In 1870, German chemist Erich Von Wolf calculated the iron content in various vegetables.  Thanks to a misplaced decimal in his notebook, he made the iron content of spinach ten times of what it was in reality, and spinach has been hailed as a super-food ever since.  Spinach might not be as iron-packed as Von Wolf led us to believe, but a recent study at MIT suggests that it has a previously unknown, and even more amazing, property.  By embedding spinach leaves with carbon nanotubes, engineers have transformed the plants into sensors that can detect explosives and transmit that information to a handheld device.  This is one of the first demonstrations of “plant nanobionics”, or engineering electronic systems into plants.  

Plant nanobionics is meant to introduce nanoparticles into the plant, giving it non-native functions.  Plants in this case were designed to detect chemical compounds that are often used in landmines and explosives, known as nitroaromatics.  When one of these chemicals is in the groundwater naturally sampled by the plant, then the carbon nanotubes embedded in the plant emit a fluorescent signal, which can be read with an infrared camera, which in turn can be attached to a small handheld device that sends an email to the user.  

In the first demonstration of plant nanobionics two years earlier, researchers used nanoparticles to enhance the photosynthesis of a plant and then turn them into nitric oxide sensors.  Plants are particularly well-suited to monitor the environment, since they already take in a good amount from their surroundings.  Their extensive root network in the soil means that they’re constantly sampling groundwater.  

In this new study, the researchers embedded sensors into the leaves of spinach plants.  Using vascular infusion, a technique that involves applying a solution of nanoparticles to the underside of the leaf, they placed the sensors into a layer of the leaf where most photosynthesis takes place.  They also embedded carbon nanotubes emitting a constant fluorescent signal that serves as a reference, allowing the researchers to compare the two signals.  The researchers would shine a laser onto the leaf to read the signal, which prompted the nanotubes in the leaf to emit near-infrared fluorescent light that can be detected through a small infrared camera connected to a handheld device called a “Raspberry Pi”.  The researchers can use this setup to pick up a signal from about one meter away from the plant.  

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