Like a flood of needles, an onslaught of carbon nanotubes seeps in through a plant's leaves' and beelines toward each cells' chloroplasts—the hub where sunlight transforms into chemical energy. These nanotubes, thin straws of latticework atoms, pierce the energetic chloroplasts and imbed themselves like a thousand splinters. But rather than irritating or damaging the cell, the foreign particles are giving the plant a hardware upgrade.
A team of MIT scientists in the science journal Nature Materials that by integrating these nanotubes into the leaves of several lab plants, they were able to markedly enhance each plant's ability to perform photosynthesis. By creating the world's first bionic plants, they increased the plants' light absorption by 30 percent.
"You could really use any nanoparticle," says Michael Strano, a chemical engineer with the team. "We're just picking nanotubes because they have an interesting property. They absorb light, and by changing the diameter of the nanotubes you use, you can tune the [colors] of light that they'll absorb."
Key to the experiment's success was the scientists' ability to direct the nanotubes through the pores on the plant's leaves and into the chloroplasts without causing damage. A bit of serendipity was on the scientists' side: Quite by chance, the researchers discovered that by painting each nanotube in a film of DNA, they not only encouraged the chloroplasts to snatch up the nanotubes, but also allowed the nanotubes to merge through the chloroplasts' fragile membranes without breaking them.
"That was a remarkable and surprising finding," says Juan Pablo Giraldo, a plant biologist with the team. "At first we didn't think they'd be able to penetrate. There was some proof that it was possible, but it seemed like a wild idea."
There is plenty the researchers still don't know about their bionic plants. While the scientists are sure that the nanotubes allow the plants to absorb more light, chiefly ultraviolet and green light, how exactly that sunlight energy is being transferred downstream is still up for debate. "There are a few different mechanisms that could be being used," Strano says. "The nanotubes could be directly producing electrons, or they could be doing something called resonance energy transfer," in which they produce an energetic particle called an exciton, he says. "But we're not exactly sure and I don't want to speculate too much."
Near infrared fluorescence of carbon nanotubes (orange) infiltrated inside leaves (green) could boost photosynthesis and enable the detection of biochemicals and pollutants. Credit: Juan Pablo Giraldo and Nicole M. Iverson
The preliminary results suggest that the addition of carbon nanotubes has no effect on plant health or longevity, according to the team. But, Ramaraja Ramasamy who studies nanomaterials in biological systems at the University of Georgia and was not involved in the experiment, insists that longer-term research will be needed before we know for sure."When a foreign object enters a biological cell, it's not treated lightly by the biological system," Ramasamy says.
While carbon nanotubes and similar nanomaterials have been around for a few decades, the researchers admit it's no surprise that nobody had succeeded in an experiment like this before, given how little research overlap exists between materials science and plant biology. Asked how this research would resonate with the larger scientific field, Strano said, "Well, that's a tough question because there really is no field."
But the researchers are excited about the potential applications. Strano already has built a plant that uses similar carbon nanotubes that react to chemicals in the air, meaning the plant itself acts as a chemical detector. The nanotube produces energy when it comes in with the chemical, which can be picked up by a watching infrared camera. He says this concept could be modified to detect anything from explosives to poisonous gasses.
As for the cost, Giraldo says this is far from being a Six Million Dollar Plant. Including the cost of the DNA coating, he says, a liter of the nanoparticle solution that he applied to his plant, in which the carbon nanotubes are diluted, could be made for roughly ten dollars. Considering Giraldo used only 100 microliters per leaf, that's a windfall at about a tenth of a cent per plant. Ramasamy, though he applauds this work and is admittedly excited, pumps the breaks on the enthusiasm for soon-to-come applications. "I can't imagine that we'll be seeing commercial applications from this technology sooner than 5 to 10 years from now. And as for chemical detectors, the real question is if the plants function better than the currently available options."
Strano and Giraldo admit that what they've accomplished is really half the equation for engineering more efficient and powerful plants. Absorbing light in only one part of the photosynthesis process, Giraldo says. Using nanomaterials to increase the speed at which a plant can store that energy is a separate project. But the work they've done is a strong first step.
"Blending this world of inorganic and organic matter will allow us to combine the various qualities of different nanomaterials and with the enormous diversity plants," Giraldo says. "The applications for this research are potentially endless. Who knows what we'll find?"