Surface plasmon resonance
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The excitation of surface plasmons by light is denoted as a surface plasmon resonance (SPR) for planar surfaces or localized surface plasmon resonance (LSPR) for nanometer-sized metallic structures.
Surface plasmons, also known as surface plasmon polaritons, are surface electromagnetic waves that propagate parallel along a metal/dielectric interface. For electronic surface plasmons to exist, the real part of the dielectric constant of the metal must be negative and its magnitude must be greater than that of the dielectric. This condition is met in the IR-visible wavelength region for air/metal and water/metal interfaces (where the real dielectric constant of a metal is negative and that of air or water is positive). Electronic and magnetic surface plasmons obey the following dispersion relation:
Typical metals that support surface plasmons are silver and gold, but metals such as copper, titanium, or chromium can also support surface plasmon generation. Surface plasmons have been used to enhance the surface sensitivity of several spectroscopic measurements including fluorescence, Raman scattering, and second harmonic generation. However, in their simplest form, SPR reflectivity measurements can be used to detect DNA or proteins by the changes in the local index of refraction upon adsorption of the target molecule to the metal surface. If the surface is patterned with different biopolymers, the technique is denoted as Surface Plasmon Resonance Imaging (SPRI).
For nanoparticles, localized surface plasmon oscillations can give rise to the intense colors of solutions of plasmon resonance nanoparticles and/or very intense scattering. Nanoparticles of noble metals exhibit strong ultraviolet-Visible absorption bands that are not present in the bulk metal. Shifts in this resonance due to changes in the local index of refraction upon adsorption of biopolymers to the nanoparticles can also be used to detect biopolymers such as DNA or proteins. Related complimentary techniques include plasmon waveguide resonance, QCM and Dual Polarisation Interferometry
Binding constant determination
When the affinity of two ligands has to be determined, the binding constant must be determined. It is the equilibrium value for the product quotient. This value can also be found using the dynamical SPR parameters and, as in any chemical reaction, it is the association rate divided by the dissociation rate.
For this, a so-called bait ligand is coated to the gold surface of the SPR crystal. Through a microflow system, a solution with the prey ligand can flow over the bait layer and bind. Binding will make the SPR signal change to an equilibrium. After some time, a solution without the prey is applied, and a new equilibrium will be reached. From these association ('on rate', von)and dissociation speeds ('off rate', voff), the binding constant can be calculated.
The actual SPR signal can be explained by the electromagnetic 'coupling' of the incident light with the surface plasmon of the gold layer. This plasmon can be influenced by the layer just a few nanometer across the gold-solution interface i.e. the bait protein and possibly the prey protein. Binding makes the reflection angle change.;
Magnetic Plasmon Resonance
Recently, there has been an interest in magnetic surface plasmons. These require materials with large negative magnetic permeability. A property that has only recently been made available with the construction of metamaterials. The only reported demonstration of magnetic plasmon resonance has been at Duke University.
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- Hydrogen microsensor
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