A matter for reflection

SPR is an optical phenomenon that occurs under conditions of total internal reflection of polarised light at an interface between two media of different refractive index, coated with an electrically conducting film. In this case the media are the glass of the sensor chip (high refractive index) and the sample (low refractive index). The conducting film is a thin gold layer on the surface of the chip. In Biacore, the light source is a high efficiency light-emitting diode with a wavelength near the IR region. A wedge of light is directed towards the sensor surface covering a range of incident angles, and the reflected light intensity is monitored. An evanescent wave created by total internal reflection interacts with and is absorbed by free electron clouds in the gold, generating plasmons and causing a drop in the intensity of the reflected light. The angle (resonance angle) at which this intensity minimum occurs is sensitive to the refractive index near the metal surface on the gold side of the chip. When material binds to the surface, the refractive index increases and the resonance angle is altered. The angle shift is proportional to mass change near the sensor surface. The angle is monitored continuously as the SPR signal. A plot of the signal against time on a graph called a sensorgram shows the changes in refractive index as material binds to the prepared surface.