13 July 2008

Artificial Photosynthesis: So Much for Doom

Photosynthesis uses the freely available energy from the sun to power important chemical conversions. It is the basis for the entire human food and bioenergy chain. If you can mimic photosynthesis well enough, you can directly channel the energy of the sun to synthesise all types of energy-requiring chemical reactions. You can make any biofuel you can conceive of. You can produce unlimited hydrogen while removing carbon dioxide from the atomosphere hand over fist. Artificial photosynthesis is one of the keys to the treasure chest of solar energy.

Scientists have been working on artificial photosynthesis since at least the 1980s. Step by step, science is unlocking the secrets of bioenergy and mimicking the process via other means. A Chinese team led by Xian-Fu Zhang at the Hebei Normal University of Science and Technology in Qinhuangdao, China, has settled on the carbon nanotube as a central piece of its neo-photosynthetic model.
A carbon nanotube can accept one electron for every 32 carbon atoms it contains, and so even a short nanotube accepts many electrons, says Zhang. That means a carbon nanotube could act as the receiver molecule in artificial photosynthesis.

Although there are no known small molecules capable of releasing a large number of electrons after absorbing visible light, a class of molecule called the phthalocyanines (PCs) does release a single electron when it absorbs light.

Zhang's team realised that by covalently bonding a large number of PC molecules to a carbon nanotube, they could create a multiple electron system activated by visible light...They found that they could bond 120 PC molecules to a nanotube just 1 micrometer long, and that about 25% of the electrons donated from those PCs end up being stored in the nanotube.

"We decided to create this system initially simply to efficiently convert solar energy into electricity," says Zhang.

But he thinks the nanosystem could form a key component of an artificial photosynthesis model. The extra electrons stored in the nanotubes could be used to convert a chloroplast chemical called NADP into NADPH, which could then reduce carbon dioxide to carbohydrates. _NS_via_kurzweilai.net
The ability to bind the photo-electron donor to the nanotube electron acceptor is the first step. Then the electrons are used to convert NADP to NADPH, which reduces CO2 into more complex hydrocarbons such as carbohydrates or biofuels. It is just the bare beginning, but carbon nanotubes provide a great scaffolding for entire synthetic pathways--biochemical factories.


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