The development from a team at MIT is said to be one of the first demonstrations of engineering electronic systems into plants, an approach dubbed plant nanobionics.
"The goal of plant nanobionics is to introduce nanoparticles into the plant to give it non-native functions," said Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT and the leader of the research team. Strano is the senior author of a paper describing the nanobionic plants in Nature Materials. The paper's lead author is Min Hao Wong, an MIT graduate student who has started a company called Plantea to further develop this technology.
The plants were designed to detect 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.
Strano's lab has previously developed carbon nanotubes that can be used as sensors to detect a wide range of molecules, including hydrogen peroxide, TNT, and 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 as a reference.
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. The signal could also be detected with a smartphone by removing the infrared filter that most camera phones have, the researchers said.
"This setup could be replaced by a cell phone and the right kind of camera," Strano said. "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 one metre away from the plant, and they are now working on increasing that distance.