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Surface plasmon resonance (SPR) is a phenomenon that occurs when light interacts with metal nanoparticles and excites collective oscillations of conduction electrons at the metal-dielectric interface. This resonance creates an evanescent electromagnetic field that extends into the dielectric medium near the metal surface. The resonance condition is sensitive to changes in the refractive index of the dielectric, which allows SPR to be used for label-free detection of biomolecular binding events at a sensor surface.
How Does SPR Biosensing Work?
In typical SPR biosensing applications, a thin gold or silver film is deposited on the sensor surface of a prism or optical fiber. Monochromatic polarized light is made to pass through the prism and reflect off the internal surface of the metal film. At a specific incident angle, known as the resonance angle, Surface Plasmon Resonance are excited at the metal-dielectric interface, causing a dip in the reflected light intensity. Any analyte binding to the sensor surface alters the local refractive index and shifts the resonance angle. By monitoring these angular shifts in real-time, the association and dissociation of biomolecular interactions can be observed without the need for fluorescent or radioactive labels.
Advantages of Label-Free Biomolecular Detection
One of the major advantages of SPR biosensing is its ability to perform direct, label-free detection of biomolecules. This eliminates many of the drawbacks associated with traditional methods that rely on labels, such as fluorophores or radioactive isotopes, for detection. Label-free detection does not require time-consuming preparation steps to conjugate labels to biomolecules and reduces the risk of altering their native structures and binding properties. It also allows kinetic and affinity data to be collected in real-time throughout the association and dissociation phases of biomolecular interactions. This makes SPR well-suited for applications involving protein-protein binding studies, antibody screening, and development of biosensors.
Various Sensor Surface Chemistries for Biomolecular Immobilization
To employ SPR for detecting specific biomolecular interactions, one type of bioactive molecule first needs to be immobilized on the sensor surface while allowing binding partners to freely flow over it. A variety of surface chemistries have been developed to covalently attach different proteins, antibodies, nucleic acids, biomarkers, and other molecules to gold surfaces. Some common approaches involve the use of self-assembled monolayers (SAMs) containing tail groups that readily bind to gold, such as alkanethiols and alkaneselenols, along with active terminal moieties like carboxylates or amines for subsequent biomolecule coupling. Thiolated DNA probes and protein A/G surfaces that enable oriented antibody immobilization through their Fc region are also widely employed. The choice of surface chemistry depends on the type of biomolecule being immobilized, desired surface density, and resistance to nonspecific binding.
Quantitative Analysis of SPR Sensorgrams
The output data collected from SPR biosensors is presented as sensorgrams which plot response units (RU) on the y-axis against time on the x-axis. Real-time binding events manifest as an ascending phase when analytes associate with surface ligands, followed by a decaying dissociation phase when unbound analytes are washed away. Various kinetic parameters can be extracted by fitting these binding curves to different reaction models using data analysis software tools. For simple 1:1 interactions the association rate constant (ka) and dissociation rate constant (kd) are obtained, allowing calculation of affinity (KD = kd/ka). Additionally, the maximum response at equilibrium (Rmax) reports the binding capacity of the surface. Such quantitative analysis provides valuable mechanistic insights into biomolecular interactions and enables monitoring dynamic changes caused by external factors like pH, salt concentrations, or drug candidates.
Widening Scope of SPR Applications
Since its commercial introduction in the 1990s, SPR has evolved into a robust and versatile analytical technique applied across diverse fields from basic research to clinical diagnostics. SPR is heavily utilized in pharmaceutical industries for drug discovery purposes like screening compound libraries for lead candidates and characterizing their binding modes. It finds applications in food safety testing for detecting allergens, toxins, and pathogens. Environmental monitoring employs SPR biosensors to identify water contaminants. Furthermore, SPR immunoassays have potential for point-of-care testing of various disease biomarkers. The integration of SPR into lab-on-a-chip microfluidic platforms and its miniaturization on optical fibers also widens the scope for portable and multiplexed detection outdoors as well as in resource-limited settings. Continuing advancements in sensor surfaces, instrumentation, and analysis algorithms ensure Surface Plasmon Resonance biosensing remains a premier technique for label-free biomolecular interaction analysis going forward.
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1. Source: Coherent Market Insights, Public sources, Desk research
2. We have leveraged AI tools to mine information and compile it
