By James Snodgrass
In the fridge of the future, we won't be looking for "Use By" or "Best Before" dates. We will be able to tell whether food is fresh, or has turned for the worse, with a quick visual inspection of the fridge. In-mould labels made from a photonic gel will change colour when exposed to chemicals associated with a foodstuff going off. So we will have an instant visual warning if our food's gone bad.
This is just one of many applications for a novel, extremely thin colour-changing films that have been developed by materials scientists at Rice University in Texas and the Massachusetts Institute of Technology (MIT), both in the US. The research has been published in the nanotechnology journal of the American Chemical Society, ACS Nano.
Leading the project is Ned Thomas, Dean of Rice University's George R.Brown School of Engineering (and formerly chair of the department of Materials Science and Engineering at MIT). Thomas and his team have combined polymers into a self-assembled metamaterial that changes colour when exposed to ions dissolved in solution - or ions in the environment at large. The material changes colour according to the ions' ability to penetrate the hydrophilic layers.
The micron-thick material, made from nanoscale layers of hydrophobic (water resisting) PS and hydrophilic (water loving) poly(2-vinyl pyridine) (P2VP), is called a photonic gel. Thinner than a human hair, Thomas believes the photonic gels will be extremely inexpensive to produce. "We could cover an area the size of a football field [5,350m2] with this film for about $100 (€78.58)," said Thomas.
Given this estimate, the cost per square metre would be little more than €0.01, meaning the cost of an in-package sensor could - theoretically - be infinitesimally small whilst delivering demonstrable added value. Though, naturally, the cost of bringing the technology to market, and the associated costs of getting EU and international regulatory approval for food contact, may impact the technology's viability.
Thomas continued: "If [the food sensor] is 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."
He explained that this makes it easier to check food freshness than conventional methods, "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."
Exposing the films to various solutions, the researchers discovered that they changed colour in proportion to the amount of solvent absorbed by the P2VP. From the film's neutral, clear, state the researchers turned the film blue (using thiocyanate), green (iodine), yellow (nitrate), orange (bromine) and red (chlorine). All these interactions were reversible; the film returned to its transparent state in the absence of contaminants.
The change in colour is caused by expansion of the P2VP layers. Different thicknesses of layers within the film change the way that light is refracted through the film. Thomas suggests that these changes can be exploited in such a way as to make gels that are "tuned" to react in specific ways.
Thomas 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."
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.