Carbon Nanotubes Could Make Artificial Photosynthesis Possible
by Jeremy Elton Jacquot, Los Angeles on 07.13.08
Image from Wikimedia Commons
Scientists may be getting closer to unraveling the secrets of photosynthesis, reports New Scientist Tech's Colin Barras. In a new study published in the journal ChemPhysChem, a team of Chinese scientists from the Hebei Normal University of Science and Technology has found that carbon nanotubes, which have been used in many nanotechnology applications including solar energy and adhesive material, can mimic a key step of the process.
Image from Calamari's Pics
Researchers have been trying to replicate photosynthesis for years. Many believe that artificial photosynthesis could produce enough hydrogen over the long term to launch a "hydrogen economy" in which all vehicles could run on hydrogen-powered fuel cells.
Another clear benefit is that artificial photosynthesis could help absorb a large chunk of atmospheric carbon emissions. Barras explains why scientists have failed so far to recreate this vital process:
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.
To get past this problem, the Chinese team decided to substitute single-walled carbon nanotubes for the electron receiver molecule in photosynthesis. Because they also needed a molecule that could release electrons when exposed to visible light, they covalently bonded a number of phthalocyanines (PCs) -- each of which releases one electron after absorbing light -- to the nanotube.
This allowed them to make a multiple electron system in which around 25% of the electrons released by PCs could be stored in a single nanotube. While only one step of a long, complex process, it could provide a needed source of electrons to convert NADP, a chloroplast chemical, into NADPH. In the Calvin Cycle, NADPH helps fix carbon dioxide to form glucose -- the end product of photosynthesis.
Via ::New Scientist Tech: Nanotubes bring artificial photosynthesis a step nearer (news website)
Other technologies made possible by carbon nanotubes
::Algae Towers, Carbon Nanotubes Coming to a City Near You
::Carbon Nanotubes and Solar Cells: A Winning Combination
::MIT: Move Over Batteries, Here Come the Nanotube-Enhanced Capacitors
This is absolutely incredible. If we could eventually get to the point where we can replicate photosynthesis on a large scale, could we simply remove the excess CO2 from the atmosphere? If so, the global warming debate/crisis would be over. Amazing stuff if we could get to that point.
Although nanotubes can do a lot and have a lot of promise, I'm not convinced something this simple will get us a system that can mimic photosynthesis with any great efficiency. Maybe it can, and the complexity of photosynthesis is an evolutionary artifact, but I am not sure. In cells light passes energy to electrons which produce intermediate energy carriers, ATP, which are consumed to enable production of sugars.
Although actually, a number of systems like this already exist in labs. They are hypotheses for future-generation solar cells. One logical next step to getting higher efficiency than silicon is cells with multiple junction types with different band gap energies, and a lot of work is being done there. But it is believed that after that, organic molecules offer the best hope for even higher conversion efficiencies. Cells have been demonstrated that use one photon to excite multiple electrons, or use multiple photons to excite one really energetic electron, and each of these increases the maximum possible conversion efficiency.
I guess my point is, is it necessarily more efficient to have a cell that produces hydrogen or something similar directly, versus a cell that produces electricity with the maximum possible electricity and then uses the electricity to produce whatever compound is needed? Of course, since this is research, by definition we don't know the answer yet. Guess I'll have to wait and see!
There must be thousands of purported applications for these nanotubes, from hyperspace drives to hyper-intelligent toothbrushes. However, is there any product on the market yet?