Dancing with light

Dancing with light Iridescent nano-sculpted substrate, repelling water droplet sat on top.

University of Southampton

The ancient art of casting is being used at the nanometre scale at the University of Southampton to develop ultra sensitive detectors that are being tested for health screening. Jeremy Baumberg, Phil Bartlett, Andrea Russell and their team of nano-scientists have developed the technique of templated electrodeposition to make nanoscale gold structures that enable the detection of tiny numbers of molecules by laser light. Working with a spinout company from the University, Mesophotonics Ltd, and Gloucester Hospitals NHS Foundation Trust, they are testing the technique as a diagnostic tool for conjunctivitis using human tear fluid.

'The Mesopotamian civilization made moulds from sand to cast molten copper', explains Jeremy. 'We use nanoscale plastic spheres as moulds and electroplating techniques to build up our structures'. The spheres are suspended in water, a drop of which is evaporated on gold-coated glass leaving a single layer of spheres. The gold is then grown up around the ball 'mould' using electroplating techniques. Finally the balls are dissolved, leaving a gold metal structure with nano-dishes and cavities.

'It is the optical properties of the structure that are key', says Phil. 'The tiny cavities are on the scale of the wavelength of light, so they trap the light and concentrate its energy with extraordinary efficiency'. The concentrated energy enhances a phenomenon known as Raman scattering by more than a million times, enabling the reliable detection of molecules at very low concentrations.

Raman scattering produces a kind of molecular fingerprint when light in the form of a laser is focused on a sample. The bonds of the molecules in the sample vibrate and absorb some of the light and scatter it so that the light emitted from the sample changes colour in a characteristic way depending on the molecules present. A Raman spectrometer is used to measure this effect. The problem however is that Raman scattering is very weak and hard to detect, so that on its own is of little practical use in diagnostics. The University of Southampton's gold nano-materials amplify Raman scattering so that the molecular fingerprints can easily be detected, even when only tiny traces of substances are present. Their techniques have allowed complete control of the amplification of Raman scattering for the first time.

Results using this new system are impressive. 'Repeating measurements on the same sample gives the same results within a few percent', says chemist Mamdouh Abdelsalam. 'Previously you were lucky to be within 1000% when repeated'.

There are many applications for seeing molecules so sensitively. 'Understanding how molecules bind to surfaces is key to unraveling the mysteries of catalysis a multi-billion-pound industry. Environmental monitoring of pollutants or bio-hazard detection are other possibilities', says Andrea. If successful for diagnosing conjunctivitis, this technique could save the NHS an estimated £471million over ten years through savings in drugs, laboratory time, and the number of patient visits.