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Photonic gels make colourful sensors

11 October 2012

Researchers have created very thin colour-changing films that may serve as part of inexpensive sensors for food spoilage or security, or multiband optical elements in laser-driven systems.

A photonic gel self-assembles from long polymer molecules (photo courtesy of Joseph Walish/MIT)

New work, by materials scientists at Rice University and the Massachusetts Institute of Technology (MIT) and led by Rice materials scientist Ned Thomas, combines polymers into a self-assembled metamaterial that, when exposed to ions in a solution or in the environment, changes colour depending on the ions’ ability to infiltrate the hydrophilic layers.

The micron-thick material called a photonic gel, far thinner than a human hair, is so inexpensive to make that an area the size of a football field could be covered with this film for about a hundred dollars!

The films are made of nanoscale layers of hydrophobic polystyrene and hydrophilic poly(2-vinyl pyridine). In the liquid solution, 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 layered structure.

The polystyrene molecules clump together to keep water molecules out, while the poly(2-vinyl pyridine), or P2VP for short, forms its own layers between the polystyrene. On a substrate, the layers form into a transparent stack of alternating “nano-pancakes.”

The researchers exposed their films to various solutions and found different colours depending on how much solvent was taken up by the P2VP layers. For example with a chlorine/oxide/iron solution that is not readily absorbed by the P2VP, the film is transparent.

The researchers progressively turned a clear film 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.

The direct exchange of counter-ions from the solution to the P2VP expands those layers and creates a photonic band gap — the light equivalent of a semiconducting band gap — that allows colour in a specific wavelength to be reflected. “The wavelengths in that photonic band gap are forbidden to propagate,” says Ned Thomas, which allows the gels to be tuned to react in specific ways.

“Imagine a solid in which you create a band gap everywhere but along a 3-D path, and let’s say that path is a narrowly defined region you can fabricate within this otherwise photonic material. Once you put light in that path, it is forbidden to leave because it can’t enter the material, due to the band gap.

“This is called moulding 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 band gaps.”

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