[tt] Nanotubes bring artificial photosynthesis a step nearer

[Eugen Leitl]

http://technology.newscientist.com/article.ns?id=dn14297&print=true

Nanotubes bring artificial photosynthesis a step nearer

    * 10:20 11 July 2008

    * NewScientist.com news service

    * Colin Barras

Carbon nanotubes are the crucial chemical ingredient that could make
artificial photosynthesis possible, say a team of Chinese researchers. The
team has found that nanotubes mimic an important step in photosynthesis that
chemists have been unable to copy until now.

Artificial photosynthesis has the potential to efficiently produce hydrogen
that could be used as a clean fuel for vehicles. It could also be used to mop
up carbon dioxide from the atmosphere.

Photosynthetic organisms use the energy from light to break down water into
oxygen and hydrogen. The hydrogen then reacts with carbon dioxide to help
synthesise carbohydrates, the molecules organisms use to store energy.

Chemists have long tried in vain to reproduce the process, but one key step
in particular has proven impossible to copy.

Visible photons can only contribute a limited amount of energy towards a
chemical reaction. This energy is absorbed by electrons involved in the
reaction.  Elusive goal

Reactions that require more energy, such as the synthesis of carbohydrates,
can only proceed when several energised electrons are available to
contribute. For that reason, chemists say the photosynthesis falls into a
class of reactions known as multiple electron systems.

But nobody has succeeded in making artificial multiple electron systems that
could provide the necessary energy for artificial photosynthesis.

Such a system would comprise of a donor molecule that can absorb visible
light and release many electrons, and a receiver molecule capable of
accepting and storing those electrons. Existing systems can donate and
receive only one electron at a time.  Nanotube key

Now, a team led by Xian-Fu Zhang at the Hebei Normal University of Science
and Technology in Qinhuangdao, China, has found that single-walled carbon
nanotubes could act as the chemical heart of a multiple electron system.

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.

'Basic requirement'

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.

James Barber at Imperial College London, UK, is an expert in photosynthesis.
"A lot of people working in this area don't address a basic requirement –
that you need to have multiple electrons in photosynthesis," he says. "I
think these researchers are right to make this an issue."

Journal Reference: ChemPhysChem (DOI: 10.1002/cphc.200800191)

Related Articles

    * Revealing the dawn of photosynthesis

    * http://technology.newscientist.com/article/mg19125654.200

    * 19 August 2006

    * Technology: Synthetic molecule mimics photosynthesis

    * http://technology.newscientist.com/article/mg12617183.600

    * 26 May 1990

    * Take a leaf out of nature's book to tap solar power

    * http://technology.newscientist.com/article/mg19225776.700

    * 13 November 2006

Weblinks

    * Xian-Fu Zhang, Hebei Normal University of Science and Technology in
China

    * http://w3.hevttc.edu.cn/gjhz/yy.htm

    * James Barber, Imperial College London

    * http://www.bio.ic.ac.uk/research/barber/people/jbarber.html

    * Phthalocyanines, Wikipedia

    * http://en.wikipedia.org/wiki/Phthalocyanine