Since its discovery in 1974, surface enhanced Raman spectroscopy (SERS) has been used to detect and characterise a variety of chemicals and biological species. At NPL, our research focuses on improving the repeatability and reproducibility of SERS and understanding the mechanisms underpinning the method. This is targeted at overcoming barriers for the uptake of this technique as a powerful commercial tool for trace analysis.

Raman Spectroscopy

Raman spectroscopy is a well-established optical method that involves excitation of a sample by a light source of a particular frequency and the subsequent collection of the scattered radiation. A small portion of this scattered radiation (approximately 1 in 10⁷ photons) has frequencies different from that of the incident beam, as energy exchanges occur between the incident photons and the molecule. The shift in light frequency is measured and this forms the basis of the Raman effect. The resultant spectrum (of intensity vs. frequency shift) therefore provides a unique fingerprint to that molecule and hence, Raman is often employed as a quick tool to identify unknown substances.

However, one main disadvantage of Raman is its low sensitivity due to the extremely small cross section of the Raman process (10^-31 to 10^-29 cm² per molecule). This often means the requirement for highly concentrated samples or the use of powerful and costly laser sources for excitation, to improve sensitivity and obtain a typical detection limit in the parts per thousand range.  Another limitation with Raman is fluorescence interferences, which if present can completely mask the Raman spectrum. To overcome such problems, specialised Raman techniques are needed; one such technique is SERS (Surface Enhanced Raman Spectroscopy).

Surface Enhanced Raman Spectroscopy

The basis of the SERS technique is the capacity of metallic substrates to support the propagation of surface plasmons with resonant frequencies in the visible region of the electromagnetic spectrum.  These surface plasmons act to enhance the native Raman signal by producing an increased electric field in the vicinity of the target molecule.

Silver and gold are the most widely used materials for SERS substrates and are most commonly used in the form of spherical particles on the nanometre scale.  In addition to these metal nanoparticles, other configurations, such as nano-structured surfaces, and the tip-surface geometries offered by scanning probe microscopies, are increasingly being used for SERS experimentation. The signal enhancement obtained means that Raman spectra can be recorded over a greater range and at much lower concentrations.

In recent years, it has been reported that single molecule detection is possible by SERS, suggesting that the enhancement factor (EF) can be as high as 10¹⁴ – 10¹⁵.  In addition to signal enhancement, SERS can also greatly reduce the problems of fluorescence interferences often observed in Raman spectroscopy, by good adsorption of the analyte to the metal surface. Overall, the ability of SERS to quench fluorescence in combination with the large signal enhancement means that we can produce high quality Raman spectra at low concentrations in the range of parts per billion.

Since its discovery in 1974, SERS has been used to detect and characterise a variety of chemicals (hazardous compounds such as environmental pollutants, explosives and chemical warfare agents) and biological species (proteins, lipids and common pathogens). Although the potential applications of SERS are vast and the technique holds considerable promise as a powerful tool for trace analysis, low repeatability and reproducibility often observed with SERS have limited its commercial applications.  Additionally the mechanisms underpinning SERS are not fully understood.  NPL recently co-organised a Faraday Discussions to debate these topical issues (Faraday Discussions, 2006, volume 132).  Our research in this area has two main strands, both targeted at overcoming some of these barriers:

• Design and modelling of novel structured substrates for SERS.
• Improving the reproducibility of SERS using metal nanoparticles.

References

Brown, R J C, Wang, J, Tantra, R, Yardley, R E, Milton, M J T “Electromagnetic modelling of Raman enhancement from nanoscale substrates: a route to estimation of the magnitude of the chemical enhancement mechanism in SERS” Faraday Discuss., 2006, 132, 201-213

Tantra, R, Brown, R J C, Milton, M J T, Wang, J “Improving the reproducibility of surface enhanced Raman spectroscopy (SERS)” VAM Bulletin, 2006, 35, 27-30

Maher, R C, Hou, J, Cohen, L F, Le Ru, E C, Hadfield, J M, Harvey, J E, Etchegoin, P G, Liu, F M, Green, M, Brown, R J C, Milton, M J T “Resonance contributions to anti-Stokes/Stokes ratios under surface enhanced Raman scattering conditions” J. Chem. Phys., 2005, 123, 084702

Etchegoin, P, Cohen, L F, Hartigan, H, Brown, R J C, Milton, M J T, Gallop, J C “Localized plasmon resonances in inhomogeneous metallic nanoclusters” Chem. Phys. Lett., 2004, 383, 577-583

Maher, R C, Cohen, L F, Etchegoin, P, Hartigan, H J N, Brown, R J C, Milton, M J T “Stokes/anti-stokes anomalies under surface enhanced Raman scattering conditions” J. Chem. Phys., 2004, 120, (24), 11746-11753

Maher, R C, Dalley, M, Le Ru, E C, Cohen, L F, Etchegoin, P G, Hartigan, H, Brown, R J C, Milton, M J T “Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids” J. Chem. Phys., 2004, 121, (18), 8901-8910

Etchegoin, P, Liem, H, Maher, R C, Cohen, L F, Brown, R J C, Milton, M J T, Gallop, J C “Observation of dynamic oxygen release in hemoglobin using surface Raman scattering” Chem. Phys. Lett., 2003, 367, (1-2), 223-229

Etchegoin, P, Maher, R C, Cohen, L F, Hartigan, H, Brown, R J C, Milton, M J T, Gallop, J C “New limits in ultrasensitive trace detection by surface enhanced Raman scattering (SERS)” Chem. Phys. Lett., 2003, 375, (1-2), 84-90

Etchegoin, P, Cohen, L F, Hartigan, H, Brown, R J C, Milton, M J T, Gallop, J C “Electromagnetic contribution to surface enhanced Raman scattering revisited” J. Chem. Phys., 2003, 119, (10), 5281-5289