Surface plasmon resonance (SPR) is a phenomenon occurring at metal surfaces(typically gold and silver) when an incident light beam strikes the surface at a particular angle.Depending on the thickness of a molecular layer at the metal surface,the SPR phenomenon results in a graded reduction in intensity of the reflected light.Biomedical applications take advantage of the exquisite sensitivity of SPR to the refractive index of the medium next to the metal surface, which makes it possible to measure accurately the adsorption of molecules on the metal surface an their eventual interactions with specific ligands. The last ten years have seen a tremendous development of SPR use in biomedical applications.
The technique is applied not only to the measurement in real time of the kinetics of ligands receptor interactions and to the screening of lead compounds in the pharmaceutical industry, but also to the measurement DNA hybridization, enzyme- substrate interactions, in polyclonal antibody characterization, epitope mapping, protein conformation studies and label free immunoassays. Conventional SPR is applied in specialized biosensing instruments. These instruments use expensive sensor chips of limited reuse capacity and require complex chemistry for ligand or protein immobilization. Laboratory has successfully applied SPR with colloidal gold particles in buffered solutions. This application offers many advantages over conventional SPR.
SPR Resonance Wavelength
Structure of the metal's surface
The nature of the medium in contact with the surface
To be useful for SPR, a metal must have conduction band electrons capable of resonating with light at a suitable wavelength. The visible and near-infrared parts of the spectrum are particularly convenient because optical components and high performance detectors appropriate for this region are readily available. A variety of metallic elements satisfy this condition. They include silver, gold, copper, aluminum, sodium, and indium. There are two critical limitations on the selection of a metal for sensor construction. The surface exposed to light must be pure metal. Oxides, sulfides and other films formed by atmospheric exposure interfere with SPR. The metal must also be compatible with the chemistries needed to perform assays. Specifically, the chemical attachment of antibodies or other binding molecules to the metal surface must not impair the resonance
The resonance condition that permits energy transfer from photons to plasmons depends upon a quantum mechanical criterion related to the energy and momentum of the photons and plasmons. Both the energy and momentum of the photons must match exactly the energy and momentum of the plasmons. For a flat metal surface, there is no wavelength of light that satisfies this constraint. Hence, there can be no surface plasmon resonance. However, there are three general configurations of SPR devices that alter the momentum of photons in a way that fulfills the resonance criterion, namely, prisms, gratings and optical waveguide-based SPR system (Figure 3). All three have been used to generate SPR
Plasmons, although composed of many electrons, behave as if they were single charged particles. Part of their energy is expressed as oscillation in the plane of the metal surface. Their movement, like the movement of any electrically charged particles, generates an electrical field.
The plasmon's electrical field extends about 100 nanometers perpendicularly above and below the metal surface. The interaction between the plasmon's electrical field and the matter within the field determines the resonance wavelength. Any change in the composition of the matter within the range of the plasmon's field causes a change in the wavelength of light that resonates with the plasmon. The magnitude of the change in the resonance wavelength, the SPR shift, is directly and linearly proportional to the change in composition
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