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Thin color-changing films in sensors to detect food spoilage and security

Thin color-changing films in sensors to detect food spoilage and security

14-Jan-13

Food is lost or wasted throughout the supply chain, from initial agricultural production down to final household consumption. Studies by The Swedish Institute for Food and Biotechnology (SIK) on request from the Food and Agriculture Organization of the United Nations (FAO), suggest that roughly one-third of food produced for human consumption is lost or wasted globally, which amounts to about 1.3 billion tpa, as per worldfoodscience.org. Overall, on a per-capita basis, much more food is wasted in the industrialised world than in developing countries. It is estimated that the per capita food waste by consumers in Europe and North-America is 95-115 kg/year, while this figure in Sub-Saharan Africa and South/Southeast Asia is only 6-11 kg/year. The causes of food losses and waste in medium/high-income countries mainly relate to consumer behaviour as well as to a lack of coordination between different actors in the supply chain. Food can be wasted due to quality standards, which reject food items not perfect in shape or appearance. At the consumer level, insufficient purchase planning and expiring ‘best-before-dates’ also cause large amounts of waste, in combination with the careless attitude of consumers and their wasteful habits. In a bid to reduce food wastage, developments are being introduced in active and intelligent packaging. The packaging made of intelligent plastics lets consumers know when the food is close to spoiling because of damaged wrappers, the expiration or “best before" date is passed, or has not be stored at the proper temperature.

In a new development for food packaging, in-mold labels made from a photonic gel will change color when exposed to chemicals associated with a foodstuff rotting. This will provide an instant visual warning if the food has gone bad. Very thin color-changing films that may serve as part of inexpensive sensors for food spoilage or security, multiband optical elements in laser-driven systems and even as part of high-contrast displays have been created by Materials scientists at Rice University and the Massachusetts Institute of Technology (MIT). 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 (water-loving) layers. The research was published in the American Chemical Society journal ACS Nano. The micron-thick material called a photonic gel, far thinner than a human hair, is inexpensive. Film to cover an area the size of a football field would cost about a hundred dollars. When used in food sensor- the color would change from blue to red, if it is inside a sealed package and the environment in that package changes because of contamination or ageing or exposure to temperature. 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 said. 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. When that film is taken out, washed, and introduced in a new solution with a different ion, the color changes.  

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. 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 color 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. Co-authors of the paper are Rice research scientist Jae-Hwang Lee and MIT postdoctoral researchers Ho Sun Lim and Joseph Walish. The work was supported by the U.S. Army Research Office, the U.S. Air Force and the Korea Research Foundation, funded by the Korean government. Other potential applications for the technology include security (instant, visual testing for drugs, explosives or biological contaminants) or as an alternative technology to e-ink or thin film transistor displays.


 
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Windmoller  and Holscher 5 layer cast film line

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