Photo finish

The value of platinum isn't only in its scarcity and purity. Like many relatively heavy transition metals, its ability to catalyse organic reactions makes it indispensable for the chemical industry. Researchers from the US Department of Energy's Sandia National Laboratories have developed a technique which could enhance the catalytic activity even further, borrowing from one of nature's oldest tricks to make microscopic structures of platinum powered by light.

Team leader John Shelnutt, working with colleagues from the University of New Mexico and the University of Porto in Portugal, has mimicked the function of the proteins involved in photosynthesis to manipulate platinum into nanometre-scale lace-like or tentacled spheres. 'We can have control over the size, porosity, composition, surface species, solubility, stability and other functional properties of these metal nanostructures,' he says.

In photosynthesis, plants use proteins containing ring structures called porphyrins to convert carbon dioxide into sugars. Shelnutt uses a similar mechanism, where the porphyrins become machines for depositing platinum atoms onto a template formed from molecules of detergent.

When dissolved in high enough concentrations, detergent molecules tend to organise themselves into structures - micelles, where the organic tail-sections of the molecules stick together forming a ball with the ionic sections on the outer surface; sheets where the heads and tails of the molecules align; and liposomes, where the sheets form a spherical membrane with water inside and out.

The structures formed depend on the concentration of the detergent. Shelnutt's technique mixes a solution of ascorbic acid, which has detergent properties, with a platinum salt and porphyrin molecules. When light is shone on the solution, the porphyrins become reducing agents, converting the platinum ions into metallic atoms and depositing them on the surface of the detergent structures.

If the detergent forms into micelles, the platinum forms into a multi-armed ball. If the detergent is in liposome form, the platinum grows across the membrane surface, forming a lace structure. And if the concentration is high enough for the liposomes to aggregate, the platinum grows across the interfaces of the spheres, forming a foam. The size of the structures can be controlled by the concentration of the platinum and porphyrin, and by the amount of light illuminating the solution. 'It's so simple, it's amazing,' Shelnutt says.

There are many applications for this technique, Shelnutt says. Catalysis, sensors and optoelectronic devices are obvious choices for active metals in this form. Another interesting property of the solutions is that the platinum can convert water into hydrogen and oxygen when illuminated, which could be useful in fuel-cell powered cars.