Surface plasmon resonance (SPR) is an optical technique utilized for detecting molecular interactions. Binding of a mobile molecule (analyte) to a molecule immobilized on a thin metal film (ligand) changes the refractive index of the film. The angle of extinction of light, reflected after polarized light impinges upon the film, is altered, monitored as a change in detector position for the dip in reflected intensity (the surface plasmon resonance phenomenon). 
SPR is a label-free technique and does not require additional reagents, assays, or laborious sample preparation steps. The major benefits associated with this technique are that it simply responds to changes in refractive index induced by molecular binding events.
SPR sensors have widely spread in many application areas as they are specific, sensitive, and quantitative. Any kind of target can be detected, from small molecules such as methane, carbon dioxide, metallic ions, polycyclic aromatic hydrocarbons (PAHs), pesticides, phycotoxin, and nucleic acid up to microorganisms.
Read more