A rust-coloured sheet of porous gold covered in an invisible coating of plant proteins provides a whole new way to tap solar energy (Image: Peter Ciesielski)
Gold leaf doesn't grow on trees, but it can now harvest power from the Sun. A team of US chemical engineers has extracted photosynthetic molecules from plants and attached them to thin sheets of gold, creating a photosynthesising cyborg.
Organisms have been photosynthesising for at least 3.5 billion years, and over that time have developed elegant combinations of protein and light-absorbing dyes to help convert sunlight into power.
Rather than reinventing the wheel, Kane Jennings and Peter Ciesielski's team at Vanderbilt University in Nashville, Tennessee, decided to take those proteins to build their own photosynthetic devices.
The idea grew from the work of Elias Greenbaum at the Oak Ridge National Laboratory, also in Tennessee, who in the late 1990s showed that a protein complex, known as PS1, extracted from spinach leaves remained active when immobilised on a gold surface.
Since then the process for extracting PS1 from plants has been perfected, says Jennings, laying the ground for his group to use those light-harvesting proteins to make an artificial leaf.
Jennings and Ciesielski made their device using commercially available gold-silver alloy leaf.
Concentrated nitric acid was used to dissolve away the silver to leave gold leaf with nanoscale pores. This gave it a high surface area that allows a large amount of PS1 to be attached. It also made the leaf thin enough for light to penetrate. The finished material was stretched over a thicker gold substrate for support.
PS1 complexes were attached to the leaf by first coating the porous gold in thiols - chemical molecules that have a free end able to form strong bonds with the proteins.
When the complete cyborg leaf is placed under light, the PS1 complexes generate electrons that flow into the gold and can be harvested as electric current.
In a living plant, those electrons would be used to reduce compounds as part of a chemical chain that produces new energy stores in the form of carbohydrates.
The most rigorously tested artificial leaf produces a current of around 800 nanoamps per square centimetre. That is far from efficient enough to be economic, but the researchers are already experimenting with a new model.
"We are currently investigating PS1 films up to one micrometer thick," says Jennings. "These films can generate up to 2 microamps per centimetre square, and can power an inexpensive calculator."
Although the cyborg devices are still far behind the best silicon-based solar cells the new approach may become more competitive in future. The new design is relatively simple, and sourcing cheap plant leaves and a suitable substrate to make cyborg leaves from should be easy.
The system is still too delicate to be exposed to direct sunlight, which would burn out the PS1 proteins. Finding a way to protect them, and building leaves that pack in more of them will improve output further. That might even turn the artificial leaves green - they currently appear rusty red due to the properties of the nanoporous gold.
Greenbaum is impressed with the direction his original discovery has taken. "This is very nice work by an outstanding group," he says. "The results represent an important research advance in bio-inspired solar energy conversion."