Dr. Timothy Swager is the John D. MacArthur Professor of Chemistry at MIT.
Photo courtesy of Justin Knight

by Enrico Villamaino

A team of researchers from the Massachusetts Institute of Technology has developed a sensor that could have large implications for the agricultural industry.

As plants bloom and fruits ripen, they produce and discharge a colorless, sweet-smelling gas called ethylene. While the release of ethylene is a normal part of many plants’ life cycles, environmental conditions, including drought, salinity and pathogens, can also cause levels of the hormone to fluctuate.

A reliable way of monitoring ethylene’s release in real time could address both challenges of preventing food spoilage and maintaining plant health. Early detection of changes in the release of this gas could allow farmers to take preventative actions that restore plant health, reducing crop losses. Transporters who deliver fruit would better be able to monitor their shipment during transit, and would be able to take action to keep the ethylene levels low. American supermarkets lose an estimated 12% of their fruit and vegetable stock to spoilage annually.

A major obstacle to this goal is that to date, existing sensors have limitations that make them impractical for use in the field.

Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT, has been leading a research team funded by the Natural Sciences and Engineering Research Council of Canada Postdoctoral Fellowships Program, the São Paulo Research Foundation, the National Science Foundation and the U.S. Army Engineer Research and Development Center. The team’s goal was to make a sensor that could sensitively detect changes in ethylene levels in the field.

“There still is not a good commercial sensor for ethylene,” explained 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.”

Prior ethylene sensors contained combinations of tens of thousands of carbon nanotubes, which allow electrons to flow along them. Added copper atoms slow down the electron flow. When ethylene is present, it binds to the copper atoms and slows down electrons even more, allowing for a measurement of how much ethylene is present. This type of sensor, however, is limited to detecting ethylene levels to 500 parts per billion. Another difficulty is that because the sensors contain copper, they are likely to eventually become corroded by oxygen and stop working.

The sensor developed by Swager and his team can detect ethylene in concentrations as low as 15 ppb. The research team’s newly developed sensor also utilizes carbon nanotubes, but works by an alternative mechanism, known as Wacker oxidation. Instead of incorporating copper, they use a palladium that adds oxygen to ethylene. As the palladium catalyst performs this oxidation, it temporarily gains electrons. These extra electrons are passed on 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,” said Darryl Fong, a member of the MIT research team. “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, and once ethylene is no longer there, it goes from that transient, electron-rich state back to its original state.”

This new sensor is intended to be a more practical apparatus for field use. The MIT team has filed for a patent on the new device.