Generating photocurrent with protein semiconductors

Russian researchers have discovered a novel way to generate photocurrent by using proteins found in the sensory and energy-storing systems of photosynthetic bacteria.

Specialists at the Russian Academy of Sciences, Moscow, have inserted proteins from the reactionary centre for purple bacteria photosynthesis into porous, nano-crystalline films of titanium oxide (TiO₂).

Photosynthesis proteins, such as bacteriochlorophyll, bacteriopheophytin and ubiquinones, are natural bioaccumulators of solar energy. They have attracted the attention of biophysicists due to the high quantum yield and relative stability of the charge produced by their reaction.

The scientist's approach differs from the conventional method of creating hybrid light-storing and light-sensitive devices based on bacterial proteins. Conventionally, a layer of photosensitive molecules is applied to a gold, platinum or graphite plate. In this method, the proteins need to be orientated on the plate surface in a certain way. Depending on the disposition of the protein, the electrode provides either a cathodic charge (carrying the electron from the electrode to the proteins), or an anodic charge if the current is reversed.

In the method pioneered by the Russians, the proteins are applied on meso-porous semiconductors made of metal oxide. This allows the excited protein molecules to transmit very quickly - within fractions of a picosecond - an electron into the semiconductor's conduction band. Also, the pores are so tightly stuffed with proteins that they do not need to be specifically oriented in any particular direction - the proteins orientate spontaneously on the electrode hydrophobic surface.

The photocurrent density is directly dependent on both the porous film structure and on the quantity of protein molecules on the electrode. The scientists therefore created a 4 µm thick film of TiO₂ designed to maximise protein sorbtion.

The TiO₂ meso-porous films are made from nano-crystalline powders combined with a paste, which was then layered on glass with a conducting covering of titanium-indium oxide. The resulting film was dried and calcined for 30-60 mins at 550°C, to increase its mechanical strength.

The plate was soaked in the photosynthetic proteins solution and the researchers managed to get a film with small pores and large specific surface area (300 m²/g). The film depth and porosity allowed a large number of proteins to attach, providing a sample concentration 160 times higher than that in the solution.

Illuminating the electrode with red light excited the proteins and generated an anodic photocurrent of almost 2 µA. TiO₂ also reacts to white light, but presence of the photosynthesis proteins more than doubled the photocurrent.

The proteins on the main electrode preserved activity even after two weeks' storage in a refrigerator.

The researchers noted that nano-porous semiconductors possess not only a very high sorbing ability, but also tremendous energy diversity of surface states, which significantly affected the electron transmission process.

Adapted from information provided by Informnauka (Informscience) Agency. www.informnauka.ru/eng/