Mar 19 2020
Flowers and fruits emit a colorless and sweet-smelling gas, known as ethylene, as they bloom and ripen.
To test the new 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—red carnations and purple lisianthus. Image Credit: Stock imagery edited by MIT News.
Chemists at Massachusetts Institute of Technology (MIT) have developed a miniature sensor that is capable of detecting this gas even in very low concentrations, for example, 15 parts per billion, which according to them, could help prevent food spoilage.
According to Timothy Swager, the John D. MacArthur Professor of Chemistry at MIT, the sensor, which is produced from semiconducting cylinders known as carbon nanotubes, can potentially be used for examining vegetables and fruits as they are transported and stored and thus help reduce food waste.
There is a persistent need for better food management and reduction of food waste. 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.
Timothy Swager, John D. MacArthur Professor, Department of Chemistry, MIT
Ethylene serves as a natural plant hormone and is also the most extensively produced organic compound in the world. It is also used for manufacturing clothing and plastic products.
According to the scientists, an ethylene detector could also prove helpful for monitoring this kind of industrial production of ethylene.
The study was recently published in the ACS Central journal. Swager is the study’s senior author, while Darryl Fong, a MIT postdoc, is the lead author. Other authors of the study included Shao-Xiong (Lennon) Luo, a graduate student from MIT, and Rafaela Da Silveira Andre, a visiting scholar.
Ripe or Not
A majority of the plants produce ethylene and use it as a hormone to trigger ripening, growth, and other important phases of their life cycle. For example, as bananas ripen and become brown, they create an increasing amount of ethylene, and flowers also produce this gas as they prepare to bloom.
Stressed flowers and produce can generate ethylene in excess amounts and this causes them to wilt or ripen ahead of time. According to the U.S. Department of Agriculture, supermarkets in the United States are estimated to lose approximately 12% of their vegetables and fruits every year, because of spoilage.
Swager’s laboratory created an ethylene sensor in 2012. This sensor included arrays of tens of thousands of carbon nanotubes that enable electrons to flow along with them; however, the addition of copper atoms to this by the researchers slowed down the electron flow.
When ethylene gas is present, it binds to the copper atoms and further slows down the flow of electrons. Measurement of this slowdown can help to identify the amount of ethylene present. But this sensor can only identify ethylene concentrations as low as 500 parts per billion, and since the sensor includes copper, it may be ultimately corroded by oxygen and may cease to work.
There still is not a good commercial sensor for ethylene. 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.
Timothy Swager, John D. MacArthur Professor, Department of Chemistry, MIT
A new type of ethylene sensor was eventually developed by Swager and Fong which is also based on carbon nanotubes. But this sensor works by Wacker oxidation, which is entirely a different mechanism compared to conventional sensors. Rather than integrating a metal like copper that directly couples to ethylene, the researchers utilized a metal catalyst, known as palladium, that introduces oxygen to ethylene during the oxidation process.
The palladium catalyst momentarily gains electrons as it carries out this oxidation. This catalyst subsequently passes these additional electrons to carbon nanotubes and thus renders them more conductive. The scientists can identify the presence of ethylene by quantifying the ensuing change in current flow.
Within a few seconds of exposure, the sensor reacts to ethylene and returns to its baseline conductivity in just a few minutes as soon as the gas vanishes.
“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,” stated Fong.
The repurposing of the Wacker oxidation catalytic system for ethylene detection was an exceptionally clever and fundamentally interdisciplinary idea. The research team drew upon recent modifications to the Wacker oxidation to provide a robust catalytic system and incorporated it into a carbon nanotube-based device to provide a remarkably selective and simple ethylene sensor.
Zachary Wickens, Assistant Professor, Department of Chemistry, University of Wisconsin
Wickens was not involved in the research.
In Bloom
To test the capabilities of the new sensor, the scientists deposited the carbon nanotubes and other components of the sensor onto a glass slide. They subsequently used the sensor to track the production of ethylene in two kinds of flowers—purple lisianthus and carnations. The researchers quantified the production of ethylene over a period of five days, enabling them to monitor the association between the levels of ethylene and the flowering of plants.
In their research on carnations, the scientists identified that ethylene levels increased rapidly during the first day of the experiment, and the flowers blossomed soon after that, and all this occurred within one day or two.
Purple lisianthus flowers exhibited a slower increase in ethylene concentration that began on the first day and continued until the fourth day, when it began to reduce. Correspondingly, the blooming of flowers was spread out across a number of days, and a few had still not bloomed towards the end of the experiment.
In addition, the scientists explored whether the plant food packets that came along with the flowers had any effect on the production of ethylene. They discovered that plants that received the food displayed insignificant delays in the production of ethylene and blooming; however, this was not a major impact (just a few hours).
The MIT researchers have now filed for a patent on the latest sensor. The study was financially supported by the U.S. Army Engineer Research and Development Center Environmental Quality Technology Program, the National Science Foundation, the Sao Paulo Research Foundation, and the Natural Sciences and Engineering Research Council of Canada.