domingo, 14 de octubre de 2007

Helical nanofibres: A twist to follow

Scientists in China have synthesized left- and right-handed helical conducting nanofibres
Helical conducting nanofibres are useful materials for making optically active devices that can twist the polarization of light. However, it is a challenge to synthesize these fibres in one helical form, that is, either 'left-' or 'right-handed'. Zhixiang Wei and co-workers at the National Center for Nanoscience and Nanotechnology of China in Beijing have overcome this challenge by introducing left-or right-handed impurities into the synthesis.

The researchers added camphorsulfonic acid (CSA), which comes in left- and right-handed forms, to the polymerization of aniline at different molar ratios of CSA to aniline, ranging from 0.5 to 160. The addition of left-or right-handed CSA, resulted in the formation of predominantly left-or right-handed nanofibres, respectively. They also found that when the ratio was less than one, polyaniline nanotubes with diameters of 120–550 nm formed but a ratio greater than one resulted in polyaniline nanofibres with diameters of 30–80 nm instead of nanotubes.
The nanofibres can further self-assemble into rope-like helical bundles of the same handedness (pictured) owing to aromatic stacking interactions between the polyaniline nanofibres.

Reference: Yan, Y., Yu, Z., Huang, Y., Yuan, W. & Wei, Z. Helical polyaniline nanofibers induced by chiral dopants by a polymerization process. Adv. Mater. doi: 10.1002/adma.200700846 (2007).

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