Plants make life as we know it possible. It all starts with the tiny organelles within a plant's leaves, known as chloroplasts. These chloroplasts—diminished descendants of the first photosynthesizers, cyanobacteria—use incoming sunlight to split water molecules and then knit together the energy-rich carbon and hydrogen compounds found in everything from food to fossil fuels. The leftover “waste” is the oxygen that we and the rest of the animal kingdom depend on to survive and thrive.



But chloroplasts aren't very efficient. They do not absorb green light (which is why most plants appear green) as well as the sun's heat, also known as infrared light. They generally waste a lot more sunlight than they use; photosynthesis maxes out at roughly 10 percent of the incoming sunshine. So why not give flora, and the chloroplasts within their leafy photosynthetic machines, a boost?



That's exactly what a group of chemical engineers and biochemists attempted in a new study, embedding single-walled carbon nanotubes—microscopic tubes thinner than a human hair that can also absorb sunlight and convert it to electron flow—in living chloroplasts. The paper is published in Nature Materials on 16 March. (Scientific American is part of Nature Publishing Group.)



"Plants have, for a long time, provided us with valuable products like food, biofuels, construction materials and the oxygen we breathe," notes plant biologist turned chemical engineer Juan Pablo Giraldo, a postdoctoral fellow in the research lab at the Massachusetts Institute of Technology who did the work. "We envisioned them as new hybrid biomaterials for solar energy harnessing, self-repairing materials [and] chemical detectors of pollutants, pesticides, [and] fungal and bacterial infections."



First, the researchers removed the chloroplasts from some spinach leaves and put it in a sugary solution. The researchers then introduced the carbon nanotubes, which embedded in the cell's fatty walls when treated with DNA to take on a negative charge or chitosan (a derivative of the material comprising insect exoskeletons) for a positive charge. This penetration happens within seconds and doesn't require heat, a catalyst or anything else, according to the researchers. The move also appears to be irreversible and complete. No nanotubes remained floating outside the chloroplasts in these experiments.



Even better, the trick also works on chloroplasts in living plants. Introduced carbon nanotubes found the chloroplasts in the leaves of an Arabidopsis thaliana, a small flowering plant often used in such studies. Perhaps more important, it did not kill the leaves or the plant over a period of several weeks.



That discovery alone may prove of interest if the nanotubes could be used to deliver packets of DNA for specific functions into the chloroplast. For example, the researchers used the nanotubes to carry nanosize particles of ceria, a compound composed of the rare earth metal cerium and oxygen, into the chloroplasts. The nanoceria then seemed to help remove some of the oxygen created by the process of photosynthesis that often ends up wrecking the cellular machinery.



But the carbon nanotubes themselves also somehow seemed to make photosynthesis better, all on their own. After six hours chloroplasts with carbon nanotubes in them boasted enhanced photosynthesis rates three times higher than untreated chloroplasts, including increasing the transport of electrons freed by sunshine by 49 percent. "We have shown that carbon nanotubes promote the conversion of light energy into electron transfer within the chloroplasts," Giraldo explains. However, "it is unknown if they can impact the ultimate products of photosynthesis," meaning the impact on the ultimate production of plant food—sugars like glucose—remains unclear.



The technique worked both in isolated chloroplasts as well as ones still in leaves, although the exact mechanism is also unknown. The mostly likely explanation is that the carbon nanotubes absorb more sunlight and donate electrons to the photosynthetic process, according to Giraldo and the rest of the team, or the nanotubes may be speeding up the transfer of electrons within the photosynthetic processes. But proof will require more work.



The nanotubes also showed that they could act as sensors. Foliage with nanotubes fluoresce under infrared light, but stop emitting light in the presence of nitric oxide, a compound common both to plants and pollution. Such “bionic” plants could be used as "biochemical detectors for monitoring environmental conditions in cities, crop fields, airports or high-security facilities," Giraldo says, noting that such sensors could also be self-repairing the way plants are.



In the end, the research suggests that greenery enhanced with carbon nanotubes could potentially produce more from sunlight, air and water, although adding such nanomaterials would be both laborious and may have unknown long-term impacts on the vegetation as a whole as well as on the environment. It may prove a long time before expensive carbon nanotubes find a home in dirt to help boost the function of crops that take them up through their roots.



In the meantime one idea might be to take the research a step back and see if carbon nanotubes have similar impacts in the root of all chloroplasts: cyanobacteria, according to Giraldo. After all, if the nanoparticles can boost photosynthesis in those simple cellular organisms, then fuel, food and other products derived from algae might become more viable.

