ABSTRACTSurface plasmon resonance (SPR) is a phenomenon occurringat metal surfaces(typically gold and silver) when an incidentlight beam strikes the surface at a particular angle. Dependingon the thickness of a molecular layer at the metal surface,theSPR phenomenon results in a graded reduction in intensity ofthe reflected light. Many applications take advantage of theexquisite sensitivity of SPR to the refractive index of themedium next to the metal surface, which makes it possible tomeasure accurately the adsorption of molecules on the metalsurface an their eventual interactions with specific ligands.
INTRODUCTIONSurface Plasmon resonance (SPR) can be described as theresonant, collective oscillation of valence electrons ina solid stimulated by incident light. The resonance condition isestablished when the frequency of light photons matches thenatural frequency of surface electrons oscillating against therestoring force of positive nuclei. SPR in nanometer-sizedstructures is called localized surface plasmon resonance.First optical chemical sensors were based on themeasurement of changes in absorption spectrum and weredeveloped for the measurement of CO2 and O2 concentration.
DESCRIBTIONSPR PRINCIPLE Surface Plasmon Resonance is an quantum optical-electricalphenomenon arising from the interaction of light with a metalsurface. Surface Plasmon, also known as surface plasmon polaritons, are 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.
Surface Plasmon Resonance stems one of the basic principlesof optics, that of total internal reflectance (or TIR). • Occurs when a thin conducting film is placed at the interface between the two optical media. • At a specific incident angle, greater than the TIR angle, the surface plasmons in the conducting film resonantly couple with the light because their frequencies match.
Propagation Constant k denotes the free space wave number, εm the dielectric constant of the metal (εm = εmr+iεmi, real and imaginary components, respectively) and ns the refractive index of the dielectric.
SPR Resonance WavelengthFactors 1. Metal 2. Structure of metal’s surface 3. The nature of the medium in contact with the surface
1. METALTo be useful for SPR, a metal must have conduction bandelectrons capable of resonating with light at a suitablewavelength.A variety of metallic elements satisfy this condition. Theyinclude silver, gold, copper, aluminum, sodium, and indium. Gold is the most practical.The SPR generating surface is usually composed of ca. 50nanometers thick layer of gold deposited on a glass or plasticsupport.
2. SURFACEFor a flat metal surface, there is no wavelength of light thatsatisfies this constraint.there are three general configurations of SPR devices thatalter the momentum of photons in a way that fulfills theresonance criterion, namely, prisms, gratings and opticalwaveguide-based SPR system
3. MEDIUMPlasmons, although composed of many electrons, behave as ifthey were single charged particles.The plasmons electrical field extends about 100 nanometersperpendicularly above and below the metal surface.The interaction between the plasmons electrical field and thematter within the field determines the resonance wavelength.The amount of binding that occurs is proportional to theconcentration the analyte in the sample. This changes thecomposition of the medium at the surface and produces a SPRshift.
a) Surface – Prism An apparatus known as a Kretschmann prism is often used for SPR sensors. It uses a prism to couple some light to the SP film and reflect some to an optical photodetector.
b) Surface – GratingsLower production costs and more latitude in selection ofconstruction materials favor the use of gratings.Sensor to sensor reproducibility depends upon the fidelity ofgrating production.
c) Surface – optical waveguidesA light wave is guided by the waveguide and, entering theregion with a thin metal overlayer, it evanescently penetratesthrough the metal layer.
SPR Analytical Applications1. In adequate optics and imaging sensors in metallurgy and inspection and testing application.2. Gas phase detection.3. Electro-chemistry.4. Food, Environment science and Life science.5. Surface sensitivity measurements of several spectroscopic such as fluorescence, Raman scattering, and second harmonic generation. .
CONCLUSIONDuring the last 10 years the surface plasmon resonancetechnique has been developed into a very useful technology withnumerous applications . in order to illustrate the potential of SPRsensing devices, major applications areas of SPR sensors havebeen outlined. We envisage that progress in SPR sensortechnology will further improve detection of SPR sensors. Italso allow sensitive, fast and cost effective chemical analysisboth in laboratories and in field this development will furtherextend the potential of SPR sensing technology and allow SPRsensors to be used for more widely.