Surface plasmon resonance oscillations

Fig. 2 Surface plasmon resonance (SPR) principle. Surface plasmons are excited by the light energy at a critical angle (9) causing an oscillation and the generation of an evanescent wave. Under this condition a decrease in the reflected light intensity is observed. The angle 9 depends on the dielectric medium close to the metal surface and therefore is strongly affected by molecules directly adsorbed on the metal surface. This principle allows the direct detection of the interaction of the analyte and the antibody...

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]. 

Figure 20-22a shows essentials of one common surface plasmon resonance measurement Monochromatic light whose electric field oscillates in the plane of the page is directed into a prism whose bottom face is coated with a thin layer (—50 nm) of gold. The bottom surface of the gold is coated with a chemical layer (—2-20 nm) that selectively binds an analyte of inter-... 

In addition to the surface plasmon resonance, other phenomena contribute to the field enhancement. The oscillating dipole of the molecular vibration induces an image field in the metal, sometimes called the antenna effect (1). These effects have also been treated theoretically, leading to a combined theory that predicts overall field enhancement. Detailed discussions of these considerations have been presented and should be consulted by the interested reader (6,7). 

Surface plasmon resonance (SPR) biosensors exploit special electromagnetic waves-surface plasmon-polaritons-to probe interactions between an analyte in solution and a biomolecular recognition element immobilized on the SPR sensor surface. A surface plasmon wave can be described as a light-induced collective oscillation in electron density at the interface between a metal and a dielectric. At SPR, most incident photons are either absorbed or scattered at the metal/dielectric interface and, consequently, reflected light is greatly attenuated. The resonance wavelength and angle of incidence depend upon the permittivity of the metal and dielectric. 

It is clear from the foregoing considerations that the surface plasmon is shifted by interaction with the oscillatory modes of the adsorbed layer, and new coupled modes are introduced. In fact, the adsorbed layer substantially changes all the dielectric response properties of the substrate in accordance with Eq.(22). In consequence of this, its optical properties are modified, in particular in surface plasmon resonance experiments (as well as in all other probes). Analysis of such modifications reflect on the nature of the oscillatoiy modes of the adsorbate, which can identify it for sensing purposes. It should be noted that the determination of the screening function K (Eq.(22), for example) not only provides the shifted coupled mode spectram in terms of its frequency poles, but it also provides the relative oscillator strengths of the various modes in terms of the residues at the poles. The analytic technique employed here for the adsorbate layer (in interaction with the substrate) can be extended to multiple layers, wire- and dot-like structures, lattices of such, as well as to the case of a few localized molecular oscillators. It can also take account of spatial nonlocality, phonons, etc., and the frequencies of the shifted surface (and other) plasmon resonances can be tuned by the application of a magnetic field. 

Surface plasmons, also known as surface plasmon polaritons, are surface electromagnetic waves that propagate parallel along a metal/dielectric (or metal/vacuum) interface. Since the wave is on the boundary of the metal and the external medium (air or water for example), these oscillations are very sensitive to any change in the boundary, such as the adsorption of molecules to the metal surface. Surface Plasmon Resonance (SPR) is the most popular method for optical biosensing. 

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