Engineerng a "nano-leaf" based on hydrogenase and PSII for hydrogen production from solar energy

The harvesting and storage of solar energy is a major challenge for technology, in order to be able to tackle fossil fuel shortage in the next future. A cutting-edge approach in this field involves the application of nanotechnology together with strategies mimicking the most efficient solar conversion process available in nature: photosynthesis. The natural enzymes are extremely well-refined nanocatalysts and by studying the complex structure-function interplay underpinning their mechanism it is possible to enhance their technological exploitation. Protein engineering can be applied to achieve a productive interaction of enzymes with electrode surfaces of inorganic conductors and semiconductors. In such a system an hydrogenase would represent the cathodic side working in combination with a separate anodic electrode binding Photosystem II (PSII) so as to generate hydrogen using electrons and protons derived from light-driven water splitting (Fig. 1). The “nano-leaf” will therefore represent an electrochemical converter of solar energy into valuable fuel. The research aims at devising the methods for production of natural hydrogenases and photosystem II (PSII) and immobilisation onto electrodes with efficient electronic coupling. The heterologous expression gives the opportunity of genetically engineering the enzymes to achieve better stability/activity and lower oxygen sensitivity as well as enhancing and orienting protein/electrode interaction for optimised device performances. Results support the technical feasibility of the “enzyme approach” in conversion of solar energy to hydrogen. High amounts of pure hydrogenase were obtained with a straightforward and efficient procedure. The enzyme displays a good activtiy, an excellent stability to temperature and a temperature-dependent increase of activity up to 60°C. These two latter properties are extremely important in view of the assembly of a prototype converter, in which the anode bearing the PSII is exposed to light and the temperature of the entire setup is therefore relatively high. The interaction of the pure hydrogenase with electrode surfaces is currently under testing. The succesful immobilisation and efficient electronic coupling of PSII on nanostructured titanium oxide, with photocurrents of up to 0.36mA cm-2 and the great stability generated by the binding of the PSII in to the pores, offer great potential for the use of this approach for the construction of the device. Further work is undergoing to increase the photocurrent efficiency and to measure the other product of the water splitting, namely oxygen and protons.