Surface-Enhanced RAMAN SPECTROSCOPY.

This article presents information on surface-enhanced Raman scattering (SERS). SERS can be exploited for sensitive and selective molecular identification. Recently, SERS has been used extensively as a signal transduction mechanism in biological and chemical sensing. The mechanism of SERS enhancement remains an active research topic. Since the discovery of the phenomenon, two major factions have disagreed on the relative significance of the proposed mechanisms, chemical enhancement, and electromagnetic enhancement. In the chemical mechanism, which is now thought to contribute an average enhancement factor of 100, a charge-transfer state is created between the metal and adsorbate molecules. In a manner analogous to that observed in resonance Raman spectroscopy, the existence of this charge-transfer state increases the probability of a Raman transition by providing a pathway for resonant excitation. This mechanism is site-specific and analyte-dependent. The molecule must be directly adsorbed to the roughened surface to experience the chemical enhancement. To understand the electromagnetic enhancement, one must consider the size, shape, and material of the nanoscale roughness features. These characteristics determine the resonant frequency of the conduction electrons in a metallic nanostructure.