The synthetic purple membrane assembly includes nanodiscs, titanium dioxide and platinum nanoparticles. (source: ANL)A synthetic purple membrane may be the Next Big Thing for hydrogen evolution technology. The system designed by researchers at the U.S. Department of Energy’s Argonne National Laboratory (ANL) uses completely synthetic bionano machinery to harvest light without the need for a living cell.

The membranes contain tiny discs of lipids, man-made proteins and semiconducting nanoparticles that, when combined, can transform sunlight into hydrogen fuel. A crucial component of the system is artificially produced bacteriorhodopsin, a protein normally found in the membranes of Halobacterium salinarum. This ancient single-celled organism lives in extreme high-salt conditions such as Utah’s Great Salt Lake and Yellowstone National Park’s hot springs, appearing as purple plumes of water.

Purple membrane bacteria use the protein to harvest energy from light. The researchers explain that their man-made system replicates this process through designed-protein expression without the need for biological cells. The lipid nanodiscs mimic the biological membrane which supports bacteriorhodopsin and enables its function.

“The process of the artificial protein synthesis was visualized with great precision using high resolution scanning probe microscopy,” said ANL materials scientist Val Novosad.

The synthetic purple membranes were assembled with nanoparticles of titanium dioxide for hydrogen evolution under visible light. The artificial system was demonstrated to use energy from the light to produce hydrogen with similar or even higher efficiency compared to systems based on bacterial purple membrane.

The system produces hydrogen at a turnover of about 240 μmol of H2 (μmol protein)−1 h–1 and 17.74 mmol of H2 (μmol protein)−1 h–1 under monochromatic green and white light, respectively, at ambient conditions, in water at neutral pH and room temperature, with methanol as a sacrificial electron donor.

Scientists from Northwestern University and Moscow Institute of Physics and Technology also participated in this research.