Scientists have made a fundamental breakthrough in their attempts to replicate photosynthesis – the ability of plants to harvest the power of sunlight – in the hope of making unlimited amounts of "green" energy from water and sunlight alone.
The researchers have assembled genetically modified viruses into wire-like structures that are able to use the energy of the sun to split water molecules into their constitute parts of oxygen and hydrogen, which can then be used as a source of chemical energy.
If the process can be scaled up and made more efficient, it promises to produce unlimited quantities of hydrogen fuel, a clean source of energy that can be used to generate electricity as well as acting as a portable, carbon-free fuel for cars and other vehicles.
Replicating photosynthesis – in which plants convert sunlight into a store of chemical energy – has been a dream of the alternative energy business for decades. The drive was given an extra boost yesterday with warnings by the US military that there could be serious global oil shortages by 2015.
Splitting water molecules into oxygen and hydrogen is seen as a critical first step in this process of artificial photosynthesis. Although it is possible to split the molecules using solar electricity, the process is not very efficient. In the latest study, scientists were able to split water directly with sunlight, without using solar panels.
Plants convert sunlight into chemical energy using the green chlorophyll pigment found in leaves, which traps packets of light and uses the energy to transport electrons from one molecular complex to another within the plant's cells. The end result is the conversion of carbon dioxide and water into glucose, which can be stored as starch or as other forms of plant carbohydrates.
Some researchers have tried to emulate this natural photosynthetic process by using the appropriate parts of plants but these structures tend to be unstable. Instead, Angela Belcher at the Massachusetts Institute of Technology borrowed the method rather than using the components of plants.
The scientists genetically engineered a harmless virus called M13, which normally infects bacteria, so that it would bind to a catalyst called iridium oxide and a biological pigment, zinc porphyrins. The viruses naturally arranged themselves into wire-like structures and the catalyst and pigments effectively harvested sunlight to split the oxygen from the water molecules.
Professor Belcher said that the role of the pigment was to act like an antenna to capture the light and then transfer the energy down the length of the virus. She said: "The virus is a very efficient harvester of light, with these porphyrins attached. We use the components people have used before but we use biology to organise them for us, so you get better efficiency."
So far, the team has only been able to split off oxygen, which is the most challenging part of the water-splitting process. The hydrogen splits into its component parts, protons and electrons, and the next stage is to complete the task of bringing these together in order to collect gas separately, Professor Belcher said.
The study, published in the journal Nature Nanotechnology, is only the first stage of a much longer process of development. In addition to refining the hydrogen-splitting part of the reaction, the scientists still have a long way to go to increase the efficiency at which sunlight is converted into chemical energy. Nevertheless, Professor Belcher predicted that it should be possible to develop the idea into a prototype commercial product that can carry out the entire water-splitting process in a durable, self-sustaining way within two years.
Professor Thomas Mallouk of Pennsylvania State University, said the research was "an extremely clever piece of work" that addressed one of the most difficult problems in artificial photosynthesis by organising molecular components in a manner that could control the critical transfer of electrons, just like real photosynthesis.
He said: "There is a daunting combination of problems to be solved before this or any other artificial photosynthetic system could be useful for energy conversion." For a start, it needs to be at least 10 times more efficient than natural photosynthesis, to use less expensive material and to be able to repeat chemical reactions over and over, billions of times.
Professor Mallouk said: "This is unlikely to happen in the near future. Nevertheless, the design idea illustrated in this [study] could ultimately help with an important piece of the puzzle."
Solar power: Photosynthesis
* Photosynthesis is probably the most critical chemical process on earth. It mostly takes place in tiny structures called chloroplasts found inside the cells of a plant's leaves.
* The energy that drives the photosynthetic process comes from sunlight and the end product is a store of carbohydrate energy.
* A green pigment called chlorophyl traps the sunlight and begins the process of photosynthesis. It is green because it absorbs light at the blue ends of the visible light spectrum.
* Photosynthesis is affected by temperature, light intensity, light wavelength and carbon dioxide concentrations.
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