Photonic Gel a Metamaterial with Myriad Uses

Thin films that change color under certain conditions and are incredibly cheap to make may inform future multiband optical elements in laser-driven systems or high-contrast displays, among other applications. The micron-thick material, called a photonic gel, combines polymers into a…

Thin films that change color under certain conditions and are incredibly cheap to make may inform future multiband optical elements in laser-driven systems or high-contrast displays, among other applications.

The micron-thick material, called a photonic gel, combines polymers into a unique self-assembled metamaterial that changes color when exposed to ions — in a solution or in the environment — based on the ion’s ability to infiltrate the hydrophilic (water-loving) layers.

The new material is so cheap to make that “we could cover an area the size of a football field with this film for about a hundred dollars,” said Rice University materials scientist Ned Thomas.

A photonic gel developed at Rice University and MIT self-assembles from long polymer molecules. Polystyrene and poly(2-vinyl pyridine) are mixed in a solution that, when evaporated, allows the polymers to quickly form into nanosized layers. The layers can be tuned to reflect specific colors when exposed to particular chemicals. Courtesy of Joseph Walish, MIT.

But for practical applications, such as in a sensor to monitor food quality, much smaller pieces would suffice, Thomas said. “If it’s inside a sealed package and the environment in that package changes because of contamination or aging or exposure to temperature, an inspector would see that sensor change from blue to red and know immediately the food is spoiled.”

Visual cues like this are good, “especially when you need to look at a lot of them,” he said. “And you can read these sensors with low tech, either with your own eyes or a spectrophotometer to scan things.”

The thin films are composed of nanoscale layers of hydrophobic polystyrene and hydrophilic poly(2-vinyl pyridine). In liquid solutions, the polymer molecules are diffused, but when the liquid is applied to a surface and the solvent evaporates, the block copolymer molecules self-assemble into a transparent stack of alternating layers, or “nanopancakes.”

Alternating, nanosized layers of hydrophilic and hydrophobic molecules self-assemble into a block copolymer called a photonic gel, developed at Rice University and MIT. It changes color depending on the amount of water absorbed by the hydrophilic layers, which can be tuned by the solvent used. Courtesy of Thomas Lab, Rice University.
“The beauty of self-assembly is that it’s simultaneous, all the layers forming at once,” Thomas said.

The team exposed the films to various solutions and discovered different colors depending on how much solvent was taken up by the poly(2-vinyl pyridine), or P2VP, layers. When a chlorine/oxide/iron solution was not readily absorbed by the P2VP, a transparent film was the result, Thomas said.

“When we take that out, wash the film and bring in a new solution with a different ion, the color changes,” he said.

A clear film was progressively turned to blue (with thiocyanate), to green (iodine), to yellow (nitrate), to orange (bromine) and finally to red (chlorine). In each case, the changes were reversible.

One photonic gel developed at Rice University was put through a series of color changes when repeatedly washed and exposed to new compounds. The gels show potential for optical elements in laser-driven systems and as inexpensive sensors and filters. Courtesy of Thomas Lab, Rice University.
The photonic gel is able to change color because the direct exchange of counterions from the solution to the P2VP expands those layers and creates a photonic bandgap — the light equivalent of a semiconducting bandgap. The wavelengths within the bandgap cannot propagate, which enables the gels to be tuned to react in specific ways, Thomas said.

“This is called molding the flow of light,” he said. “These days in photonics, people are thinking about light as though it were water. That is, you can put it in these tiny pipes. You can turn light around corners that are very sharp. You can put it where you want it, keep it from where you don’t want it. The plumbing of light has been much easier than in the past, due to photonics, and in photonic crystals, due to bandgaps.”

The metamaterial could also be used in security applications.

Thomas collaborated with Rice research scientist Jae-Hwang Lee and MIT postdoctoral researchers Ho Sun Lim and Joseph Walish. The work was supported by the US Army Research Office, the US Air Force and the Korea Research Foundation, funded by the Korean government.

The work appeared in ACS Nano (doi: 10.1021/nn302949n).

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