Carbon nanotube ethylene sensor could help reduce food waste
Made from semiconducting carbon nanotubes, the sensor can detect ethylene – a colorless, sweet-smelling gas that is emitted when flowers bloom and fruits ripen – in concentrations as low as 15 parts per billion. The sensor, say the researchers, could be used to monitor fruit and vegetables as they are shipped and stored, helping to reduce food waste.
“There is a persistent need for better food management and reduction of food waste,” says Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT. “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.”
In addition to being a plant hormone, ethylene is also the world’s most widely manufactured organic compound and is used to manufacture products such as plastics and clothing. A detector for ethylene could also be useful for monitoring this kind of industrial ethylene manufacturing, say the researchers.
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.
“There still is not a good commercial sensor for ethylene,” says Swager. “To manage any kind of produce that’s stored long-term, like apples or potatoes, people would like to be able to measure its ethylene to determine if it’s in a stasis mode or if it’s ripening.”
The new ethylene sensor works via a mechanism known as Wacker oxidation, where a palladium metal catalyst is used that adds oxygen to ethylene during oxidation. As the palladium catalyst performs this oxidation, the catalyst temporarily gains electrons. Palladium then passes these extra electrons to carbon nanotubes, making them more conductive. By measuring the resulting change in current flow, the researchers can detect the presence of ethylene.
The sensor, say the researchers, responds to ethylene within a few seconds of exposure, and once the gas is gone, the sensor returns to its baseline conductivity within a few minutes.
“You’re toggling between two different states of the metal,” says MIT postdoc Darryl Fong and lead author of a paper on the research, “and once ethylene is no longer there, it goes from that transient, electron-rich state back to its original state.”
To test the sensor’s capabilities, the researchers deposited the carbon nanotubes and other sensor components onto a glass slide, which they then used 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. The 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 had any effect on 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).
The MIT team has filed for a patent on the new sensor. For more, see “Trace Ethylene Sensing via Wacker Oxidation.”
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