SERS

Surface-enhanced Raman scattering (SERS) is a surface-sensitive spectroscopy technique that amplifies Raman scattering from molecules adsorbed on rough metal surfaces or nanostructures. The enhancement arises from the interaction between the incident light and collective oscillations of electrons in noble-metal substrates, enabling Raman signals from tiny amounts of material to be detected.

The primary mechanism is electromagnetic: localized surface plasmon resonances in metal nanostructures such as silver and gold create intense, highly confined near-field regions. Molecules in these hot spots experience greatly amplified excitation and scattering, yielding Raman signals that can be enhanced by factors between 10^6 and 10^14 under optimal conditions. A secondary, chemical mechanism involving charge-transfer between the molecule and the metal can contribute additional, often smaller, enhancement for certain species.

Substrates for SERS include roughened metal surfaces, nanoparticle aggregates, and lithographically defined nanostructures. Gaps between particles or between particles and substrates—hot spots—produce the strongest enhancements. SERS substrates are used in solution or immobilized formats and can be tailored for specific analytes.

Applications of SERS span chemical and biological sensing, environmental monitoring, food safety, and biomedical diagnostics. The technique enables trace detection, rapid spectral fingerprinting, and, in favorable cases, single-molecule sensitivity. Measurements are typically performed with visible or near-infrared excitation; spectra resemble conventional Raman spectra but may exhibit fluorescence backgrounds and extra variability due to substrate heterogeneity.

History notes that SERS was first observed in 1974 on roughened copper surfaces, revealing unexpectedly large Raman signals. The field has since matured with ongoing efforts to improve substrate reproducibility, stability, and quantitative capabilities.