With few exceptions, every single plant on this planet can convert light energy into chemical energy. And they do a pretty good job of this, even though they only use about 10 percent of the sunlight available to them. Now, researchers have found a way to enhance photosynthetic activity in plants, and the same method can be used to bolster them with new, unnatural powers -- turning them into self-powered, supercharged energy producers, biochemical detectors, and sensors for pollutants and explosives.
“Plants are very attractive as a technology platform,” MIT’s Michael Strano says in a news report. “They repair themselves, they’re environmentally stable outside, they survive in harsh environments, and they provide their own power source and water distribution.”
Using nanotechnology to enhance plants’ native functions (like energy production) and impart them with completely new functions means that one day, we can create materials that grow and repair themselves using just sunlight, water, and carbon dioxide. They’re calling this new field, “plant nanobionics.”
First, a little plant biology lesson. Chloroplasts are the organelles in plants where photosynthesis happens. When the green pigment chlorophyll absorbs light, this excites the electrons flowing through the membranes of the chloroplast. This electrical energy is captured and used to power the second stage of photosynthesis: building sugars. If removed from the plant, chloroplasts can continue performing these reactions for a few hours -- but light and oxygen will eventually damage their photosynthetic proteins.
So, how do you protect chloroplasts from breaking down outside the plant? This brings us to step 1: Deliver synthetic nanoparticles to infiltrate chloroplasts.
Ceria is a rare-earth metal oxide. Polymer nanoparticles containing ceria -- called cerium oxide nanoparticles or nanoceria -- are very strong antioxidants that scavenge oxygen radicals, highly reactive molecules produced by light and oxygen.
They wrapped the particles in polyacrylic acid (a highly charged molecule), which allows the particles to penetrate the fatty membrane that surrounds chloroplasts.
They call this delivery technique lipid exchange envelope penetration (LEEP), and levels of damaging molecules dropped dramatically in chloroplasts embedded with nanoceria.
Step 2: Enhance photosynthetic activity. Because plants typically utilize only 10 percent of the sunlight available, the team had to design some sort of “prosthetic photoabsorber.”
They developed carbon nanotubes that act as artificial antennae, allowing chloroplasts to capture wavelengths of light outside their normal range, including ultraviolet, green, and near-infrared.
Using LEEP, semiconducting carbon nanotubes coated with negative charges were embedded into the chloroplasts.
By broadening the spectrum of captured light, photosynthetic activity was 49 percent greater than in extracted chloroplasts without embedded nanotubes.
When nanoceria and carbon nanotubes were delivered together, the chloroplasts remained active for a few extra hours.
Step 3: Boost photosynthesis in living plants and give them new functions. They turned to Arabidopsis thaliana, a small flowering plant related to mustard.
They applied a solution of nanoparticles to the underside of the leaf, which contain tiny pores for carbon dioxide and oxygen to flow through. That’s where the nanoparticles penetrated.
The nanotubes moved into the chloroplasts of the plant, boosting its ability to capture light energy by 30 percent.
Then, using another type of carbon nanotube -- one that detects the presence of the gas nitric oxide, an environmental pollutant produced by combustion -- the researchers turned Arabidopsis plants into biochemical sensors.
Strano’s lab has previously developed carbon nanotube sensors for many different chemicals: hydrogen peroxide, the explosive TNT, and the nerve gas sarin. When the target molecule binds to a polymer wrapped around the nanotube, it alters the nanotube’s fluorescence. (Pictured above, imaging the fluorescence of carbon nanotubes inside leaves of an Arabidopsis plant using a near infrared microscope.) By adapting the sensors to different targets, the researchers hope to one day develop plants that could be used to monitor environmental pollution, pesticides, fungal infections, or exposure to bacterial toxins.
Janet Fang has written for Nature, Discover and the Point Reyes Light. She is currently a lab technician at Lamont-Doherty Earth Observatory. She holds degrees from the University of California, Berkeley and Columbia University. She is based in New York.
Follow her on Twitter.