Massachusetts Institute of Technology (MIT) researchers have found a novel way to mimic the process by which plants use sunlight to split water into hydrogen and oxygen. They used a modified virus as a “biological scaffold” that can assemble the nanoscale components needed to split a water molecule into hydrogen and oxygen atoms.
The new technique, described in Nature Nanotechnology, could be an important element of the much touted hydrogen economy that some energy experts believe will prevail in the future. By using sunlight to make hydrogen from water, the hydrogen can then be stored and used at any time to generate electricity using a fuel cell, or to make liquid fuels (or be used directly) for cars and trucks. While other researchers have made systems that use electricity to split water molecules, the new biologically based system skips the intermediate steps and uses sunlight to power the reaction directly.
The team, led by Angela Belcher, engineered a common bacterial virus called M13 so that it would attract and bind with molecules of a catalyst (in this case, iridium oxide) and a biological pigment (zinc porphyrins). The viruses became wire-like devices that could very efficiently split the oxygen from water molecules. Over time, however, the virus-wires would clump together and lose their effectiveness, so the researchers added an extra step: encapsulating them in a microgel matrix, so they maintained their uniform arrangement and kept their stability and efficiency.
In the team’s system, the viruses simply act as a kind of scaffolding, causing the pigments and catalysts to line up with the right kind of spacing to trigger the water-splitting reaction. The role of the pigments is “to act as an antenna to capture the light,” Belcher explains, “and then transfer the energy down the length of the virus, like a wire. The virus is a very efficient harvester of light, with these porphyrins attached. We use components people have used before, but we use biology to organize them for us, so you get better efficiency.”
Commenting on the work, Thomas Mallouk, the DuPont Professor of Materials Chemistry and Physics at Pennsylvania State University, said: “This is an extremely clever piece of work that addresses one of the most difficult problems in artificial photosynthesis, namely, the nanoscale organization of the components in order to control electron transfer rates.”
But according to Mallouk, there are still many problems to be solved before this or any other artificial photosynthetic system could actually be useful for energy conversion. “To be cost-competitive with other approaches to solar power,” he says, “the system would need to be at least 10 times more efficient than natural photosynthesis, be able to repeat the reaction a billion times, and use less expensive materials. This is unlikely to happen in the near future, nevertheless, the design idea illustrated in this paper could ultimately help with an important piece of the puzzle.”