After sensing dangerous chemicals, the carbon-nanotube-enhanced plants send an alert.
Spinach is no longer just a superfood: By embedding leaves with carbon nanotubes, MIT engineers have transformed spinach plants into sensors that can detect explosives and wirelessly relay that information to a handheld device similar to a smartphone.
This is one of the first demonstrations of engineering electronic systems into plants, an approach that the researchers call “plant nanobionics.”
“The goal of plant nanobionics is to introduce nanoparticles into the plant to give it non-native functions,” says Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT and the leader of the research team.
In this case, the plants were designed to detect chemical compounds known as nitroaromatics, which are often used in landmines and other explosives. When one of these chemicals is present in the groundwater sampled naturally by the plant, carbon nanotubes embedded in the plant leaves emit a fluorescent signal that can be read with an infrared camera. The camera can be attached to a small computer similar to a smartphone, which then sends an email to the user.
“This is a novel demonstration of how we have overcome the plant/human communication barrier,” says Strano, who believes plant power could also be harnessed to warn of pollutants and environmental conditions such as drought.
Strano is the senior author of a paper describing the nanobionic plants in the Oct. 31 issue of Nature Materials. The paper’s lead authors are Min Hao Wong, an MIT graduate student who has started a company called Plantea to further develop this technology, and Juan Pablo Giraldo, a former MIT postdoc who is now an assistant professor at the University of California at Riverside.
Two years ago, in the first demonstration of plant nanobionics, Strano and Giraldo used nanoparticles to enhance plants’ photosynthesis ability and to turn them into sensors for nitric oxide, a pollutant produced by combustion.
Plants are ideally suited for monitoring the environment because they already take in a lot of information from their surroundings, Strano says.
“Plants are very good analytical chemists,” he says. “They have an extensive root network in the soil, are constantly sampling groundwater, and have a way to self-power the transport of that water up into the leaves.”
Strano’s lab has previously developed carbon nanotubes that can be used as sensors to detect a wide range of molecules, including hydrogen peroxide, the explosive TNT, and the nerve gas sarin. When the target molecule binds to a polymer wrapped around the nanotube, it alters the tube’s fluorescence.
In the new study, the researchers embedded sensors for nitroaromatic compounds into the leaves of spinach plants. Using a technique called vascular infusion, which involves applying a solution of nanoparticles to the underside of the leaf, they placed the sensors into a leaf layer known as the mesophyll, which is where most photosynthesis takes place.
They also embedded carbon nanotubes that emit a constant fluorescent signal that serves as a reference. This allows the researchers to compare the two fluorescent signals, making it easier to determine if the explosive sensor has detected anything. If there are any explosive molecules in the groundwater, it takes about 10 minutes for the plant to draw them up into the leaves, where they encounter the detector.
To read the signal, the researchers shine a laser onto the leaf, prompting the nanotubes in the leaf to emit near-infrared fluorescent light. This can be detected with a small infrared camera connected to a Raspberry Pi, a $35 credit-card-sized computer similar to the computer inside a smartphone. The signal could also be detected with a smartphone by removing the infrared filter that most camera phones have, the researchers say.
“This setup could be replaced by a cell phone and the right kind of camera,” Strano says. “It’s just the infrared filter that would stop you from using your cell phone.”
Using this setup, the researchers can pick up a signal from about 1 meter away from the plant, and they are now working on increasing that distance.
Michael McAlpine, an associate professor of mechanical engineering at the University of Minnesota, says this approach holds great potential for engineering not only sensors but many other kinds of bionic plants that might receive radio signals or change color.
“When you have manmade materials infiltrated into a living organism, you can have plants do things that plants don’t ordinarily do,” says McAlpine, who was not involved in the research. “Once you start to think of living organisms like plants as biomaterials that can be combined with electronic materials, this is all possible.”
“A wealth of information”
In the 2014 plant nanobionics study, Strano’s lab worked with a common laboratory plant known as Arabidopsis thaliana. However, the researchers wanted to use common spinach plants for the latest study, to demonstrate the versatility of this technique. “You can apply these techniques with any living plant,” Strano says.
So far, the researchers have also engineered spinach plants that can detect dopamine, which influences plant root growth, and they are now working on additional sensors, including some that track the chemicals plants use to convey information within their own tissues.
“Plants are very environmentally responsive,” Strano says. “They know that there is going to be a drought long before we do. They can detect small changes in the properties of soil and water potential. If we tap into those chemical signaling pathways, there is a wealth of information to access.”
These sensors could also help botanists learn more about the inner workings of plants, monitor plant health, and maximize the yield of rare compounds synthesized by plants such as the Madagascar periwinkle, which produces drugs used to treat cancer.
“These sensors give real-time information from the plant. It is almost like having the plant talk to us about the environment they are in,” Wong says. “In the case of precision agriculture, having such information can directly affect yield and margins.”
Learn more: Nanobionic spinach plants can detect explosives
Unused TV spectrum and drones could help make smart farms a reality
ON THE Dancing Crow farm in Washington, sunflowers and squashes soak up the rich autumn sunshine beside a row of solar panels. This bucolic smallholding provides organic vegetables to the farmers’ markets of Seattle. But it is also home to an experiment by Microsoft, a big computing firm, that it hopes will transform agriculture further afield. For the past year, the firm’s engineers have been developing a suite of technologies there to slash the cost of “precision agriculture”, which aims to use sensors and clever algorithms to deliver water, fertilisers and pesticides only to crops that actually need them.
Precision agriculture is one of the technologies that could help to feed a world whose population is forecast to hit almost 10 billion by 2050. If farmers can irrigate only when necessary, and avoid excessive pesticide use, they should be able to save money and boost their output.
But existing systems work out at $1,000 a sensor. That is too pricey for most rich-world farmers, let alone those in poor countries where productivity gains are most needed. The sensors themselves, which probe things like moisture, temperature and acidity in the soil, and which are scattered all over the farm, are fairly cheap, and can be powered with inexpensive solar panels. The cost comes in getting data from sensor to farmer. Few rural farms enjoy perfect mobile-phone coverage, and Wi-Fi networks do not have the range to cover entire fields. So most precision-agriculture systems rely on sensors that connect to custom cellular base stations, which can cost tens of thousands of dollars, or to satellites, which require pricey antennas and data plans.
In contrast, the sensors at Dancing Crow employ unoccupied slices of the UHF and VHF radio frequencies used for TV broadcasts, slotting data between channels. Many countries are experimenting with this so-called “white space”; to unlock extra bandwidth for mobile phones. In cities, tiny slices of the white-space spectrum sell for millions of dollars. But in the sparsely populated countryside, says Ranveer Chandra, a Microsoft researcher, there is unlicensed space galore.
Learn more: Precision agriculture