Chinese researchers have made a breakthrough in artificial photosynthesis. With the aid of carbon nanotubes (falls under nanotechnology) they can reproduce an important part of photosynthesis that was hitherto not possible.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. 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.
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. 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.
Future Vision by Erwin Van Lun on this article
Most important about this development is that devices will start to disconnect from their power supply. At the moment we need electric cables everywhere. This means that cars recharge themselves in sunlight; this means that computers can be outside; that robots can recharge in the garden. For humans that means an enormous increase in our freedom and furthermore we’re relieved from the slumbering concern of energy supplies, something that will help especially the underdeveloped countries to reach ‘western’ levels.