Surface plasmon resonance (SPR) is an optical phenomenon that occurs when light interacts with a free electron metal under specific conditions. This resonance is highly sensitive to the refractive index of the environment close to the metal surface. This allows changes in refractive index to be monitored in real time.

Binding events caused by molecular interactions cause corresponding increases in refractive index making SPR a powerful non–invasive, label free, transduction principle for biosensing.