What is Surface Plasmon Resonance (SPR)?
When polarized light is shone through a prism on a sensor chip with a thin metal film on top, the light will be reflected by the metal film acting as a mirror. On changing the angle of incidence, and monitoring the intensity of the reflected light, one observes, that the intensity of the reflected light goes through a minimum. At this angle of incidence the light will excite surface plasmons inducing surface plasmon resonance, causing a dip in the intensity of the reflected light. Photons of p-polarized light can interact with the free electrons of the metal layer; inducing a wave-like oscillation of the free electrons thereby reducing the reflected light intensity.
Schematic experimental set-up of surface plasmon resonance excitation. A sensor chip with gold coating is placed on a hemisphere (or prism). Polarized light shines from the light source (star) on the sensor chip. Reflected light intensity is measured in the detector (disk).
The angle at which the maximum loss of the reflected light intensity occurs, is called resonance angle or SPR-angle. The SPR-angle is dependent on the optical characteristics of the system, e.g. on the refractive indices of the media at both sides of the metal, usually gold. While the refractive index at the prism side is not changing, the refractive index in the immediate vicinity of the metal surface will change when accumulated mass (e.g proteins) adsorb on a thin metal layer. Hence, the shift of the SPR angle is suited to provide information on the kinetics of e.g. protein adsorption to the surface.