Fundamentals of Surface-Enhanced Raman Spectroscopy

The extremely small cross sections for conventional Raman scattering, typically 10 111 to 10-25 cm2/molecule has in the past precluded the use of this technique for single-molecule detection and identification. Until recently, optical trace detection with single molecule sensitivity has been achieved mainly using laser-induced fluorescence [14], The fluorescence method provides ultrahigh sensitivity, but the amount of molecular information, particularly at room temperature, is very limited. Therefore, about 50 years after the discovery of the Raman effect, the novel phenomenon of dramatic Raman signal enhancement from molecules assembled on metallic nanostructures, known as surface-enhanced Raman spectroscopy or SERS, has led to ultrasensitive single-molecule detection. 

The size of the enhancement factor or the effective SERS cross section is a key question for the application of SERS as a tool for ultrasensitive detection. The effective cross section must be high enough to provide a detectable Raman signal from a few molecules. In the early SERS experiments, Van 

Duyne and co-workers estimated enhancement factors on the order of 105 to 106 for pyridine on rough silver electrodes. The value was obtained from a comparison between surface-enhanced and normal bulk Raman signals from pyridine by taking into account the different number of molecules on the electrode and in solution. The size of the enhancement was found to correlate with the electrode roughness, indicating that enhancement occurs via a strong electromagnetic field. On the other hand, the dependence of the 

Attenuated total reflection infrared (IR) spectra are obtained by pressing the sample against an internal reflection element (IRE) [e.g., zinc selenide (ZnSe) or germanium (Ge)]. IR radiation is focused onto the end of the IRE. 

Sample Location Anti-Stokes (cps)6 Stokes (cps)6 Ratio 

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