The basic optical configuration of an SPR biosensor consists of a
prism coated with a thin metal film (usually silver or gold)
(figure). In this configuration, which was originally proposed by
Kretschmann & Raether, the total internal reflection of light is
used to excite nonradiative surface plasmons in the metal film.
Surface plasmons are waves of oscillating surface charge density
traveling along the metal surface. The electromagnetic field
amplitudes of nonradiative surface plasmons decay exponentially with
increasing distance perpendicular to the sensor surface, with a decay
length in the biosensor of a few hundred nanometers. These plasmons
can be resonantly excited by light only at a well-defined angle of
incidence, which occurs when the wave vector of the light in the
plane of the sensor surface matches that of the surface plasmon. The
resonance causes an energy loss in the reflected light, which is
visible as a sharp minimum in the angle-dependent reflectance, which
is, experimentally, the primary recorded quantity. The resonance
angle strongly depends on the refractive index (or dielectric
constant) profile of the sample within the evanescent field above the
sensor surface. Adsorption or desorption of macromolecules at the
sensor surface change the local refractive index and produce a shift
in the resonance angle, which, to a good approximation, has been
shown to be proportional to the surface concentration of
macromolecules up to a concentration of 50 ng mm-2. For a given
refractive index increment of the macromolecules, the signal is
approximately proportional to the mass that is bound to the sensor
surface. The time-dependent change of the refractive index in the
vicinity of the surface upon binding is commonly measured in RU
(resonance units), with 1 RU corresponding to ~1pg protein mm-2.