Tobacco plant virus proteins modified for solar panels
Proteins from a common tobacco plant virus are being used to arrange gold nanoparticles into ultrathin sheets for use in solar panels.
Researchers at McGill University, Canada, suggest this will achieve cheaper and safer materials for solar panels and similar technology.
Gold nanoparticles are only effective in strengthening optical signals when the nanoparticles are arranged on a surface and spaced at exact distances. Previously, this has required harsh chemicals and lab conditions.
The McGill team has modified proteins in the tobacco plant virus to self-assemble into sheets in water and at room temperature, helping the gold nanoparticles to be properly spaced.
To build the scaffold, the team modified the virus’ protein by adding a short chain of histidine – tiny hooks that latch onto gold nanoparticles and guide the proteins to self-assemble into ultrathin sheets.
Associate Professor and co-author Amy Blum says they 'use interactions between these histidines first to attach the discs together to form sheets and then to attach gold particles to the edges of the discs. This scheme works because the histidine-histidine interactions by themselves are very weak. Just one interaction does not really impact protein behaviour.
‘Since each disc contains 17 monomers, we have a large number of histidines at the same place so that [they] can interact with each other. The assembly forms when there are enough of these interactions to hold the protein discs together’.
This only occurs when the discs are oriented to maximise the number of interactions, leading to perfectly packed sheets. The discs pack into hexagonal or square lattices tuned by pH.
Blum continues, ‘With this process, we can take a disc that is 18nm in diameter and make perfectly packed sheets that are 100μm2 or larger in size with defects only really occurring at the edges. The tight control over organisation gives our material its interesting optical properties that could be used in solar cells, sensors and other optical devices.’
Without this modification, the protein tends to clump. The weaker interactions instead encourage the proteins to lay flat, Blum notes.
She adds, ‘The tobacco mosaic virus (TMV) protein naturally makes an 18nm disc structure when held at the proper solution conditions (concentration, pH and salt concentration). We added a chain of six copies of the amino acid histidine to the end of the protein via genetic mutation. We express modified TMV protein in E. Coli.’
‘For safety, we don’t use the active virus. We just use the shell, which contains no genetic material.’
Since this process relies on interactions between proteins, it works at room temperature (or colder) in buffered water solutions. ‘We don’t need to use a lot of energy or any organic solvents. The gold particles are also made in water-based solutions, using salts as the starting materials, at room temperature,’ shares Blum.
‘If you just chuck these nanoparticles on a surface, some fraction of them will randomly cause enhancement…But if you can get them to be at a fixed good distance, then the whole surface is active.’
The sheets can then be rolled up into nanotubes when a small amount of ethanol is added to solutions at the correct pH and salt concentration. ‘We think that the ethanol increases the strength of interactions between the face of the discs and the edges of TMV rods in solution, causing the sheets to roll up around the rods.’
The team is now seeking to enhance the TMV template. Blum says, ‘We are interested in controlling sheet layering, which changes optical properties. We are also extending this system to use magnetic, iron-oxide nanoparticles…We have a very modular system that could be adapted to precisely organise…any sort of nanosized object.’
As they make assemblies in water, they are ‘solution processable’ and can be deposited on nearly any substrate compatible with water. ‘We can also coat substrates that are not flat,’ Blum concludes.
