This graphic shows how the color of the gold nanorod film responds to twisting. Image: UCR/Yadong Yin.
This graphic shows how the color of the gold nanorod film responds to twisting. Image: UCR/Yadong Yin.

A new film made of gold nanoparticles can change color in response to any type of movement. These unprecedented qualities could allow the creation of robots able to mimic chameleons and octopi – among other futuristic applications.

Unlike other materials that try to emulate nature's color changers, the new film can respond to any type of movement, like bending or twisting. Robots coated in it could enter spaces that might be dangerous or impossible for humans, and report back information based purely on the way they look.

For example, a camouflaged robot could enter tough-to-access underwater crevices. If the robot changes color, biologists could learn about the pressures facing animals that live in these environments.

Although some other color-changing materials can also respond to motion, this one can be printed and programmed to display different, complex patterns that are difficult to replicate. The new material was developed by scientists at the University of California (UC) Riverside, who reported their advance in a paper in Nature Communications.

Nanomaterials are materials that have been reduced to an extremely small scale – tens of nanometers in width and length, or about the size of a virus. When materials like silver or gold reach these scales, their colors will change depending on their size, shape and the direction they face.

"In our case, we reduced gold to nano-sized rods. We knew that if we could make the rods point in a particular direction, we could control their color," said Yadong Yin, a professor of chemistry at UC Riverside. "Facing one way, they might appear red. Move them 45°, and they change to green."

The problem facing the research team was how to take millions of gold nanorods floating in a liquid solution and get them all to point in the same direction to display a uniform color. Their solution was to fuse smaller magnetic nanorods onto the larger gold ones. The two different-sized rods were encapsulated in a polymer shield, so that they would remain side-by-side, allowing the orientation of both rods to be controlled by magnets.

"Just like if you hold a magnet over a pile of needles, they all point in the same direction. That's how we control the color," Yin said.

Once the nanorods are dried into a thin film, their orientation is fixed in place and they no longer respond to magnets. "But if the film is flexible, you can bend and rotate it, and will still see different colors as the orientation changes," Yin said.

Other materials, like butterfly wings, are shiny and colorful at certain angles, and can also change color when viewed at other angles. However, those materials rely on precisely ordered microstructures, which are difficult and expensive to make for large areas. But this new film can be made to coat the surface of any sized object just as easily as applying spray paint on a house.

Though futuristic robots are an ultimate application of this film, it can be used in many other ways as well. Zhiwei Li, a chemist at UC Riverside and first author of the paper, explained that the film could be incorporated into checks or cash as an authentication feature. Under normal lighting, the film is gray, but when putting on sunglasses and looking at it through polarized lenses, elaborate patterns can be seen. In addition, the color contrast of the film may change dramatically if the film is twisted.

The applications, in fact, are only limited by the imagination. "Artists could use this technology to create fascinating paintings that are wildly different depending on the angle from which they are viewed," Li said. "It would be wonderful to see how the science in our work could be combined with the beauty of art."

This story is adapted from material from the University of California, Riverside, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.