Surface Enhanced Raman Spectroscopy (SERS)

Example of electric field localization in colloids and sharp point samples. The field intensity depends on the inter-particle distance and particle shape

A molecule adsorbed on a corrugated metal produces a Raman signal which is 6 to 12 orders of magnitude more intense than the signal it would emit if deposited on a flat substrate. The enhancement comes from the increase of the local optical field intensity in the proximity of sharp points of textured metals [Au, Ag, Cu], or in the nano-scale gaps between colloidal particles.

It was observed for the first time in 1974 [1] on pyridine molecules absorbed unto an electrochemically roughened silver surface. To date, however, the theoretical understanding of SERS is not clear, but it is accepted that it has two linked components.

The electromagnetic contribution is due to the increase of the optical field intensity in the proximity of sharp points, whereas the chemical effect is due to the mixing of the orbital of the adsorbed molecule and the metal atoms.

How does SERS work?

The phenomena mediating the enhanced Raman scattering interaction between the laser light and the molecule is called a “surface plasmon”, and can be viewed as collective charge oscillation at the metal air interface.

SERS process steps: (1) laser light incident on the metal substrate (2) plasmons excitation (3) light scattered by the molecule (4) Raman scattered light transferred back to plasmons and scattered in air (5)
Plasmons at the metal act as antennas, which assist in coupling light into molecules close to the surface and couple out photons into specific directions. It is this enhanced coupling both into and out of the molecule that enhances the Raman signal.

The plasmon properties – such a wavelength and width of its resonance – depend on the nature of the metal surface and on its geometry. Many early SERS substrates used a random roughening of the surface so only small uncontrolled areas of the total metal surface would have the correct geometry for Raman enhancement. Other techniques have relied on aggregating gold colloids and with only some colloids in solution being SERS active. Most traditional techniques of preparing SERS surfaces have therefore been plagued by 100% variations in the raman signal across the surface and by hot spots where only small areas of the total devices had the right metal geometry for SERS enhancement.

Photonic Crystal SERS substrates

Photonic Crystal SERS substrates that are used in Klarite™ are a new class of highly engineered surfaces with sub-micron metal cavities. Instead of depending on random roughening or nanoparticle separation and sharp metallic features - as used in previous techniques - they exploit voids architecture. Such continuous flat-voids metal film can support two types of plasmons. The delocalised plasmons are distributed on the metal surface, whereas localised plasmons are trapped in the void features. Delocalised and localised plasmons interact strongly, both mutually and with the incident light. By modifying the size, separation and geometry of texture features, the properties of photonic crystal SERS substrates can be tuned, making them extremely versatile.

Furthermore by exploiting semiconductor lithographic fabrication technology the photonic crystal pattern of the Klarite™ surface can be reproducibly fabricated with high precision over large areas. By providing a uniform patterned surface, Klarite™ slides provide control of the Raman process giving consistent SERS signals from anywhere on the active surface.
Example of Photonic Crystal SERS substrates. The cross section shows the details of nano-structured metal. The top view diagram shows the uniform distribution of the localised and delocalised plasmons on the textured metal.
[1] Fleischman M, Hendra PJ, McQuillan AJ. Chem. Phys. Lett. 26, 123 (1974)

Sensitive and selective DNA detection

Surface enhanced resonance Raman scattering (SERRS) combines enhancement of the Raman signal by surface enhancement with resonance enhancement from a chromophore contained in the analyte We have shown in both DNA and antibody assays that this method of detection can be more sensitive than fluorescence and that the ability to identify and determine the concentration of several analytes in a mixture is much superior.

The advantages have been used to develop reliable and quantitative analytical procedures for disease conditions such as Cystic Fibrosis, Chlamydia and Gonorrhoea (1,2). The methods use specially designed probes which have a chromophore and surface attachment group. These are used with colloidal suspensions of silver to create SERRS. The use of colloidal suspensions has enabled us to add the superior detection capability to standard solution based molecular biology protocols.

Figure 1 Shows two probes synthesised. In probe 1 a chromophore which does not adhere well to the negative surface is forced onto it by adding positively charged propargyl amine groups and in the second, a positively charged dye which adheres directly is used.

Figure 1 The use of colloidal aggregates to obtain sensitive and specific SERRS from specially designed probes
The quantitative response is illustrated in figure 2. In one study sensitive detection limits were obtained for 8 probes (3) and we have more.

Figure 2 Quantitative response of the SERRS intensity for the rhodamine probe illustrated in figure 1.

Figure 3 Discrimination of three differently labelled probes in a mixture without separation
The huge potential for multiple analyte analysis is being exploited in the development of assay procedures suitable for use in hospital clinics.

References
1. Simple Multiplex Genotyping by Surface Enhanced Resonance Raman Scattering, SERRS Graham, D.*, Mallinder, B. J., Whitcombe, D., Watson, N. D., Smith, W. E.,Analytical Chemistry, 2002, 74, 5, 1069-1074.
2. Evaluation of Surface Enhanced Resonance Raman Scattering (SERRS) For Highly Sensitive and Quantitative DNA Analysis Faulds, K., Smith, W. E., Graham, D. Analytical Chemistry, 2004, 76, 412-417.
3. Quantitative simultaneous multianalyte detection of DNA by dual-wavelength surface-enhanced resonance Raman scattering Faulds K., McKenzie F., Smith W.E., Graham D., Angewandte Chemie, 2007, 46, 1829 -1831.