US scientists demonstrate how viruses can be used to make titania nanoparticles of fixed shapes and sizes. 

Trevor Douglas and coworkers from Montana State University, Bozeman, found that when the pH is reduced inside a viral capsid containing soluble Ti(IV) salts, insoluble titania (TiO₂) nanoparticles are formed.  Importantly the nanoparticle shape and size are controlled by the interior shape of the virus being used - the cowpea chlorotic mottle virus.

Titania nanoparticles with narrow size distributions give reliable homogeneous properties that are useful for a variety of applications including photocatalysts, sensors, semiconductors and pigments.  

The titania nanoparticles can be used directly as a photocatalyst whilst still contained within the virus capsid.  This is because the cage is transparent to light and simply acts as a template, explains Douglas.  In addition, the capsid's protein nature means its surface can be chemically or genetically modified to enable binding to other surfaces if required. The team also expects that in the future different sized protein cages could be used to make particles of different sizes.

Douglas has previously used this route to make tungsten- and vanadium-based minerals inside the same viral capsid. Douglas explains the importance of his method: 'Since we are utilising bio-inspired techniques, we are operating in aqueous media under very mild conditions.  This green approach toward synthesis could potentially reduce the use of harsh solvents and high temperatures that are often required in traditional synthesis.' 

Rajesh Naik of the Air Force Research Laboratory, Dayton, US, whose research interests include the application of biological methods to making nanoparticles, was interested by the demonstration of the photocatalytic activity: 'Improvement in the materials properties could result in the use of these materials in photovoltaic applications where one might be able to exploit the outer protein coat for functionalisation with dye molecules.'

Christina Hodkinson