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Sensors/MEMS??

Photonic gels boast numerous applications

Posted: 12 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:photonic gel? metamaterial? sensor?

Researchers at Rice University and the Massachusetts Institute of Technology (MIT) have developed very thin colour-changing films that according to them can be used as part of inexpensive sensors for food spoilage or security, multi-band optical elements in laser-driven systems and in high-contrast displays.

The 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 colour depending on the ions' ability to infiltrate the hydrophilic (water-loving) layers.

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

But for practical applications, much smaller pieces would do. "Suppose you want a food sensor," said Thomas. "If it's inside a sealed package and the environment in that package changes because of contamination or ageing or exposure to temperature, an inspector would see that sensor change from blue to red and know immediately the food is spoiled."

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

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), 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 beauty of self-assembly is that it's simultaneous, all the layers forming at once," Thomas added.


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