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Scientists create ultra-thin material that could be used as food sensor

By Joe Whitworth , 23-Oct-2012

A photonic gel developed at Rice University and MIT self-assembles from long polymer molecules. Picture: Joseph Walish/MIT
A photonic gel developed at Rice University and MIT self-assembles from long polymer molecules. Picture: Joseph Walish/MIT

Researchers have created a thin-film polymer metamaterial that can be incorporated inside packaging and that changes colour if the conditions inside the package alter.

The work by materials scientists at Rice University and the Massachusetts Institute of Technology (MIT) combines polymers into metamaterial that, when exposed to ions in the environment, changes colour depending on the ions’ ability to infiltrate the hydrophilic (water-loving) layers.

The films are made of nanoscale layers of hydrophobic polystyrene and hydrophilic poly(2-vinyl pyridine) (P2VP). The micron-thick material is called a photonic gel.

Nano-sized layers of hydrophilic and hydrophobic molecules self-assemble into a block copolymer called a photonic gel which changes colour depending on the amount of water absorbed by the hydrophilic layers.

How it works

The polystyrene molecules clump together to keep water molecules out, while the P2VP forms its own layers between the polystyrene. On a substrate, the layers form into a transparent stack of what the researchers called alternating “nano-pancakes.”

The research was published in the American Chemical Society journal ACS Nano.

The researchers exposed their films to various solutions and found different colors 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.

They turned a clear film to blue (with thiocyanate), to green (iodine), to yellow (nitrate), to orange (bromine) and finally to red (chlorine) with the changes being reversible.

Photonic gel

Ned Thomas, the William and Stephanie Sick Dean of Rice’s George R. Brown School of Engineering, confirmed that the film is easy to make and fairly inexpensive so would be able to cope with industry demand.  

Thomas said the technology is such that the shift can be to longer or shorter wavelengths, so for example, from green to blue or from green to red.

“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.

“And you can read these sensors with low tech, either with your own eyes or a spectrophotometer to scan things.”

He added that the next step was to either licence the technology, which could also be used for security and multiband optical elements in laser-driven systems, or create a start-up company.

The work was supported by the US Army Research Office, the US Air Force and the Korea Research Foundation, funded by the Korean government. 

Source: ACS Nano

Online ahead of print, DOI: 10.1021/nn302949n

"Dynamic Swelling of Tunable Full-Color Block Copolymer Photonic Gels via Counterion Exchange"

Authors: Ho Sun Lim, Jae-Hwang Lee, Joseph J. Walish, and Edwin L. Thomas