Photonic gels could yield inexpensive sensors

The new work, led by Rice materials scientist Ned Thomas, combines polymers into a unique, self-assembled metamaterial that, when exposed to ions in a solution or in the environment, changes color depending on the ions' ability to infiltrate the hydrophilic layers. The micron-thick material, called a photonic gel, is far thinner than a human hair and said to be so inexpensive to make that an area the size of a football field could be covered for around £60. 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. The team progressively turned a clear film to blue, to green, to yellow, to orange and finally to red. In each case, the changes were reversible. Thomas explained that the direct exchange of counterions 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," he said, "which allows the gels to be tuned to react in specific ways." Thomas continued: "Imagine a solid in which you create a band gap everywhere but along a 3D 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." The work was supported by the US Army Research Office, the US Air Force and the Korea Research Foundation, funded by the Korean government.