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Surface plasmon resonance (SPR) is an optical phenomenon that occurs when light hits the interface between two media with different refractive indices, such as a metal and a dielectric. At a particular incident angle, known as the resonant angle, photons from light couple with the conduction electrons known as surface plasmons at the metal surface. This causes the plasmon waves to be excited and propagate at the interface. SPR results in a narrow absorption band that can be observed as a dip in the reflected light.
How does SPR Happen?
When photons from p-polarized light hit the interface at the resonant angle, their energy matches the collective oscillation of conduction electrons known as surface plasmons. This results in resonance energy transfer from the photons to the plasmons. The excited surface plasmons then propagate along the interface as evanescent waves that penetrate approximately 100-200 nm into the medium with lower refractive index. Due to photon- Surface Plasmon Resonance coupling at resonance, the reflected light intensity decreases sharply while the reflected light angle increases. This reflected light intensity dip is the characteristic signature of SPR.
Applications of SPR Technique
The high sensitivity and label-free detection capabilities of SPR have made it a valuable analytical technique with diverse applications. Some of its major uses include:
Biosensing – SPR is commonly used for real-time analysis of biomolecular interactions like antigen-antibody binding, protein-peptide binding, and DNA hybridization. By functionalizing one of the interfaces with a ligand, the binding of analytes in solution can be detected in real-time as a change in the SPR signal.
Drug Discovery – SPR assays are utilized for screening drug candidates and analyzing their binding to target molecules like receptors, enzymes, and DNA/RNA. This accelerates the drug discovery process.
Food Safety – Pathogens detection is an important application in ensuring food safety. SPR biosensors have been developed for rapid detection of foodborne pathogens like Salmonella, E. coli and Listeria.
Environmental Monitoring – SPR sensors monitor water quality by detecting pollutants, toxins or heavy metals in water samples. This helps protect the environment.
Industrial Processes – SPR finds uses in various industrial processes like thickness monitoring, corrosion monitoring and real-time monitoring of reactions.
Principles of SPR measurement
A typical SPR setup consists of a prism with high refractive index, a thin metal film like gold deposited on one face of the prism, and a fluidics system. A monochromatic p-polarized light from a laser/LED is passed through the prism and directed towards the metal film at varying incident angles.
At the resonant incidence angle, surface plasmons are excited at the metal-sample interface which couples with the incident light. This leads to a reduction in the intensity of the reflected light beam. The reflected light intensity is measured by a detector as a function of incident angle. Any change in the local refractive index near the metal surface shifts the resonant angle which is recorded in real-time. This allows extremely sensitive detection of biomolecular interactions.
Factors Affecting SPR signals
Several parameters influence the SPR response and need optimization for high sensitivity detection:
Refractive index – Both the material and thickness of the prism and metal film affect coupling of the incident light to surface plasmons. Prisms with higher refractive index enhance sensitivity.
Metal used – Noble metals like gold and silver support surface plasmons in the visible range. Aluminum is used for UV-Vis range while copper and nickel for infrared range.
Thickness of metal layer – An optimal thickness of around 50 nm is usually selected to get sharp resonance dips.
Wavelength of light – Visible light sources like LEDs are commonly used but lasers provide monochromatic excitation for higher resolution. Near-infrared lasers are preferred to minimize optical absorption in biological samples.
Flow cell design – The shape, volume and material of the fluidic cell housing the sample affect interactions at the sensor surface and mass transport phenomena. Optimized designs ensure consistent measurements.
By carefully tuning the above experimental parameters, Surface Plasmon Resonance achieves ultra-high sensitivity to detect minute changes in local refractive index arising from binding events at sensor surfaces. This has enabled diverse real-time label-free applications of the technique.
*Note:
1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it
