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Posted: February 1, 2008
From bacterium to semiconductor
(Nanowerk News) Proteins of photosynthetic bacteria can be used to generate a photocurrent.
Researchers from different countries are accommodating to their purposes proteins of photosynthesis system bacteria. They are used as an active component of the photocurrent generation chain in the sensory and energy-storing systems. In Russia, the problem is being addressed by specialists of the Lomonosov Moscow State University, Institute of Problems of Chemical Physics (Russian Academy of Sciences), Moscow Institute of Applied-Physics and the Institute of Chemical Physics (Russian Academy of Sciences). The researchers built proteins of reactionary center for purple bacteria photosynthesis into porous nano-crystalline films of titanium oxide.
Proteins of the photosynthesis system (bacteriochlorophyll, bacteriopheophytin and ubiquinones) – are natural solar energy bioaccumulators. Excited bacteriochlorophyll molecule transmits electron along the chain to other proteins of photosystem. These proteins attract attention of biophysicists due to high quantum yield of reaction of primary charge division and relative stability of these charges. As of today, there exist two different approaches to creation of hybrid light-storing and sensitive devices based on bacterial proteins. For elements of the first type, a layer of photosensitive molecules are applied on a metal (golden or platinum) or graphite plate. At that, the proteins should be orientated on the plate surface in a certain way. Depending on the protein disposition on the electrode, there occurs either a cathodic charge (in this case, the electron is carried from the electrode to proteins), or an anodic charge, if there occurs reverse direction current. In the devices of the second type, the proteins are applied on meso-porous semicoductors made of metal oxide. In such systems, excited protein molecules transmit very quickly, within fractions of a picosecond, an electron into the semiconductor’s conduction band. Besides, the pores are so tightly stuffed with proteins, that the special procedure of their orientation may be omitted. Proteins will get orientated spontaneously on the electrode hydrophobic surface, the donor section being turned to the semiconductor.
The photocurrent density is directly dependent on both the porous film structure and on the quantity of protein molecules on the electrode. Therefore, the Russian researchers tried to obtain thick film (4 micrometers thick) made of titanium o?ide. The researchers selected a structure which is optimal to maximum protein sorbtion.
The TiO2 meso-porous films are obtained from nano-crystalline powders, which are added into special paste. They were applied on glass with a conducting covering of titanium- indium oxide. The film was dried up and calcined for 30-60 minutes at 550 degrees. Calcination adds mechanical strength to films. Then the plate was soaked in the photosynthetic proteins solution, and the main electrode was ready. The researchers managed to get a film with small pores and large specific surface area (300 m2/g). Thanks to the film depth and porosity, a lot of proteins get stuck to it, their concentration in the sample being 160 times higher than that in the solution. Proteins on the main electrode preserve activity even after the two week keeping in a refrigerator. Illuminating the electrode by red light, which only proteins react to, generates the anodic photocurrent of almost 2 microamperes. Titanium o?ide also reacts to white light, but presence of the photosynthesis system proteins in the electrode increases the photocurrent by more than twice.
The researchers note that nano-porous semiconductors possess not only a very high sorbing ability, but also tremendous energy diversity of surface states, which significantly impacts the electron transmission process.