Hydrogen could be an environmentally-friendly alternative to conventional fossil fuels, particularly if it is electrochemically produced from water in electrocatalytic or photoelectrochemical cells. These cells might be inspired by biological structures that are themselves capable of water splitting, light-harvesting or proton reduction.

Elena Rozhkova’s team, which includes scientists from Argonne’s Center for Nanoscale Materials (CNM) and Materials Science Division as well as Northwestern University and the Moscow Institute of Physics and Technology, has been developing visible-light-driven nano-bio photocatalysts for producing hydrogen based on noncovalent assemblies of the natural proton pump bacteriorhodopsin (bR) and titanium dioxide (TiO2) semiconductor nanoparticles. In nature, the protein machinery of the bR pumps and transfers protons across a membrane when exposed to sunlight. This produces an electrochemical gradient for synthesizing ATP.

An entirely man-made system

In previous work, the researchers reported using a natural transmembrane proton pump isolated from Halobacteria as a building block in nano-bio hybrid structures that produce hydrogen when exposed to visible light. They have now gone a step further by proving that it is possible to create an entirely man-made system based on this type of architecture.

“We employ an artificial membrane nanodisc technology that we know is handy for structural biology and the development of therapeutics as a functional unit of a nano-bio hybrid,” explains Rozhkova. “Thanks to the unique naturally-evolved structure of the protein and the homogeneity of the artificial membrane nano-scaffold, the synthetic bio-architecture can self-assemble with a TiO2 semiconductor nanocluster doped with platinum co-catalyst atoms. Such a system can catalyse a visible light-driven hydrogen evolution reaction in the presence of a sacrificial substrate (for example, methanol in an experimental setting or for potential commercial applications, cheaper waste, such as biomass or by-products of biodiesel production).”

Proton pump's natural function enhances catalytic activity of nano-bio hybrid

“The system produces 17.74 mmol of H2 (μmol protein)–1 h–1 under white light at ambient conditions (room temperature and neutral pH),” Peng Wang, a former postdoc at the CNM and now professor at the State Key Laboratory of Crystal Materials, Shandong University, China, tells nanotechweb.org. “The artificial membrane-protein complex enables TiO2 to become sensitive to visible light similar to light-sensitive dye molecules. Furthermore, the catalytic activity of the nano-bio hybrid is enhanced by the proton pump's natural function – that is, proton translocation across a membrane resulting in an electrochemical gradient.”

“In this work, we have shown how two modern technologies, namely cell-free synthetic biology and nanotechnology, can be combined and utilized in a mutually enhancing way to develop new materials with properties that go far beyond just those of the individual components themselves. The cell-free approach allows for a fast, labour- and cost-efficient way to manipulate the nanoparticle-biological interface so that it can be exploited as an artificial life system in applications such as energy production and catalysis”, says Rozhkova.

The researchers, reporting their work in ACS Nano DOI: 10.1021/acsnano.7b01142, say that they are now going to look at how the properties of an artificial light-driven system can be enhanced or altered using photonic nanostructures other than those they have studied. “It would also be very interesting to visualize photochemistry and biostructure–nanoparticle interactions using modern imaging methods,” adds Rozhkova.