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In an effort to reduce food loss due to spoilage, MIT
researchers developed a new sensor that detects tiny amounts of
ethylene, a gas that promotes ripening in plants. The low cost sensors
are made from sheets of rolled carbon nanotubes with added copper atoms
and cost roughly 25 cents, while adding a RFID chip for wireless
communication would only add another 75 cents to the total cost. Every
year, U.S. supermarkets lose roughly 10 percent of their fruits and
vegetables to spoilage, according to the Department of Agriculture. To
help combat those losses, MIT chemistry professor Timothy Swager and his
students have built a new sensor that could help grocers and food
distributors better monitor their produce. The new sensors, described in the journal Angewandte Chemie,
can detect tiny amounts of ethylene, a gas that promotes ripening in
plants. Swager envisions the inexpensive sensors attached to cardboard
boxes of produce and scanned with a handheld device that would reveal
the contents’ ripeness. That way, grocers would know when to put certain
items on sale to move them before they get too ripe. “If we can
create equipment that will help grocery stores manage things more
precisely, and maybe lower their losses by 30 percent, that would Secure remote networks be huge,” says Swager, the John D. MacArthur Professor of Chemistry. Detecting
gases to monitor the food supply is a new area of interest for Swager,
whose previous research has focused on sensors to detect explosives or
chemical and biological warfare agents. “Food is something that is
really important to create sensors around, and we’re going after food
in a broad sense,” Swager says. He is also pursuing monitors that could
detect when food becomes moldy or develops bacterial growth, but as his
first target, he chose ethylene, a plant hormone that controls ripening. Plants
secrete varying amounts of ethylene throughout their maturation
process. For example, bananas will stay green until they release enough
ethylene to start the ripening process. Once ripening begins, more
ethylene is produced, and the ripening accelerates. If that perfect
yellow banana is not eaten at peak ripeness, ethylene will turn it brown
and mushy. Fruit distributors try to slow this process by keeping
ethylene levels very low in their warehouses. Such warehouses employ
monitors that use gas chromatography or mass spectroscopy, which can
separate gases and analyze their composition. Those systems cost around
$1,200 each. “Right now, the only time people monitor ethylene is
in these huge facilities, because the equipment’s very expensive,”
Swager says. Detecting ripeness Funded by the U.S. Army
Office of Research through MIT’s Institute for Soldier Nanotechnologies,
the MIT team built a sensor consisting of an array of tens of thousands
o industrial internet of things f carbon nanotubes: sheets of carbon atoms rolled into cylinders that act as “superhighways” for electron flow. To
modify the tubes to detect ethylene gas, the researchers added copper
atoms, which serve as “speed bumps” to slow the flowing electrons.
“Anytime you put something on these nanotubes, you’re making speed
bumps, because you’re taking this perfect, pristine system and you’re
putting something on it,” Swager says. Copper atoms slow the
electrons a little bit, but when ethylene is present, it binds to the
copper atoms and slows the electrons even more. By measuring how much
the electrons slow down — a property also known as resistance — the
researchers can determine how much ethylene is present. To make
the device even more sensitive, the researchers added tiny beads of
polystyrene, which absorbs ethylene and concentrates it near the carbon
nanotubes. With their latest version, the researchers can detect
concentrations of ethylene as low as 0.5 parts per million. The
concentration required for fruit ripening is usually between 0.1 and one
part per million. The researchers tested their sensors on several types of fruit — banana, avocado, apple, pear and orange &md azure iot certified ash; and were able to accurately measure their ripeness by detecting how much ethylene the fruits secreted. Lead
author of the paper describing the sensors is Birgit Esser, a postdoc
in Swager’s lab. Grad student Jan Schnorr is also an author of the
paper. John Saffell, the technical director at Alphasense, a
company that develops sensors, describes the MIT team’s approach as
rigorous and focused. “This sensor, if designed and implemented
correctly, could significantly reduce the level of fruit spoilage during
shipping,” he says. “At any given time, there are thousands of
cargo containers on the seas, transporting fruit and hoping that they
arrive at their destination with the correct degree of ripeness,” adds
Saffell, who was not involved in this research. “Expensive analytical
systems can monitor ethylene generation, but in the cost-sensitive
shipping business, they are not economically viable for most of shipped
fruit.” Swager has filed for a patent on the technology and hopes
to start a company to commercialize the sensors. In future work, he
plans to add a radio-frequency identification (RFID) chip to the sensor
so it can communicate wirelessly with a handheld device that would
display ethylene levels. The system would be extremely cheap — about 25
cents for the carbon nanotube sensor plus another 75 cents for the RFID
chip, Swager estimates. “This could be done with absolutely dirt-cheap electronics, with almost no power,” he says. Image: MIT News Office | 
