Surface Plasmon Resonance (SPR)

Surface Plasmon Surface electromagnetic waves that propagate in a direction parallel to the metal/dielectric (or metal/vacuum) interface. Since the wave is on the boundary of the metal and the external medium (air or water for example), these oscillations are very sensitive to any change of this boundary, such as the adsorption of molecules to the metal surface. Electron oscillations due to charge density at the interface of a material with a positive dielectric constant and that of a negative dielectric constant: Electromagnetic field. Surface plasmon resonance (SPR) : Excitation of surface plasmon by light for planar surfaces : - Localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures. Enhancement: a factor of about 10 for a smooth flat surface, and even more for a rough surface  Surface enhanced spectroscopy. (cf: Surface Enhanced Raman spectroscopy: 1010 to 1011 enhancement)

Evanescent field Formed when waves travelling in a medium undergo total internal reflection at its boundary because they strike it at an angle greater than the so-called critical angle  Used for excitation of surface plasmons (p-polarized light (polarization occurs parallel to the plane of incidence) can excite surface plasmons) Formed at the boundary between two media with different wave motion properties  Exponentially decaying electromagnetic field with distance from the boundary

Surface Plasmon Resonance When phases of two waves are coincident, resonance occurs, resulting in minimum reflectivity

Configuration for SPR SPS excitation with a prism (Kretschmann configuration), then the observation of plasmon is possible: the metal film is thin enough to monitor the plasmons from the backside SPS excitation with a prism (Otto configuration): observation of the plasmon is impossible through infinite metal If the thickness of the metal film varies, the phase difference between the reflected light and the light re-radiated by SP's will change, and for some "magical" thickness of the evaporated gold (48nm!)  they will become completely out of phase and almost equal in amplitude. Thus they will destructively interfere, no light will come out

If we scan the angle of incidence of light onto a metal film of the right thickness, then at a certain angle the reflected light intensity will go sharply to almost zero, indicating resonant coupling to surface plasmons. This angle is always greater than the angle of total internal reflection off the prism/outer dielectric interface, and is called the attenuated total reflection (ATR) angle.

Applications Absolute quantitative analysis of binding event Analysis of biomolecular interactions Kinetic and equilibrium studies - Binding affinity Label-free biochemical assays Drug discovery Advantage : high sensitivity : - Resonance angle shift of 0.1o : 1 ng/mm 2 - Detection limit of protein : 1.2 ng /cm 2