ABSTRACT
Techniques that use ¦uorescence probes based on ¦uorescence energy transfer (e.g., molecular beacons) have been widely applied with great success in biosensing assays. Nevertheless, the need for alternative, rapid, and selective assays has continued to encourage researchers to explore other technologies having comparable sensitivity as ¦uorescence but having additional unique and complementary advantages. Raman spectroscopy is an important analytical technique for chemical and biological analysis due to the wealth of information on molecular structures, surface processes, and interface reactions that can be extracted from experimental data. ™e spectral selectivity associated with the narrow emission lines and the molecularspeci›c vibrational bands of Raman labels make it an ideal tool for molecular genotyping. However, a limitation of Raman techniques for trace detection is the very weak Raman cross section. However, Raman spectroscopy has gained increasing interest as an analytical tool with the advent of the surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS) ežects, which can produce signi›- cant enhancement of the Raman signal. It is believed that the origin of the enormous Raman enhancement is produced by at least two main mechanisms that contribute to the SERS ežect: (a) an electromagnetic ežect occurring near metal surface structures associated with large local ›elds caused by electromagnetic resonances, o²en referred to as “surface plasmons” and (b) a chemical ežect involving a scattering process associated with chemical interactions between the molecule and the metal surface. Plasmons are quanta associated with longitudinal waves
propagating in matter through the collective motion of large numbers of electrons. According to classical electromagnetic theory, molecules on or near metal nanostructures experience enhanced ›elds relative to that of the incident radiation. When a metallic nanostructured surface is irradiated by an incident electromagnetic ›eld (e.g., a laser beam), conduction electrons are displaced into frequency oscillations equal to those of the incident light. ™ese oscillating electrons, called “surface plasmons,” produce a secondary electric ›eld, which adds to the incident ›eld. ™ese ›elds can be quite large (106-107-, even up to 1015-fold enhancement at “hot spots”). When these oscillating electrons become spatially con›ned, as is the case for isolated metallic nanospheres or otherwise roughened metallic surfaces (nanostructures), there is a characteristic frequency (the plasmon frequency) at which there is a resonant response of the collective oscillations to the incident ›eld. ™is condition yields intense localized ›elds that can interact with molecules in contact with or near the metal surface (Otto 1978, Gersten and Nitzan 1980, Schatz 1984, Zeman and Schatz 1987). In an ežect analogous to a “lightning rod” ežect, secondary ›elds can become concentrated at high curvature points on the roughened metal surface. In SERRS, the energy of the incoming laser is selected such that it coincides with an electronic transition of the molecule being monitored. An advantage of SERRS over SERS is the large increase in intensity of the Raman peaks.
