Ethylene-detecting sensor could help prevent food waste

MIT chemists have created a tiny sensor that can detect ethylene, a gas that indicates when fruits ripen, which they believe could be useful in preventing food spoilage.

According to an article on MIT’s website, the sensor can detect ethylene concentrations as low as 15 parts per billion. The sensor, made from semiconducting cylinders called carbon nanotubes, could be used to monitor fruit and vegetables as they are shipped and stored, said Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT.

“There is a persistent need for better food management and reduction of food waste,” said Swager in the article. “People who transport fruit around would like to know how it’s doing during transit, and whether they need to take measures to keep ethylene down while they’re transporting it.”

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Swager is the senior author of the study, which appears in the journal ACS Central Science. Other authors include Darryl Fong (lead author), MIT graduate student Shao-Xiong (Lennon) Luo, and visiting scholar Rafaela Da Silveira Andre.

According to the authors, ethylene is produced by most plants, which use it as a hormone to stimulate growth, ripening, and other key stages of their life cycle. Produce and flowers under stress can overproduce ethylene, leading them to ripen or wilt prematurely. It is estimated that every year U.S. supermarkets lose about 12% of their fruits and vegetables to spoilage, according to the U.S. Department of Agriculture. 

In 2012, Swager’s lab developed an ethylene sensor containing arrays of tens of thousands of carbon nanotubes. These carbon cylinders allow electrons to flow along them, but the researchers added copper atoms that slow down the electron flow. When ethylene is present, it binds to the copper atoms and slows down electrons even more. Measuring this slowdown can reveal how much ethylene is present. However, this sensor can only detect ethylene levels down to 500 parts per billion, and because the sensors contain copper, they are likely to eventually become corroded by oxygen and stop working.

Swager and Fong created a new kind of ethylene sensor that is also based on carbon nanotubes but works by an entirely different mechanism, known as Wacker oxidation. Instead of incorporating a metal such as copper that binds directly to ethylene, they used a metal catalyst called palladium that adds oxygen to ethylene during a process called oxidation.

To test the sensor’s capabilities, the researchers deposited the carbon nanotubes and other sensor components onto a glass slide. They then used it to monitor ethylene production in two types of flowers—carnations and purple lisianthus. They measured ethylene production over five days, allowing them to track the relationship between ethylene levels and the plants’ flowering.

In their studies of carnations, the researchers found that there was a rapid spike in ethylene concentration on the first day of the experiment, and the flowers bloomed shortly after that, all within a day or two.

Purple lisianthus flowers showed a more gradual increase in ethylene that started during the first day and lasted until the fourth day, when it started to decline. Correspondingly, the flowers’ blooming was spread out over several days, and some still hadn’t bloomed by the end of the experiment.

The researchers also studied whether the plant food packets that came with the flowers affected ethylene production. They found that plants given the food showed slight delays in ethylene production and blooming, but the effect was not significant (only a few hours).