Photosystem-I (green) is optically excited by an electrode (on top). An electron then is transferred step by step in only 16 nanoseconds.
As plant photosynthesis is the basis of life, re-engineering of this process for power generation is a big dream of researchers around the world. A team of scientists at the TU Muenchen, led by Joachim Reichert, John Barth (Cluster of Excellence Munich-Centre of Advanced Photonics) and Alexander Holleitner (Cluster of Excellence Nanosystems Initiative Munich) in cooperation with Itai Carmeli (Tel Aviv University), has now developed such a process in the nano-scale.
In a first step they fixed molecular complexes of the plant photosystem I on a gold surface. Then they coated an extremely fine glass tip, as it is used for near-field microscopy, with an ultrathin layer of gold. While the glass tip directs the light exactly to the protein to be examined, the gold layer forms the counter electrode. Thus the photosystem I protein complex acts as a highly efficient light-driven electron pump and could serve as a power generator in nano-electrical components.
The scientist investigated the photosystem-I reaction center which is a chlorophyll protein complex located in membranes of chloroplasts from cyanobacteria. Plants, algae and bacteria use photosynthesis to convert solar energy into chemical energy. The initial stages of this process – where light is absorbed and energy and electrons are transferred – are mediated by photosynthetic proteins composed of chlorophyll and carotenoid complexes. Until now, none of the available methods were sensitive enough to measure photocurrents generated by a single protein. Photosystem-I exhibits outstanding optoelectronic properties found only in photosynthetic systems. The nanoscale dimension further makes the photosystem-I a promising unit for applications in molecular optoelectronics.
The first challenge the physicists had to master was the development of a method to electrically contact single molecules in strong optical fields. The central element of the realized nanodevice are photosynthetic proteins self-assembled and covalently bound to a gold electrode via cysteine mutation groups. The photocurrent was measured by means of a gold-covered glass tip employed in a scanning near-field optical microscopy set-up. The photosynthetic proteins are optically excited by a photon flux guided through the tetrahedral tip that at the same time provides the electrical contact. With this technique, the physicists were able to monitor the photocurrent generated in single protein units.
The research was supported by the German Research Foundation (DFG) via the SPP 1243 (grants HO 3324/2 and RE 2592/2), the Clusters of Excellence Munich-Centre for Advanced Photonics and Nanosystems Initiative Munich, as well as ERC Advanced Grant MolArt (no. 47299).
Contacts and sources:
Dr. Andreas Battenberg
Technische Universitaet Muenchen
nature nanotechnology, 30. Sept. 2012 – DOI: 10.1038/nnano.2012.165