Surface plasmons, nanostructured metals

The electromagnetic enhancement mechanism features the major contribution to the overall enhancement of SERS. It is based on the generation of an electromagnetic field at the surface of nanostructured metal surfaces due to the interaction of an incident electromagnetic field and the excitation of localized surface plasmon polaritons. To explain this phenomenon in more detail, a simple model can be used. A simple metal nanosphere with a size smaller than the wavelength of the incident light is considered for this purpose. This metal nanosphere is surrounded by a medium or vacuum with a dielectric constant Eq, and all appearing processes are assumed to be quasi-static. The dielectric constant inside the metal nanosphere is independent of the size of the sphere and is described as follows ... 

When the size of metals is comparable or smaller than the electron mean free path, for example in metal nanoparticles, then the motion of electrons becomes limited by the size of the nanoparticle and interactions are expected to be mostly with the surface. This gives rise to surface plasmon resonance effects, in which the optical properties are determined by the collective oscillation of conduction electrons resulting from the interaction with light. Plasmonic metal nanoparticles and nanostructures are known to absorb light strongly, but they typically are not or only weakly luminescent [22-24]. 

Similar to zero-dimensional metal nanoparticles, most of the work on one-dimensional metal nanostructures focuses almost exclusively on gold nanorods. The high interest in anisometric gold nanoclusters arises from their unique optical and electronic properties that can be easily tuned through small changes in size, structure (e.g., the position, width, and intensity of the absorption band due to the longitudinal surface plasmon resonance is strongly influenced by the shell as well as the aspect ratio of the nanorods), shape (e.g., needle, round capped cylinder, or dog bone), and the inter-particle distance [157]. 

To understand the profound effect that nanostracturing can have on light, with the help of surface plasmons, we consider the simplest nanostructure a gap between two metals. 

Metal nanostructures (such as particles and apertures) can permit local resonances in the optical properties. These local resonances are referred to as localized surface plasmons (LSPs). The simplest version of the LSP resonance comes for a spherical nanoparticle, where the electromagnetic phase-retardation can be neglected in the quasi-static approximation, so that the electric field inside the particle is uniform and given by the usual electrostatic solution [3] ... 

In the case of using a substrate with surface-relief nanostructures, overall trends would agree with what is mentioned in (Sect. 2.1.2), i.e., momentum-matching would occur at higher plasmon momentum. Since gold nanoparticles act as a target of supermolecules, plasmon momentum would be shifted further, which may induce nonlinear plasmon characteristics. Here, the effects of particle parameters, such as particle size, concentration, shape, and interaction distance between metal surface and nanoparticles, are discussed briefly based on experimental data of the interaction between particle plasmons and conventional SPs. 

Xia, Y. N. and Halas, N. J. (2005). Shape-controlled synthesis and surface plasmonic properties of metallic nanostructures. Mrs Bulletin 30 338-344. 

The fluorescence amplification provided by the plasmonic nanostructures has been shown to be applicable to many fluorophores. Hence fluorophores currently employed in assays would still be suitable. However, the use of low quantum yield fluorophores would lead to much larger fluorescence enhancements (i.e. 1 / Qo) and could significantly reduce unwanted background emission fi om fluorophores distal fi om the metallic surface. 

Surface plasmons (SPs) are collective electronic excitations near the surfaces of metallic structures. They can usually be described well with classical electromagnetic theory and correspond to electromagnetic fields that are localized and relatively intense near the metallic surfaces [1, 2]. These properties make them potentially useful for a variety of applications in optoelectronics, chemical and biological sensing, and other areas. Metallic nanostructures such as metal nanoparticles and nanostructured thin metal films, particularly those composed of noble metals such as silver or gold, are of special interest because often their SPs can be excited with visible-UV light and are relatively robust. 

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