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SPIDERS WEAVE A WEB OF LIGHT
WHAT do you get if you give a delicate thread of spider's silk a glassy coating- and then extract the silk by baking? Yushan Yan reckons you will solve a major problem in photonics: how to make ultra-thin, hollow optical fibres narrow enough to carry light beams around the fastest nanoscale optical circuits. Using this technique, Yan and a team of engineers from the University of California at Riverside say they soon expect to be able to make hollow fibres with cores just 2 nanometres wide- or 50,000 times thinner than a human hair. In addition to photonics, the spider silk-based fibres could also be used to boost the resolution of optical microscopes. Or they could be turned into nanoscale test tubes in a new breed of sensors that can suck up single molecules of a particular chemical, say. The hollow fibres are produced in a beguilingly simple process, similar to the way candles are made by dipping a wick in wax. The team took 1-centimetre-long lengths of spider silk from Nephila madagascariensis, the giant orb-weaving spider of Madagascar. They glued the ends to a piece of card and repeatedly dipped the silk into a solution of tetraethyl orthosilicate. They then left the coated fibres to dry and baked them at 420°C to burn away the silk- the coating shrinks fivefold in the oven, leaving hollow silica tubes with a diameter of just 1 micrometre. The next step will be to produce even finer fibres, using the thinnest known silk with a diameter of only 10 nanometres from the spider Stegodyphus pacificus, a native of the Middle East and South Asia. Allowing for shrinkage, that means silk from S. pacificus should produce fibres with a diameter of around 2 nanometres. The thinnest hollow fibre cores produced by conventional methods are around 25 nanometres wide. Philip Russell, a physicist at Bath University in Britain, who pioneered hollow optical fibres for the telecommunications industry, is impressed by the cheapness and simplicity of Yan's technique. He believes the technique, which will be described in a forthcoming edition of the Journal of Materials Chemistry, will be useful for building tiny sensors that exploit the strange 'supramolecular' chemistry that occurs when substances are confined in very small spaces. In these circumstances, rates of reaction speed up significantly and completely different reactions happen. Carbon nanotubes are often used in this field, but they only come in a relatively narrow range of shapes, sizes and textures. The spider fibres offer a wider range of structures that can be more easily fine-tuned. Christopher Viney, a chemist at Heriot-Watt University in Edinburgh, says that the new fibres could also be turned into finer fibre-optic probes for near-field microscopes. These are used by biologists who want to look at features smaller than the wavelengths of light without damaging the sample as an electron microscope does. At present, such microscopes rely on 'lenses' made from optical fibres drawn-out from ultra-thin glass tubes- but these are relatively wide at around 100 nanometres. 'These new fibres will open up whole new vistas for biologists,' Viney predicts.
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