If you've ever accidentally put spoilt milk in your tea or on your cereal you'll know it's an experience you don't want to repeat. One way to stop this culinary disaster is to have sophisticated electronic sensors that can pick up chemicals produced by milk when it spoils and then display this on the side of the package. Unfortunately, this is a rather expensive gizmo to add to every single carton of milk given the millions manufactured every day.
Chinese scientists think they have a cheaper solution to this problem. And they're relying on the same phenomenon that medieval stained glass window artisans have made use of for centuries - plasmonics. The new sensor doesn't pick up on noxious volatiles from food, but instead uses a simpler method that displays - with a simple colour change - how old the milk is, and to temperatures to which it has been exposed.
Gold nanorods suspended in a silver solution are the basis for the sensor. Thenanorods appear red rather than the gold we'd expect because, at the nanoscale, metals can interact with light in unusual ways, absorbing and scattering some wavelengths and reflecting others. Stained glass window makers were unwitting nanotechnologists and made use of this effect by putting metal salts into their glass: the vivid ruby reds in these windows are produced by gold nanoparticles, for example.
In the sensor, the silver ions slowly coat the nanorods, altering the way they interact with light and changing their colour from red to green. This change happens much faster when it's hotter, and so gives the consumer a brief history of the milk in a simple display. This display needn't just be used for milk, and would be especially useful for products like meat or seafood that can make you very ill if they warm up enough for bacteria that can cause food poisoning to multiply. The authors hope to roll out their sensor as soon as possible - you just need to remember that in this case red means go while green is the signal to stop eating!
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