Sensitising Raman: Forensic and diagnostic boost

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  • Published: Dec 1, 2010
  • Author: David Bradley
  • Channels: Raman
thumbnail image: Sensitising Raman: Forensic and diagnostic boost

Low concentration problem

The efforts of forensic investigators are often stymied by the diverse and complex nature of sample mixtures they must analyse but low analyte concentration is probably the most problematic issue they must address. Now, a US team has found a way to improve Raman spectroscopy for low-concentration analysis. Their approach could also improve medical diagnosis, drug/chemical development, and in national security.

The specificity and non-invasive nature of Raman spectroscopy has led it to become a tool of choice for many applications, but its shortcoming is its lack of sensitivity and its poor reliability at extremely low concentrations. Now, Tiziana Bond of the Lawrence Livermore National Laboratory's Center for Micro and Nano Technology and her colleagues Elaine Behymer, Hoang Nguyen, Jerald A Britten, Cindy Larson, Robin Miles, Mihail Bora, Allan S-P Chang have worked with Manas Gartia, Zhida Xu and Logan Liu of the Micro and Nanotechnology Laboratory, of the University of Illinois at Urbana-Champaign, on a new approach to surface-enhanced Raman spectroscopy (SERS). SERS commonly improves Raman signals by orders of magnitude by using roughened metal surfaces, but often remains unreliable at very low concentrations. Of course, for more amenable samples, tunable SERS substrates, such as UK company Renishaw Diagnostics' Klarite, have been commercially available for several years. 

Roughened treatment

The roughened surface enhances the interaction of the analyte molecule with the metal. A substrate with uniform topographic features would yield more consistent signal enhancements. The researchers have worked on several techniques to achieve a more robust and uniform substrate that maintains high sensitivity and reproducibility. The team suggests that electromagnetic and chemical enhancements are two factors to boost the SERS signal. To exploit electromagnetic effects, the team had to improve the design of metallic nanostructures on the surface. Previously, they had investigated an innovative design using a vertical a gold-coated nanowire array substrate that could provide strong and controllable enhancement. The team's innovation was the fabrication of tuneable plasmon resonant cavities in the vertical wire arrays. Tuning is achieved by controlling the dimensions of the cavity.

Resonant cavities

The team also introduced the smallest optical resonant cavity that is thousands of times smaller than wavelength of light and showed that it is possible to go beyond this diffraction limit by using surface plasmons. "By confining the light in such tight spaces we are able to create intense fields that are useful in increasing the spectroscopy signal," Bond says. These various design features offer them a number of advantages. For example, it allows them to tune the system's sensitivity to be tuned, or adapted, to different wavelengths offering researchers greater versatility.

The team's designer large-area ultrahigh-uniformity tapered silver nanopillar array was produced by laser interference lithography on the entire surface of a 4-inch wafer. In their latest paper in Nanotechnology, they also present a rigorous optical characterization method applied to the tapered nanopillar substrate to accurately quantify the Raman enhancement factor, uniformity and repeatability. "An average homogeneous enhancement factor of close to 100,000,000 was obtained for benzenethiol adsorbed on a silver-coated nanopillar substrate," the team writes.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

The efforts of forensic investigators are often stymied by the diverse and complex nature of sample mixtures they must analyse. Now, a rapid, on-site method for identifying illicit narcotics, such as cocaine, has been developed by researchers in Ireland using chemometrics methods to classify Raman spectra. The approach almost trebles success rates from 35% to 90% compared to standard methods.
SERS-boosting team

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