Surface plasmon resonance (SPR) is an optical phenomenon that occurs when light interacts with metal nanoparticles. It involves the excitation of plasma oscillations at the interface between metals and dielectrics. SPR was first observed in 1902 by Robert Wood but it was not until the early 1980s that SPR was developed into a technology for thin-film and biosensing applications. Since then, SPR biosensing has become an important tool for studying biomolecular interactions in real-time without any labeling requirements.

How does SPR work?

At the interface between a metal and a dielectric medium, such as air or water, confined electromagnetic Surface Plasmon Resonance can be excited by light under certain conditions. As light strikes the interface at the appropriate angle, known as the resonance angle, energy is transferred from the photons to electrons in the metal nanoparticles, exciting the surface plasmons. This leads to a dip in the light's reflectivity that is strongly dependent on changes in the local refractive index near the surface. In SPR sensors, one of the interfacing materials is functionalized with biological molecules to allow binding interactions to occur on the sensor surface. Changes in the local refractive index due to molecule binding cause a shift in the resonance angle that can be measured quantitatively in real-time.

Components of a typical SPR instrument

A typical SPR instrument consists of three main components:

1. Metal film: Most commonly gold or silver, deposited as a thin film (~50 nm thick) on a glass substrate via vacuum deposition methods. The metal film supports the excitation of surface plasmons.

2. Prism: A high refractive index prism is required to couple incident light into the SPR angle via total internal reflection. Most often BK-7 glass or SF-10 glass prisms are used.

3. Light source and detector: A monochromatic light source such as a laser is directed through the prism towards the metal film at varying angles. Reflected light is collected and detected, usually with a CCD camera. The light source is kept at a fixed wavelength while the angle of incidence is varied to produce a reflectivity curve.

Signal detection and analysis

SPR detection is based on monitoring changes in the reflectivity curve as binding occurs on the sensor surface. Software analyzes the resonance angle shifts in real-time, converting the data into sensograms that display response units plotted against time. The resolution of the instrument depends highly on parameters like light source wavelength/power and the detector but high precision angular resolution detectors allow detection of binding down to picogram/mm2 levels or better. Multiplexed detection of several interactions on an array further increases assay throughput and capabilities.

Applications of SPR

Due to its label-free, real-time detection capabilities, SPR biosensing has widespread applications in areas like:

- Protein-protein interactions: Surface Plasmon Resonance is commonly used to study binding kinetics and affinity between biomolecules like antibodies, receptors, antigens. It allows characterizing protein-protein and protein-ligand complexes.

- Drug discovery: SPR helps screen compound libraries for drug candidates, determine binding epitopes, and perform lead optimization studies during drug development.

- Food safety: Pathogen detection, toxin detection, and allergen testing are important applications in ensuring food safety. SPR biosensors provide rapid, sensitive screening.

- Environmental monitoring: Detection of pollutants, heavy metals monitoring, and water quality testing take advantage of SPR's portability and ease of use.

- Diagnostics: Development of point-of-care SPR tests for biomarkers, antibodies, and infection screening has potential for use in hospitals, clinics and even homes.

That covers the basics of SPR technology including its working principles, instrumentation components, measurement techniques, and wide range of applications impacting many areas of research, industry and healthcare. SPR continues enhancing as a highly sensitive detection platform allowing rich characterization of binding interactions.

Summary

In summary, surface plasmon resonance (SPR) is an optical technique that exploits the excitation of surface plasmons at a metal-dielectric interface under specific conditions. Changes in the refractive index near the surface upon molecular binding events are sensitively detected in real-time without labeling requirements. SPR biosensors utilize this phenomenon along with components like a prism, metal film and light source-detector setup to quantitatively study biomolecular interactions. The technique has proven very useful in areas like protein interaction analysis, drug discovery, food and environmental testing and diagnostic development due to its high sensitivity, specificity, real-time monitoring and label-free nature. SPR technology continues advancing further capabilities and new applications across various fields.