If a light beam passes a medium (e.g.: glass) with higher refractive index (ռ1) and hits a medium (e.g.: water) with lower refractive index (ռ2) at different angles of incidence, one part of the light will be reflected; another part of the light penetrates the lower refractive medium.
At the critical angle, the light will be totally reflected (Figure 1-2). Even at Total Internal Reflection (TIR), the so-called evanescent wave penetrates the lower refractive medium, see Figure 1-1.
If there is a thin metal layer (e.g.: silver or gold) attached to the medium (e.g.: glass) with higher refractive index (ռ1) as illustrated in Figure 1-3, the relation between the reflected light intensity and the angle of incidence is changing.
At another angle (> critical angle), the so-called SPR-angle, a minimum in the reflected light intensity can be observed, due to resonance of surface plasmons in the metal film and photons of the incident light (Figure 1-4). In a broader sense surface plasmons can be seen as delocalized electron oscillations at the interface of the metal film. Almost the whole energy of the incident light is absorbed by surface plasmons in the metal film, when wave-vector and the electrical field of the surface plasmons equals the wave-vector and the electrical field of the photons of the incident light.
In a SPR biosensor a microfluidic channel is attached to the gold surface in order to facilitate sample delivery to and from the gold surface (Figure 1-5).
Furthermore, if the wavelength of the incident light, the temperature and thickness of the metal film are kept constant, the angle at which surface plasmon resonance takes place depends only on refractive index changes of the lower dense medium (e.g.: water). The association and dissociation of biomolecules to or from the sensor surface leads to continuous changes of the refractive index, which result in changes of surface plasmon resonance conditions or changes in SPR angle (Figure 1-6).
Most of SPR biosensors report the signal as RU (resonant unit) over time. Thereby 1 RU corresponds to ~10-4 angular shift of the SPR-Dip or 1 RU is approximately ~10-6 refractive index change. The collected data (RU vs. time) is called sensorgram (Figure 1-7).