A team led by Joachim Reichert from Munich Technical University and Itai Carmeli of Tel Aviv University studied a single functionalized photosynthetic protein system, known as the photosystem-I reaction centre. This is a protein complex made of chlorophyll that is found in the chloroplast membranes of cyanobacteria – microorganisms that harness sunlight to produce chemical energy. The initial stages of this process occur thanks to the photosynthetic proteins absorbing light and transferring energy and electrons.

Photosystem-I has excellent optoelectronic properties but is only found in nature. Ideally, scientists would like to use such a system in artificial photosynthetic devices that might be used to power nanoscale circuits of the future. However, to do this, they must first be able to measure and monitor the photocurrents generated by single molecules.

Measuring the photocurrent in single proteins

Reichert and Carmeli’s teams have developed a technique to electrically contact the single-protein molecules in photosystem-I (which they assembled on a plain gold substrate) to a metal-coated glass fragment. The glass fragment serves as a counter electrode and a light source at the same time and the protein molecules are functionalized with cysteine groups to covalently bind the electrodes.

The researchers measured the photocurrent through the photosynthetic proteins using a scanning near-field optical microscope set-up. The proteins are first optically excited by a flux of photons guided through the microscope tip, which simultaneously provides the electrical contact in the set-up, and the resulting photocurrent is then monitored.

“Our proof-of-principle experiments show that we can measure the electric current generated by a single molecule – in our case it is around 10 pA for the covalently bound single-protein junctions (a figure that is in agreement with the internal electron transfer times of photosystem-I),” Reichert told nanotechweb.org. “This current might now be exploited using these, or related chemical structures, in molecular circuits that are directly addressable with light.”

The work is detailed in Nature Nanotechnology.