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Optical Methods for Chemical Sensing

Surface-Enhanced Raman Scattering

Raman scattering has generated substantial interest as a spectroscopic technique, due to its generally well-resolved spectra that provide a unique fingerprint for every Raman active substance or compound. Raman scattering is observed when light is inelastically scattered by vibrating molecules, resulting in a shift to both shorter and longer wavelengths. Surface-enhanced Raman scattering (SERS) is attributed to electromagnetic and chemical interactions between a surface with nanoscale metal structures and the target molecule absorbed onto or in close proximity to the surface. The SERS effect can lead to a millionfold increase in scattering intensity over the relatively weak normal Raman scattering.

For some time now, there has been an interest in using SERS as a means to detect and monitor Blood Glucose Levels (BGL) to aid in the treatment of diabetes. The main stumbling block has been the inability to build the SERS active surface in a small enough and stable enough form to be implanted inside a patient's body.

Traditionally, generating the nanoscale metal structure has required the use of expensive processes such as e-beam lithography or self-assembled or randomly deposited structures which, by their very nature, have little sample-to-sample reproducibility. These issues have limited the viability of using SERS sensors in medical and industrial applications.

At CAOUS, we have developed a technique for building nanoscale metal structures onto the tips of optical fibres. Such an approach is relatively cheap and lends itself to both high sample-to-sample repeatability and the potential for commercial manufacturing. An example of the sensor is pictured above.

One of the key advantages of the SERS optical fibres is that they can be built very small. The fibre pictured has a diameter of 40 microns (1 micron = 1 millionth of a metre) which is less than half the width of an average human hair! To date, we have build sensors ranging in size from 15 micron up to 70 micron. Although the sensor needs to be implanted just under the skin (remember that in order for the SERS effect to take place, the sensor must be close contact with the target molecule), such small sizes means this would be an almost painless procedure.

Detail of the surface structure showing the nanoscale metal 'honeycomb'. Each pyramid is about 120nm wide.

On the right is an example spectra for the reference molcule thiophenol (chemical formula C6H6S) taken from the surface of one of our SERS fibres. Pictured is the spectrum of the Stokes radiation which is the shift to longer wavelengths. Measuring the location and heights of the peaks gives information on the bonds between the atoms in the molecule.

This work is supported by grants from:

  • Biopharmica Ltd
  • Diabetes Australia (Victoria)