Raman spectroscopy is a technique that measures the energy of photons generated by the inelastic scattering of monochromatic excitation photons. A specialized version of Raman spectroscopy is surface enhanced Raman spectroscopy (SERS).  SERS utilizes the unique optical properties of nanostructured metallic substrates to enhance the intensity of the native Raman signal of an adsorbed analyte by several orders of magnitude. Nanostructured substrates composed of noble metals (primarily silver and gold) are capable of supporting localized surface plasmon resonance (LSPR). LSPR is a condition induced by the resonant excitation of surface-bound substrate electrons which generates a highly enhanced localized electromagnetic field. Raman scattering intensity is governed by the polarizability of the analyte being interrogated as well as the localized electromagnetic field experienced by the analyte as a result of laser excitation. LSPR results in a significant increase in magnitude of the local electromagnetic field near the surface of a nanostructured substrate. Adsorption of an analyte onto a LSPR-supporting nanostructured substrate followed by excitation with a laser of appropriate wavelength leads to an increase in Raman signal intensity by several orders of magnitude and a significant increase in sensitivity compared to traditional Raman spectroscopy.

SERS utilizes a conventional Raman spectrometer and a SERS substrate for sample preparation.  The success of a SERS measurement is critically dependent on the ability of the nanostructured substrate to support LSPR. Therefore, the development of robust, reproducible, and highly-active SERS substrates is an active research topic in the academic and commercial communities. There are several commercially available SERS substrates, mostly utilizing a chip-based array of metallic nano-features fabricated using lithographic techniques common to the semi-conductor processing industry. There are several other SERS substrate configurations in use including metallic nanoparticles in solution and thin, nanostructured metallic films supported on dielectric substrates. Substrate suitability is dependent upon several application-specific criteria including wavelength of the Raman excitation source used and adsorption affinity of the analyte for the substrate. Customized substrates can be fabricated to induce analyte-specificity by including a molecular recognition moiety such as an antibody/antigen or an analyte-specific aptamer.  While SERS has been reported for decades, it is an emerging commercial area.  A number of commercial substrates are available and cost is dependent upon substrate configuration.