Electromagnetic enhancement predictions

The natural geometry of biomolecules can be exploited to clarify the SERS sensitivity dependence on distance from the surface. This aspect of SERS spectroscopy is important for the interpretation of the SERS spectrum of a biopolymer. The electromagnetic model predicts very rapid decay of SERS with increasing distance Thus, in small molecules with a dimension of approximately 0.6 nm (benzene) all vibrations of the molecule can be enhanced. In large biomolecules with diameters of about 6 nm (hemoglobin protein) only groups which are attached directly to the surface will yield SERS. This important aspect is illustrated by SERS studies of three examples mono-, di-, and polynucleotides. 

SERS is an interface-sensitive technique, with molecules adsorbed in the first layer at the surface showing the largest enhancements. However, electromagnetic enhancement has a long-range property (i.e., it decays exponentially with increasing distance from the surface, 1-10 nm) [45-48], which predicts that SERS does not require the adsorbate to be in direct contact with the surface but rather within a certain sensing volume. 

When the hydrophobic and electrical properties do not favor enolization and ionization of Ce=0 then an orientation effect should be considered. Indeed, another possible interpretation is based on the short-range effects 2 of fhe SERS enhancement factors and on specific orientations of the guanine derivates at the silver surface. As the electromagnetic model predicts, a very rapid decrease of SERS effects with distance on the A scale would limit the enhancement factors to bond vibrations in the immediate vicinity of the silver surface. The dispari-tion of the carbonyl stretching vibrations in SERS spectra of 3-Me-Gua and 9-Me-Gua would thus suggest that the C6=0 bond lies far away from the surface. The hypothesis of an "inactive" SERS carbonyl vibration would consider orientations for both methylted guanines in the adsorbed state different from the other derivatives. In 1-Me-Gua and 7-Me-Gua the Ce O... 

The electromagnetic field enhancement provided by nanostructure plasmonics is the key factor to manipulate the quantum efficiency. However, as it is illustrated in the unified theory of enhancement, since both the radiative and non-radiative rates of the molecular systems are affected by proximity of the nanostructure, the tuning has to be done on a case by case basis. In addition, there are factors due to molecule-metal interactions and molecular orientation at the surface causing effects that are much more molecule dependent and as are much more difficult to predict. Given the fact that fluorescence cross sections are the one of the highest in optical spectroscopy the analytical horizon of SEF or MEF is enormous, in particular in the expanding field of nano-bio science. 

Since enhanced electromagnetic fields in proximity to metal nanoparticles are the basis for the increased system absorption, various computational methods are available to predict the extent of the net system absorption and therefore potentially model the relative increase in singlet oxygen generation from photosensitizers. " In comparison to traditional Mie theory, more accurate computational methods, such as discrete dipole approximation (DDA/ or finite difference time domain (FDTD) methods, are often implemented to more accurately approximate field distributions for larger particles with quadruple plasmon resonances, plasmon frequencies of silver nanoparticles, or non-spherical nanoparticles in complex media or arrangements. ... 

However, there are few studies which predict such large chemical enhancements while several similar TDDFT studies on different molecules predict much lower enhancements [72-74]. For example, a study of pyridine complexed to a 20 atom Ag cluster by the same group predicted much lower enhancement factors. Factors on the order of 10 for the static chemical enhancement, 10 for charge transfer effects, and 10 for the electromagnetic contribution emphasized that the chemical enhancement is a much smaller contribution in general [72]. 

The possibility of utilizing the surface electromagnetic field to enhance the Raman scattering from overlayers on metals in an ATR configuration was proposed before the SERS era by Chen, et 375,376 predicted, for silver, enhancements of about 300 when... 

Thermal emission from a single-wall carbon nanotube (CNT) has been analyzed in the dipole approximation using the fluctuation-dissipative theorem. A strong resonance enhancement of the thermal radiation intensity of metallic CNTs in the far zone is predicted at frequencies of the electromagnetic surface wave resonances. 

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