Seira

SEIRA, or Surface-Enhanced Infrared Absorption, is a spectroscopy technique that boosts the infrared absorption signals of molecules by placing them on plasmonic nanostructures. The enhanced electromagnetic field near metallic nanostructures amplifies vibrational transitions, enabling detection of low-concentration species in the mid-infrared region, where many molecules exhibit characteristic bands.

The underlying mechanism combines an electromagnetic enhancement from localized surface plasmon resonances in metals (typically gold, silver, or other plasmonic materials) with a possible chemical contribution from molecule–substrate interactions. The electromagnetic effect is the primary source of signal amplification, which increases the effective absorption cross-section of the analyte.

Substrates used for SEIRA include nanostructured metal films, nanoantennas, metasurfaces, and other engineered plasmonic architectures designed to concentrate infrared fields at specific frequencies. Typical spectral range for SEIRA measurements lies in the mid-infrared (roughly 2.5 to 25 micrometers, or 4000–400 cm−1). Enhancement factors commonly reported span from about 10^2 to 10^4, depending on design, gaps, and molecular coverage.

Instrumentation often couples SEIRA substrates with Fourier-transform infrared spectroscopy or infrared microscopy, sometimes integrated with microfluidics for small-sample analysis. SEIRA provides label-free, surface-selective detection that complements conventional FTIR measurements, particularly for trace analysis and where targeted vibrational modes are enhanced by the substrate.

Applications are widespread in chemical sensing, biosensing, environmental monitoring, food safety, and forensic analysis. Limitations include variability and reproducibility of nanostructured substrates, substrate background signals, and the need for careful substrate design to avoid spectral interference. SEIRA development began in the late 1990s and has continued through ongoing advances in plasmonics and nanofabrication.