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2015
May 29

Surface Enhanced Raman Scattering (SERS) technique can be used for label-free identification of materials within analyte. Theoretically, it is applicable to any type of materials, however, for some analytes it is difficult to achieve good adsorption and proper orientation of molecules to be analyzed.Raman scattering signal represents vibrational information of molecules, i.e. the energy of atomic vibrations contributes to shifts in a spectrum of exciting radiation. Photon energy of incident laser radiation is changed upon interaction with vibrating ions. For low (room) temperature materials, the energy is usually absorbed, thus energy of incident photons is reduced and the scattered light spectrum shifts to the side of longer wavelengths (the Stokes shift). And if the material is heated prior to illumination, the ions might release energy and the photons are gaining more energy, thus the spectra is shifting towards short wavelength range (the anti-Stokes shift).Under regular conditions, i.e. when analyte is directly illuminated with laser radiation, the Raman scattering process is rather weak and inefficient. Efficiency grows with increasing number of photons, which collide with molecules, however simply focusing the laser beam is not an option, as a high fluence might affect the analyte material – evaporate, degrade or bleach. Therefore various enhancement techniques are preferred, such as surface- or tip-enhanced Raman scattering.SERS is a popular method for enhancing usually weak Raman scattering signals. It is used in spectroscopy, where small concentrations of materials have to be detected in solutions, powders or gasses. Colloidal nanoparticles or special configuration of nano-engineered surfaces are covered with a thin layer of noble metals, such as silver or gold. Analyte particles attached to the metal surface are exposed to large scale amplification of its Raman spectra. Localized Surface Plasmons (LSP) are playing a major role in such enhancement processes. It is important to understand, that SERS substrates enhance not the Raman spectra itself, but the incident laser radiation. Irradiated electromagnetic field is concentrated on separate noble metal-plated nanoparticles and especially in small gaps between them, thus photon-molecule interaction efficiency is increased, leading to higher intensity of scattered radiation.