Surface-enhanced Raman spectroscopy SERS intensity

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

In surface-enhanced Raman spectroscopy (SERS) samples are adsorbed onto microscopically roughened metal surfaces. Spectra are the intensities and frequencies of scattered radiation originating from a sample that has been irradiated with a monochromatic source such as a laser. SERS spectra are of molecules that are less than 50 A from the surface. 

Since most biomolecules normally exhibit medium or low Raman cross sections, an enhancement of the signal intensity for the ability to characterize even low concentrations would be preferable. Besides the application of resonance Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS) is a promising alternative. In doing so the vicinity of molecules to rough noble metal surfaces leads to Raman enhancement factors of 106-108 and even up to 1014 leading to a single molecule detection limit [9]. 

Surface-enhanced Raman spectroscopy (SERS) provides a means of obtaining the Raman spectmm of a monolayer. Although this method requires careful preparation of a roughened surface (necessary for intensity enhancement), and the absorbance may vary from sample to sample, it is very sensitive to the functionalities located close to the metal surface. The enhancement factor depends inter alia on the distance between the functional group and the metal surface on one hand and the surface coverage on the other. A typical distance at which the enhancement factor decreases to half of its initial value in a well-packed gold-thiol monolayer is about 3.5-7. This technique provides useful... 

Surface-enhanced Raman spectroscopy (SERS) " involves obtaining Raman spectra in the usual way on samples that are adsorbed on the surface of colloidal metal particles (usually silver, gold, or copper) or on roughened surfaces of pieces of these metals. For reasons that are finally becoming understood, at least semiquantitatively, the Raman lines of the adsorbed molecule are often enhanced by a factor of 10 to lO. When surface enhancement is combined with the resonance enhancement technique discussed in the previous section, the net increase in signal intensity is roughly the product of the intensity produced by each of the techniques. Consequently, detection limits in the range of 10 to 10 " M have been observed. 

Metal nanocrystals also interact strongly with electromagnetic waves and offer remarkable properties due to the localized surface plasmon resonance (SPR) that induces, through optical excitation, very intense local electrical fields. This property can be exploited for surface-enhanced Raman spectroscopy (SERS) and SPR-based... 

One way to try and compensate for the very low excitation intensities currently inherent in the RSNOM technique is to enhance the Raman signal from the sample. A number of reports have been published in which surface-enhanced Raman SNOM measurements have been made [34,42,44]. This is particularly interesting as the surface topography of a substrate for surface-enhanced Raman spectroscopy (SERS) plays a critical role in the SERS enhancement process. RSNOM provides both a Raman map with spatial resolution comparable to the surface roughness and also a simultaneous topographic image of the SERS substrate. 

Nanoporous platforms recently have found utility in the fields of plasmonics and optical detection. Nanoporous gold fllms and metallic-coated nanopores - have been applied to techniques like surface-plasmon resonance (SPR) and surface-enhanced Raman spectroscopy (SERS). Additionally, nanoporous metal has been demonstrated to enhance single-molecule fluorescence intensity of immobilized fluorophores due to the enhanced localized plasmon field present within the nanopores. Optically transparent alumina membranes have been developed and found utility as optical biosensors. Additionally, nanoporous gold has been demonstrated to optically detect Hg + ions at concentrations smaller than parts per trillion. A fiber-optic ultrasound generator has been developed from the excitation of gold nanopores with a nanosecond laser. ... 

SERS Surface-enhanced Raman spectroscopy [214-217] Same as RS but with roughened metal (usually silver) substrate Greatly enhanced intensity... 

STM-Raman spectroscopy utilizes the effect that Raman scattering is enhanced for a molecule in the vicinity of a metal nanostructure. This enhancement effect is generally called surface-enhanced Raman scattering (SERS). When a sharp scanning probe, such as a tunneling tip for STM, is used as a metal nanostructure to enhance Raman intensity, it is called tip-enhanced Raman scattering (TERS). The concept of STM combined with Raman spectroscopy is presented in Figure 1.1. 

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