Researchers have developed a new technique for making metamaterials with nanoscale structures that can be tuned with strange optical properties. Using gold nanoparticles attached to strands of DNA that would shrink and stretch on demand, the team was able to change the color of the material, but future versions could use the mechanism to turn any color, acting as an environmental or medical sensor, or even a cloaking device.

To develop their new material architecture, the researchers combined top-down lithography with a new DNA-driven technique. The end result is a "superlattice" of stacks of nanoparticles held together with strands of DNA. The lengths of those strands can be changed in response to certain stimuli, so the optical properties of the materials can be tweaked as needed.

"Architecture is everything when designing new materials, and we now have a new way to precisely control particle architectures over large areas," says Chad Mirkin, co-corresponding author of the study. "Chemists and physicists will be able to build an almost infinite number of new structures with all sorts of interesting properties. These structures cannot be made by any known technique."

To make the superlattices, the researchers start by drilling tiny holes in a polymer resist using lithography techniques similar to those used to make computer chips. Since these holes are just one nanoparticle wide, they form landing pads for nanoparticles to be dropped into. These particles have been modified with strands of DNA, and when a second or third are dropped in on top, they all bind together, with the landing pads keeping them in a vertical stack.

For their prototype, the Northwestern researchers used nanoparticles of gold. When they exposed the material to solutions containing different concentrations of ethanol, they found that the DNA strands would change length, in turn changing the color of the material from black, to red, to green.

"Tuning the optical properties of metamaterials is a significant challenge, and our study achieves one of the highest tunability ranges achieved to date in optical metamaterials," says Koray Aydin, co-corresponding author of the study. "Our novel metamaterial platform – enabled by precise and extreme control of gold nanoparticle shape, size and spacing – holds significant promise for next-generation optical metamaterials and metasurfaces."

The researchers say that using their method, scientists can exert huge amounts of control over the precise layout of particles over a relatively large area. By changing the size, shape and type of nanoparticle used, material engineers could design completely new metamaterials with virtually any optical properties needed. That could include devices that turn virtually any visible color, or even those that can bend light to "cloak."

The study was published in the journal Science.

Source: Northwestern University