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Surface Plasmon Resonance

The Surface plasmon resonance (SPR) phenomenon has been known for a long time. However, its application in biosensing is relatively new. A surface plasmon is an electromagnetic surface-bound wave (p-polarized) propagating along the interface between a metal and a dielectricum. The bonded wave has an evanescent field associated with it, decaying exponentially perpendicular to the surface.

By varying either the angle of incidence or the wavelength of a light source, excitation of a surface plasmon is possible (resonance), whereby the measured reflected intensity of the lightsource sharply decreases. This resonance angle or wavelength is a function of the refractive index of the dielectricum. Since the surface plasmon wave decreases exponentially from the surface (probe depth ~350nm), it is a surface sensitive technique.

Modifying the metal surface with organic thin films (SAMs) and subsequently biomolecules (i.e. proteins, ligands and DNA), one obtains a biosensor, able to monitor interactions with complementary biomolecules, since these interactions will result in a shift in the refractive index near the surface. Figure 1 gives a comprehensive explanation to the SPR-biosensor.

Figure 1 Schematic illustration of Surface Plasmon Resonance. The sensor surface is gold with antibodies attached to it. As the complementory antigen (analyte) binds to the antibodies, the refractive index shifts and the SPR-dip moves to larger angles. The movement of the SPR-dip is the actual monitored signal, and the movement over time forms the sensorgram.

Advantages of SPR is that it allows to study biomolecular interactions in real-time. There is also no need for labelling (with e.g. radioactivity or fluorescence), which, in addition to saving time, helps in preserving biomolecular nativity.

We have in our lab two custom-built SPR-instruments, one is able to resolve the surface in one dimension (over a line), and the other is capable of completely resolving the surface over two dimensions. Our instruments are versatile and flexible and can perform measurements for many different biochemical systems. We have for instance successfully monitored the coagulation of whole-blood on different surfaces. For now we are trying to characterize protein-array chips with these instruments.

For more information, contact Karin Enander (karen@ifm.liu.se).

Responsible for this page: Erik Martinsson
Last updated: 08/28/13