Solar: Efficient Light Harvesting Using “Sun Sponges”

A team from the US have designed a semi-synthetic ring of pigment-attached protein which can be used to efficiently absorb more of the solar spectrum than naturally-occurring complexes.

Using a mixture of synthetic and naturally-occurring materials, a team from Washington University in St. Louis (WUStL) have designed “sun sponges” – complexes that self-assemble into a structure that soaks up sunlight efficiently. And in doing so, they have developed a molecular test platform that can be used to rapidly-prototype future versions of these high-efficiency light-harvesting antennas (LHAs).

Nature has evolved many systems to capture the sun’s energy, but they all depend on pigments (or LHAs) that absorb a particular wavelength of the solar spectrum and take the first step in converting sunlight into usable energy. Naturally-occurring antennas consist of protein scaffolding that holds pigment molecules in specific positions to capture and transfer the sun’s energy. The number and type of pigment molecules determines how much of the sun’s energy the antennas can grab and dump into an energy trap.

In plants, the native antenna complex is chlorophyll, whose green colour is a result of the molecule absorbing the red and violet parts of the solar spectrum, but reflecting the green. Plants do not absorb any light in the middle part of the visible spectrum, or at long wavelengths. But some naturally-occurring bacteria contain a complex called bacteriochlorophyll that absorbs light in the near-infrared region, giving them colours ranging between purple, red, brown, and orange.

Taking inspiration from nature, the team from WUStL hope to develop biohybrid complexes that can absorb more sunlight than the fully natural structures found in plants or photosynthesising bacteria. Their design, published in the August edition of Chemical Science, is based on a part-synthetic, part-natural ring of protein, to which pigments can be attached. Their prototype antenna complexes provide use two pigments, providing spectral coverage across the visible and near-infrared, outperforming natural antennas in each of these regions.

The team also demonstrated that this semi-synthetic structure can be used as a ‘test-bed’ to further develop more complex antennas that work via an energy-transfer cascade – where several pigments can be used in tandem to absorb even greater proportion of the solar spectrum.

It is hoped that this development will lead to a new generation of small-scale solar light harvesters, and eventually to a system based on their semi-synthetic antenna that could use the sun’s energy to directly split water (REF: Chemical Science (2013) DOI: 10.1039/C3SC51518D)