Surface-enhanced Raman spectroscopy

Abstract

Modern analytical tools should enable highly specific identification and characterization of inorganic and organic matter with minimal effort for sample preparation. Raman spectroscopy is one such highly specific method that enables identification of molecules through their specific molecular fingerprint information. Unfortunately the sensitivity of Raman spectroscopy is quite low, meaning that it cannot be used for the analysis of samples with low analyte concentration. One possible solution to this problem is the use of metal nanostructures or particles to enhance the intrinsically weak Raman effect. This method is known as surface enhanced Raman spectroscopy (SERS) and it combines the specificity of Raman with high sensitivity which enables analysis of samples with minimal analyte concentration. However, to develop SERS further and to convert it in a standard analytical tool several problems have to be solved. A major issue for routine application of SERS is the production of reproducible SERS substrates, which have predictable and reliable enhancement factors, because the enhancement of the Raman effect is highly dependent on the structure of the SERS substrates. The rapid development of nanotechnology has helped develop new concepts for the production of reproducible SERS substrates. In their review Ren et al. describe and compare different methods for production of SERS substrates. Further, they analyze existing methods for determination of the enhancement factors of substrates and propose, on the basis of the results from this analysis, guidelines to obtain these factors. Extremely high enhancement factors can be achieved by using dimers of gold nanoparticles, whereby the enhancement factor depends on the spacing between the two particles. Using finite element method calculations Schatz et al. investigate the optimal spacing of particles. Another possible SERS substrate consists of planar gold nanostructures, which are made by Electron Beam Lithography. These structures can be reproducibly fabricated but have an interparticle spacing which is much bigger than the optimum described by Schatz and co-workers. Using different analytical methods, the plasmon dynamics and evanescent field distributions of these substrates have been investigated. Besides employing SERS substrates, an alternative approach for the realization of SERS sensors is the use of optical fibres. Inspired by the success of optical fibre systems implementing conventional Raman spectroscopy, there is growing interest in the development of SERSactive fibres. In their review Stoddart and White discuss the development of technologies for the production of such fibres and also show the potential and challenges in these recent developments. Probably one of the most widely used type of SERS substrate employs metal colloids and their aggregates. These colloids can be used in solution and also quench the fluorescence signal of the analyte in the course of surface enhanced resonance Raman spectroscopy. This enables the highly sensitive detection of dyes in aqueous solution as described by Shadi et al. By combination of metal colloids and microfluidics, tools can be created that enable the automated and sensitive detection of substances. In their contribution Choo et al. use this approach to detect Anal Bioanal Chem (2009) 394:1717–1718 DOI 10.1007/s00216-009-2864-z