In 2010, Stanford University researchers reported harnessing energy directly from chloroplasts, the cellular "power plants" within plants where photosynthesis takes place. Now, by embedding different types of carbon nanotubes into these chloroplasts, a team at MIT has boosted plants' ability to capture light energy. As well as opening up the possibility of creating "bionic plants" with enhanced energy production, the same approach could be used to create plants with environmental monitoring capabilities.
Chloroplasts are self-contained units that contain all the machinery required for photosynthesis – the conversion of sunlight into chemical energy. Although they can still function when removed from plants, they start to break down after a few hours because light and oxygen damage the photosynthetic proteins. This damage is usually repaired by the plants, but chloroplasts are unable to do this on their own.
As part of an attempt to enhance the photosynthetic function of chloroplasts that were extracted from plants for possible use in solar cells, the MIT research team led by Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering, embedded them with cerium oxide nanoparticles. These nanoparticles, which are also known as nanoceria, are very strong antioxidants and the hope was that they would protect the chloroplasts from damage and prolong their productivity by scavenging oxygen radicals and other highly reactive molecules produced by light and oxygen.
The nanoceria were delivered into the chloroplasts using a new technique called lipid exchange envelope penetration (LEEP), which was developed by the team. This involves wrapping the nanoparticles in polyacrylic acid, a highly charged molecule, which allows the particles to penetrate the fatty, hydrophobic membranes that surrounds the chloroplasts. Using this technique, the researchers were able to significantly reduce the levels of the damaging molecules.
Building on this research, the team then used the LEEP technique to embed semiconducting carbon nanotubes coated in negatively charged DNA into the chloroplasts. The scientists believed that the carbon nanotubes could allow the plants to make use of more than the 10 percent of sunlight they usually make use of by acting as artificial antennae that would capture wavelengths of light beyond their normal range, such as ultraviolet, green and near-infrared.
Measuring the rate of electron flow through the thylakoid membranes within the chloroplasts, the researchers saw an increase in photosynthetic activity of 49 percent compared to isolated chloroplasts without the embedded nanotubes. Chloroplasts to which both nanoceria and carbon nanotubes were delivered together also remained active for a few hours longer than normal.
To test the approach on living plants, the team then used a technique called vascular infusion to deliver nanoparticles to Arabidopsis thaliana, a small flowering plant commonly known as thale cress. This involved applying a solution of nanoparticles to the underside of the leaf, where it penetrated the plant through tiny pores through which the plant usually takes in carbon dioxide in and expels oxygen. The nanotubes made their way to the chloroplasts, resulting in a boost in photosynthetic electron flow of about 30 percent.
Photosynthesis involves two stages. The first sees green chlorophyll pigments absorb light, which excites electrons that flow through the thylakoid membranes within the chloroplasts. This electrical energy is then captured by the plant to power the second stage – the production of sugars. The researchers say it is still unclear how boosting the electron flow using nanoparticles affects the plants' sugar production.
Plants as chemical detectorsThe MIT team says the same approach used to enhance the Arabidopsis thaliana plants' energy production could also be used to turn them into chemical sensors. MIT researchers have previously developed carbon nanotube sensors that can identify various different chemicals, including hydrogen peroxide, TNT and sarin. These consist of carbon nanotubes that glow when a polymer in which they are wrapped binds with the target molecule.
"We could someday use these carbon nanotubes to make sensors that detect in real time, at the single-particle level, free radicals or signaling molecules that are at very low-concentration and difficult to detect," says postdoc and plant biologist Juan Pablo Giraldo.
Similar to the EU-funded PLants Employed As SEnsing Devices (PLEASED) project, which aims to develop plants capable of measuring a variety of chemical and physical parameters, such as pollution, temperature, humidity, sunlight, acid rain, and the presence of chemicals in organic agriculture, the MIT team hopes to create plants that acts as biosensors to monitor environmental pollution, pesticides, fungal infections, or exposure to bacterial toxins.
"Plants are very attractive as a technology platform," says Strano. "They repair themselves, they’re environmentally stable outside, they survive in harsh environments, and they provide their own power source and water distribution."
As well as working towards self-powered chemical detectors, the researchers say they are also working to incorporate electronic nanomaterials and devices, such as graphene, into plants.
The team's paper detailing their "bionic plants" appears in the journal Nature Materials.