to hold instruments properly, for example. "I can imagine lots of people using it," Yin said.