Surface Plasmon Resonance (SPR)

Surface Plasmon Resonance (SPR) is an optical based label-free detection method utilizing a form of total internal reflectance.  For normal total internal reflection light traveling through an optically dense medium at an angle of incidence that is greater than the critical angle, will be totally reflected when reaching an interface with an optically less dense medium.

Although the light is totally reflected, a component of the light called the evanescent wave will penetrate into the less dense medium a distance of approximately one wavelength of the incident light.  If the light is monochromatic and p-polarized, and the interface between the two different media is coated with a thin metal film, the evanescent wave will under certain conditions with the free oscillating electrons of the metal film.

When surface plasmon resonance occurs, light energy is lost to the metal film and the reflected light intensity is decreased.  When all else is kept constant the angle of the resonance phenomenon is dependent on the refractive index of the media close to the metal surface.  Changes in this refractive index can then be characterized by monitoring the resonance angle over time.

In practice an SPR sensor is usually a gold coated glass slide.  In biosensor applications, the binding of molecules to the sensor surface, or to molecules attached to the surface, changes the local refractive index and these changes are monitored over time.  By measuring these changes in refractive index information such as binding specificity, concentration of target molecules, kinetic rates, and affinity constants for molecular interactions can be determined.