Although the best-before dates on our foods may stop us from eating spoiled items, they can also cause us to toss out food that's really still good. New phone-powered labels could help, by notifying users when the food is actually starting to go "off."

The prototype labels are being developed by a team at Britain's Imperial College London, led by Dr. Firat Güder. Scientifically known as "paper-based electrical gas sensors" (PEGS), they're made mainly of readily-available cellulose paper, onto which are printed conductive carbon-ink electrodes.

Each label is non-toxic, biodegradable, and currently costs just two US cents to make.

Added to a disposable NFC (near-field communication) chip and then integrated into food packaging, the labels' paper fibers take in water vapor given off by the food. If water-soluble gases associated with food spoilage are present – gases such as ammonia, trimethylamine or carbon dioxide – the electrical conductance of the paper increases, with the extent of the increase determined by how much gas is present.

The amount of power transmitted between the electrodes to the NFC chip lessens as that conductance increases, until it reaches a threshold where the chip stops working completely. When users wirelessly scan/power-up the label with an NFC-capable smartphone, an app lets them know if the chip is still working, and thus if the food is still OK to eat.

According to the university, the PEGS have several advantages over other experimental spoilage-detecting food labels. These include the facts that they work at almost 100-percent humidity, they function at room temperature (some others need to be heated), and they react only to spoilage-related gases.

They should also be cheap and easy to manufacture. "We believe our very simple technique could easily be scaled up to produce PEGS on a mass scale by using existing high-volume printing methods such as screen printing and roll-to-roll printing," says Güder.

The scientists are now looking into other possible applications for the technology, such as detecting disease markers in breath or pollutants in the air. They are also developing arrays of the labels, in which each one is capable of detecting a different gas.

A paper on the research was recently published in the journal ACS Sensors.

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