Surface-Enhanced Raman Spectroscopy

Abstract

mekal theoretically predicted inelastic light scattering in 1923 (1). Raman and Krishnan first experimentally observed the phenomenon and reported in their 1928 Nature paper that the inelastic scattering effect was characterized by "its feebleness in comparison with the ordinary scattering" (2). This "feeble" phenomenon is now known as Raman scattering. The change in wavelength that is observed when a photon undergoes Raman scattering is attributed to the excitation (or relaxation) of vibrational modes of a molecule. Because different functional groups have different characteristic vibrational energies, every molecule has a unique Raman spectrum. In accordance with the Raman selection rule, the molecular polarizability changes as the molecular vibrations displace the constituent atoms from their equilibrium positions. The intensity of Raman scattering is proportional to the magnitude of the change in molecular polarizability. Thus, aromatic molecules exhibit more intense Raman scattering than aliphatic molecules.Even so, Raman scattering cross sections are typically 14 orders of magnitude smaller than those of fluorescence; therefore, the Raman signal is still several orders of magnitude weaker than the fluorescence emission in most cases. Because of the inherently small intensity of the Raman signal, the sensitivity limits of available detectors, and the intensity of the excitation sources, the applicability of Raman scattering was restricted for many years. However, its utility as an analytical technique improved with the advent of the laser and the evolution of photon detection technology.In 1977, Jeanmaire and Van Duyne demonstrated that the magnitude of the Raman scattering signal can be greatly enhanced when the scatterer is placed on or near a roughened noble-metal substrate (3). Strong electromagnetic fields are generated when the localized surface plasmon resonance (LSPR) of nanoscale roughness features on a silver, gold, or copper substrate is excited by visible light. When the Raman scatterer is subjected to these intensified electromagnetic fields, the magnitude of the induced dipole increases, and accordingly, the intensity of the inelastic scattering increases. This enhanced scattering process is known as surface-enhanced Raman (SER) scattering-a term that emphasizes the key role of the noblemetal substrate in this phenomenon.

TiO2 nanofibrous interface development for Raman detection of environmental pollutants

“…The inherent nanofibrous porosity also controls molecular diffusion which is critical in micro-spot sized optical spectroscopy techniques. 20,21 In this study, we have demonstrated controllable surface roughness and porosity which was directly related to SERS activity of BPA and DCFNa. These findings impact the SERS sensor pad development towards chemical and bio-molecular sensing applications.…”

“…Since strong BPA adsorption is expected, fluorescence is attributed to a reduction in nanoparticle size. 21 As opposed to BPA, DCFNa was detected better under dry residue conditions given its high solubility under aqueous conditions. DCFNa structure can have two nonsymmetrical isomers with 58 irreducible vibration modes.…”

Gold nanostructures: a class of multifunctional materials for biomedical applications

“…Interested readers are directed to ref. 50 and 51 for more information about this technique. The sensitivity of LSPR to changes in dielectric environment and aggregation has also been harnessed to fabricate colorimetric sensors.…”

The Role of Hybrid Modes in Extraordinary Optical Transmission Through a Plasmonic Nanohole Array

“…Nanometallic objects are able to trap incident radiation, focusing it into very small volumes. Gold nanoparticles have been used to concentrate electromagnetic fields to increase Raman scattering in a molecular identification technique known as surface-enhanced Raman spectroscopy (SERS) [8]. Nanoparticles may be used to control membrane processes via thermoplasmonics [9,10].…”