It has been well-known for a long time that as flowers bloom and fruits ripen, they emit a colorless, sweet-smelling gas called ethylene. MIT chemists have now created a tiny sensor that can detect this gas in concentrations as low as 15 parts per billion, which they believe could be useful in preventing food spoilage.
The sensor, which is made from semiconducting cylinders called carbon nanotubes, could be used to monitor fruit and vegetables as they are shipped and stored, helping to reduce food waste, says 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," says Swager. "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 its natural role as 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, the researchers say.
Swager is the senior author of the study, which appears today in the journal ACS Central. MIT postdoc Darryl Fong is the lead author of the paper, and MIT graduate student Shao-Xiong (Lennon) Luo and visiting scholar Rafaela Da Silveira Andre are also authors.
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.
"There still is not a good commercial sensor for ethylene," Swager says. "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."
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.
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 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.