What is Surface Plasmon Resonance?

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- GoPhotonics

Jun 12, 2023

Surface Plasmon Resonance (SPR) is an optical technique used for the real-time measurement of molecular interactions. SPR occurs when a metal film, subjected to total internal reflection, is exposed to plane-polarized light. The resonance condition for SPR is met when photons, having the appropriate energy, interact with the metal surface at a specific angle of incidence. The incident photons transfer energy to the free electrons present within the metal lattice, which are capable of moving relatively freely. As a result, the free electrons undergo a collective oscillation, resulting in the formation of surface plasmons.

Surface plasmon resonance is a phenomenon in which incident photons at a specific angle of incidence excite electrons within the metal surface layer. These excited electrons then propagate parallel to the metal surface because of the nature of the excitation mechanism and the properties of the metal. The occurrence of SPR is dependent upon the refractive index of the material adjacent to the metal surface, alongside a constant light source wavelength and a thin metal surface. This unique characteristic of SPR becomes advantageous for analyte detection, as even a minor alteration in the refractive index of the sensing medium can hinder SPR. 

Principle of Surface Plasmon Resonance

Surface plasmons are collective oscillations of electrons at the interface between a metal and a dielectric or vacuum. When light interacts with a metal surface, it can excite surface plasmons by transferring energy to the conduction electrons of the metal. This energy transfer occurs when the frequency of the incident light matches the resonant frequency of the surface plasmons. The excited surface plasmons propagate along the metal-dielectric interface and decay through various processes, such as scattering or absorption.

Experimental Setup

SPR assays refer to the experimental setups and procedures used to perform real-time molecular interaction analysis using the SPR technique. These assays utilize the principles of SPR to monitor and characterize various biomolecular interactions, including protein-protein interactions, receptor-ligand binding, DNA hybridization, and small molecule interactions. 

The key components of an SPR assay include a sensor chip, a flow system, a light source, and a detector. The sensor chip is typically coated with a thin metal film, such as gold or silver, which supports the generation and detection of surface plasmons. The metal film is functionalized with ligands or capture molecules that specifically interact with the target analyte of interest. The analyte solution is introduced over the sensor chip surface through the flow system in a controlled way. 

After the analyte solution has been introduced, the flow system facilitates washing steps. These steps help remove any non-specifically bound molecules or debris that might interfere with the accurate detection of binding events. The washing process ensures a clean and optimized surface for molecular interactions. Then continuous monitoring of binding events in real-time is done.

The light source provides the incident light that interacts with the metal film, leading to the excitation of surface plasmons. The light is usually monochromatic and polarized, allowing for precise control of the SPR conditions. The reflected light intensity or the changes in the resonance angle are measured by the detector, which directly correlate with the molecular binding events occurring on the sensor chip surface. By monitoring these changes over time, the kinetics, affinity, specificity, and concentration of the interacting molecules can be determined.

SPR Biosensors

An SPR biosensor is a device that utilizes Surface Plasmon Resonance (SPR) to detect and analyze molecular interactions in real-time. The SPR biosensor typically consists of a thin metal film, commonly gold or silver, deposited on a transparent substrate. The metal film is designed to support the propagation of surface plasmons when exposed to specific incident light conditions.

There are SPR biosensors that are used for the detection of clinically relevant biomarkers. Researchers have developed various types of SPR biosensors, each catering to specific detection requirements. Also, the integration of nanomaterials has played an important role in enhancing the performance and sensitivity of SPR biosensors bringing new pathways for innovation and improvement in this field.

SPR sensors have become popular in many fields because they are accurate, sensitive, measure amounts precisely, and don't need labels on the molecules being detected. They can detect various targets, ranging from small molecules like methane and carbon dioxide to metallic ions, PAHs, pesticides, phycotoxins, nucleic acids, and even microorganisms. SPR sensors offer exciting possibilities for studying the ocean and its properties.

Applications of Surface Plasmon Resonance

Surface plasmon resonance has found numerous applications in diverse areas such as biotechnology, pharmaceuticals, environmental monitoring, and material science. SPR has emerged as a fundamental tool for studying biomolecular interactions, including protein-protein interactions, protein-ligand binding, antibody-antigen interactions, and DNA hybridization. By immobilizing one binding partner on a sensor surface and flowing the other partner, researchers can monitor real-time binding events and quantify binding kinetics, affinity, and specificity. This ability to analyze biomolecular interactions has profound implications in drug discovery, diagnostics, and understanding biological processes.

It is used in pharmaceutical research and development by providing critical insights into drug-target interactions. SPR enables the identification and characterization of lead compounds, determination of binding affinity, and assessment of drug selectivity. Researchers can optimize drug candidates, screen small molecules for potential therapeutic applications, and evaluate the efficacy of drug formulations using this technique. This expedites the drug development process and improves the success rate of clinical trials.

SPR-based biosensors have revolutionized the field of diagnostics, enabling rapid, sensitive, and label-free detection of various analytes. The immobilization of specific biomolecules on the sensor surface allows for the detection of target analytes in complex biological samples. SPR biosensors have been used for detecting pathogens, allergens, toxins, and biomarkers associated with diseases such as cancer, cardiovascular disorders, and infectious diseases. Their high sensitivity and real-time monitoring capabilities make them invaluable tools for early disease diagnosis and personalized medicine.

It is also used in environmental monitoring and pollutant detection. They can detect and quantify a wide range of contaminants, including heavy metals, pesticides, toxins, and pollutants in water, air, and soil samples. Environmental scientists can obtain real-time measurements, leading to more accurate risk assessment and efficient remediation strategies by leveraging the selectivity and sensitivity of SPR.