Tiny gel tags indicate when packaged food has spoiled
By David Szondy
March 18, 2014
We're all familiar with the sell-by dates stamped on groceries. They're supposed to protect us, but in practice, they can be a bit of a coin toss. Now a research team led by Chao Zhang of Peking University in Beijing, China has come up with a color-coded smart tag that uses nanotechnology to tell when the food or drugs in a package are in danger of spoiling.
Sell-by tags are meant to tell us when the milk will go off or when it's time to toss the veal. The problem is that they operate on a good deal of guesswork. This is because these tags are based on a lot of assumptions about how the product is stored. If the storage conditions match the assumptions, then fine, but if the goods are stored better or worse than the label makers allowed for, that throws the whole thing off.
What's needed is something that reacts to the actual conditions that the product is stored in, rather than what it might have been stored in. Presented at the 247th National Meeting & Exposition of the American Chemical Society (ACS), the gel-like Peking tags are the size of a kernel of corn, inexpensive, safe, and even edible. They react to the ambient temperatures around them and change color in a programmed fashion that mimics spoilage with red for fresh, orange and yellow for getting a bit old, and green for gone off.
According to Zhang, the tags can be widely programmed to mimic all ambient temperatures, and tell whether or not the product has gone off regardless of what the date on the label says. The tab technology uses gold nanorods, which are red, which is why the the researchers don't use the more conventional green for fresh. In the gelatin there's also silver chloride and vitamin C. These interact with the nanorods over time, coating them with silver. This changes the composition and shape of the rods and therefore their color.
Temperature is a variable in this, so the tags change color at different rates depending on how they’re stored. The current version of the tabs was developed and tested using E. coli bacteria in milk at various temperatures as a reference model, and Zhang says that similar models could be crafted for other products.
According to Zhang, despite the precious metals used in their manufacture, the tags cost only US$0.002.
“In addition, all of the reagents in the tags are nontoxic, and some of them (such as vitamin C, acetic acid, lactic acid and agar) are even edible,” he says.
Zhang went on to say that the process has been patented in China and that the next step is to find a manufacturer to make it practical.
The team's results were published in ACS Nano.
The video below introduces the new tags.