A polar solvent is the key to making polymeric photonic crystals that could be used as biological sensors, say scientists from Taiwan and the US.

Alexander Cartwright, at the State University of New York, Buffalo, US, Vincent Hsiao, at the National Chi Nan University, Puli, Taiwan, and colleagues used a method known as holographic photopatterning to make their photonic crystals.

The team mixed formamide, a highly polar solvent, with a non-polar solution of monomers and surfactant. The formamide formed emulsion droplets within the solution and the mixture was sandwiched between two glass slides. They shone a laser through a prism on to the slides, which created a pattern of incident and reflected beams. In the pattern's bright regions, the monomers polymerised, driving the formamide droplets into the dark regions. After switching off the laser, the team opened the sandwich to evaporate the formamide , leaving behind a polymeric film with a periodic structure.

Scientists have previously reported holographic patterning using liquid crystal additives but Cartwright says using formamide is simpler. 'We wanted to develop a more direct route to the production of these porous structures,' he explains.

Cefe López, an expert in photonic crystals at the Institute of Materials Science of Madrid, Spain, says the use of the formamide additive is significant but adds, 'What is more interesting is the smart way of preparing a multilayer by herding the [formamide] micelles into dark regions while, at the same time, the light regions are crosslinked.'

A photonic crystal's periodic structure disrupts the passage of light through it. By altering the periodicity, scientists have made a range of photonic crystals that reflect different colours. Cartwright says he hopes to optically tune his polymeric photonic crystals so they can be used as biosensors.

'The ability to produce large areas of the films will be a challenge,' predicts Cartwright. 'More importantly, we would like to include additional functionality within the film but this will require a change in the way we fabricate the structures.'

Mary Badcock

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