Plants plus water is a powerful recipe

A limitless supply of energy from ordinary water could be a reality within five years. It would work by harnessing technology that appears, ready-made, in plant and algae cells. The method could also produce imaging devices and minute switches triggered by light.

Elias Greenbaum, leader of biotechnological research at Oak Ridge National Laboratory in the United States, has been exploring the unique ability of plants to split water, using sunlight as the energy source. The electron released by splitting the water is used as energy to power photosynthesis.

The key link in this chain of reactions is at the beginning, when the plant keeps apart the positively and negatively charged parts of the water molecule for long enough to siphon off the negatively charged electron. "We want to capture the electron before it does what it's meant to do," said Dr Greenbaum. "Instead, it would be used to provide electrical power."

The first step was made a few years ago when he and his team chopped up some spinach, isolated the chloroplasts, where the trapping of light occurs, and managed to deposit a layer of platinum on them. They placed the platinum so close to the system that splits the water that the electrons released flowed into the platinum.

Now the team has isolated the reaction centres in the chloroplasts where the key charge transfer occurs. They have orientated the reaction centres so that the platinised sides point in the same direction.

"These have technological potential because they are very small, very fast, they have good electrical properties and we have made a first step in how to wire them up," said Dr Greenbaum.

He said the tiny systems could be used to make optoelectronic switches with very fine resolution. Chloroplasts are five nanometres in length and each takes one millionth of a millionth of a second to generate one volt. "Here we're using individual molecules, which is a key reason why it's a revolutionary concept."

The biggest scientific problem he foresees is developing tiny molecular wires which will connect the reaction centres without interfering with each other.

Dr Greenbaum has also been harnessing single-cell algae to split water to produce hydrogen as an energy source. "We have even found certain marine algae that can split sea water. We're getting to a stage where we can custom-tailor designer algae."

Splitting water to produce energy is being attempted by other scientists using methods such as silicon photovoltaic devices. On the algae approach, Dr Greenbaum says: "The scientific problems will all be worked out within five years. But the next step will be the more difficult political problems."