How MIT researchers taught spinach to detect bombs

Could plants become a new line of national defense? Yes, with a little help from nanotechnology, suggest researchers from the Massachusetts Institute of Technology.

The researchers took ordinary spinach plants and embedded them with carbon nanotubes capable of detecting nitroaromatics, compounds often used in bombs and land mines. If these chemicals are present in groundwater, the plant will take in the water and begin to emit a fluorescent signal within 10 minutes, alerting observers to any potential problems.

Plants already detect even subtle changes in their environment. With nanotechnology, plants can now signal these changes to humans. Having that information may allow humans to quickly address a security issue or prepare for situations like droughts, the researchers say.

“Plants are very environmentally responsive. 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,” said Michael Strano, a professor of chemical engineering at MIT and the leader of the research team.

The field, which has come to be known as "plant nanobionics," allows a plant embedded with certain nanoparticles to assume a new function. Two years ago, Dr. Strano and his colleagues made the first demonstration in the field, using nanoparticles to improve plants’ photosynthesis. The plants also acted as sensors for nitric oxide, which is produced by burning fossil fuels.

How does the sensing process work? In this case, the research team applied a solution of nanoparticles to the underside of the spinach leaves. As the plant takes on water, any molecules of nitroaromatics attach themselves to a polymer wrapped around the nanotubes, causing a fluorescent glow within about 10 minutes of the plants absorbing water.

An infrared camera picks up the change. Some plants glow constantly, and this "control" mechanism makes it easy to tell when there’s a signal coming from many plants.

The camera relays the signal to a pocket-sized Raspberry Pi computer, which is available for as little as $35. The computer, in turn, sends an email that lets observers know what’s happening.

For now, the camera can read a signal from about 1 meter (3.3 feet) away. The researchers are working on improving that distance so that a single camera can survey more plants.

Any living plant can be deployed in this way, the researchers say. And different nanotubes glow when they detect different compounds, meaning that plants can be designed to communicate just about any change in their environment. 

The researchers are working on developing other applications for the technology, including warning humans about droughts. Professor Strano told the BBC that plants in public spaces could be used to detect terrorism-related activities, as well as monitoring seepage from buried munitions. The plants, he said, can pick up changes in the air as well as in water.

“This is a novel demonstration of how we have overcome the plant-human communication barrier,” Strano told MIT News.

The researchers’ new paper is available in the October 31 issue of Nature Materials.