Researchers from the University of California created a film of gold nanoparticles that changes color in response to movement.

The material mimics qualities of chameleons and octopi in response to bending, twisting or pressure. The nanofilm can be printed and programmed to display different and complex patterns.

Graphic depicting gold nanorod film's color response to twisting. Source: UCR/Yadong Yin

When materials like gold or silver become tiny, their colors change depending on their size, shape and the direction they are facing.

To create the new material, the team reduced gold into nanosized rods. When facing a certain direction, the rods appear red and when turned 45° the other way they appear green. Next, the team had to figure out how to make millions of gold rods floating in a liquid solution face the same direction.

The solution was to fuse the smaller magnetic nanorods to the longer gold ones. The rods are encapsulated in a polymer shield that allows them to bind together. Their orientation is controlled by magnets. When the nanorods are dried into a thin and flexible film, the orientation is fixed in place and they no longer respond to magnets. But the film changes color when viewed at different angles.

The nanomaterial has a variety of uses. It could enable robots to enter spaces that may be dangerous or impossible for humans to explore, including tough to access underwater crevices. Biologists can use the nanomaterial to learn about the underwater pressure an animal is feeling. It could be incorporated into cash and checks for authentication because it appears grey under normal light, but when viewed with polarized glasses intricate designs are revealed. Artists can use the nanomaterial in their pieces. There are basically unlimited uses for the color-changing material.

A paper on the gold nanomaterial was published in Nature Communications.