Surface Plasmons Resonance:

SPR resonance is a sensitive probe for an assessment of the interfacial architecture.
The mass coverage, thickness and optical constants are measured with high precision. A time resolution in the sub-milliseconds regime and sub-monolayer sensitivity is achieved. SPR is ideally suitable for the investigation of fast kinetic processes and has become a standard technique in the field of biosensing. SPR experiments are fairly simple and robust.

The main advantage of the SPR resonance is the localization of the electric field close to the interface. The plasmon is only sensitive to changes at the interface and the bulk has only little impact on the plasmon. Unfortunately surface plasmons can only be formed in a couple of metals such as gold or silver.

The following review article can be recommended: 

Raether, Surface plasmons, Springer, Heidelberg
Homola et al. Surface plasmon resonance sensors: Sensors and Actuators B 54 (1999), 3-15.

Basics

SPR spectroscopy is an optical reflection technique with a high sensitivity to the prevailing interfacial architecture. A surface plasmon is a charge-density oscillation that may exist at the interface of two media with dielectric constants of opposite signs, for instance, a metal and a dielectric. The charge density wave is associated with bound TM-polarized electromagnetic wave at the metal dielectric interface. The electric field of this wave has its maximum at the interface and decays evanescently into both media. Any change in refractive index of the bulk or the binding events lead to a shift in the SPR resonance.

The excitation of a surface plasmon requires a special geometry. It turns out that is impossible to excite a surface plasmon within a simple reflection experiment. One of the mandatory condition for the excitation of SPR resonance is that the projection of the wavevector k_x of light matches the one of the plasmon. As sketched in the following figure it turns out that the dispersion relation of light in air omega(k) has no intersection with the corresponding curve of the plasmon. As a consequence the reflection of light at a metal surface does not lead to the formation of a plasmon provided that air is the ambient media. A trick helps to fulfil the conditions. If light is incident in a media with a higher refractive index than air the slope of the straight line is changed and a intersection between the plasmon dispersion relation may occur. This is utilized in the so-called Kretschmann configuration.

Angular resolved techniques using a prism in anATR configuration have become the most common arrangement. The metal layer is at the base of a prism and the reflected intensity is measured as a function of the angle of incidence. The angle scan changes the projection of the wavevector k_x onto the prism base in a similar fashion as wavelength change.
For this reason the in a way misleading term Surface Plasmon spectroscopy is commonly used even if laser light with a single wavelength is incident on the sample.

The excitation of the plasmon occurs in the total reflection regime. The exact position of the resonance bears information on the interfacial mass coverage or the thickness of an interfacial layer.

The reflectivity of a silver coated prism is calculated as a function of the angle of incidence. The formation of the surface plasmon is indicated by a dip in the reflectivity. The laser light is then used to excite the surface plasmon and consequently a minimum in the reflectivity is observed. 

The following video illustrate some of the important features of surface plasmons. It illustrates also the handling of our data simulation and evaluation program. The video clip requires the realplayer, soundcard and loudspeaker.


View video

The aim of SPR instrumentation is to determine the resonance position as precisely as possible and with the best time resolution. This task is solved in the Multiskop in four different modes.(see control software)