Surface plasmon resonance (SPR) is a powerful optical technique for studying biomolecular interactions. It uses light to detect tiny changes at metal surfaces, allowing real-time, label-free monitoring of binding events between molecules like proteins and DNA.

Plasmonic biosensors harness SPR's sensitivity to create versatile tools for medical diagnostics and research. By measuring how light interacts with nanoscale metal structures, these sensors can detect disease markers, study drug interactions, and analyze complex biological systems with high precision.

Surface Plasmon Resonance (SPR) Fundamentals

Principles of Surface Plasmon Resonance

• Surface plasmon resonance occurs when light interacts with free electrons at a metal-dielectric interface
• Collective oscillation of electrons creates electromagnetic waves called surface plasmons
• SPR requires specific conditions including polarized light and a thin metal film (typically gold)
• Resonance angle determines the angle of incidence at which maximum coupling occurs between light and surface plasmons
• Changes in refractive index near the metal surface alter the resonance conditions

Kretschmann Configuration for SPR

• Kretschmann configuration serves as the most common setup for SPR experiments
• Consists of a prism coated with a thin gold film (~50 nm thick)
• Incident light passes through the prism and reflects off the gold film
• Total internal reflection occurs at the prism-gold interface
• Evanescent wave penetrates the gold film and excites surface plasmons
• Reflected light intensity decreases at the resonance angle due to energy transfer to surface plasmons

SPR Sensing Mechanism

• Gold film acts as the sensing surface for biomolecular interactions
• Analyte binding to the gold surface changes the local refractive index
• Shift in resonance angle occurs due to refractive index changes
• Magnitude of angle shift correlates with the amount of bound analyte
• Real-time monitoring of binding events becomes possible by tracking angle changes
• SPR allows label-free detection of biomolecular interactions (no fluorescent tags required)

Localized Surface Plasmon Resonance (LSPR)

Fundamentals of LSPR

• Localized surface plasmon resonance occurs in metallic nanostructures smaller than the wavelength of incident light
• Confinement of surface plasmons to nanoparticle surfaces creates localized oscillations
• LSPR leads to enhanced electromagnetic fields near the nanoparticle surface
• Resonance wavelength depends on nanoparticle size, shape, and composition
• Surrounding medium's refractive index influences LSPR properties
• LSPR enables sensing applications with high sensitivity to local environment changes

Plasmonic Nanoparticles for LSPR

• Gold and silver nanoparticles commonly used for LSPR applications
• Nanoparticle shapes include spheres, rods, cubes, and more complex geometries
• Size control allows tuning of LSPR wavelength (smaller particles resonate at shorter wavelengths)
• Nanoparticle aggregation can lead to coupling effects and spectral shifts
• Surface functionalization enables specific binding of target analytes
• Colloidal solutions of plasmonic nanoparticles exhibit vibrant colors due to LSPR (red for gold, yellow for silver)

LSPR Sensing Mechanisms

• Refractive index changes near nanoparticle surface alter LSPR properties
• Spectral shifts in absorption or scattering spectra indicate binding events
• Single nanoparticle sensing possible with advanced spectroscopic techniques
• LSPR sensors offer advantages of miniaturization and simplified optical setups
• Integration with microfluidic devices enables lab-on-a-chip applications
• Multiplexed sensing achievable using nanoparticles with different resonance wavelengths

Plasmonic Biosensor Applications

Sensitivity and Performance Metrics

• Sensitivity factor quantifies the sensor's response to refractive index changes
• Expressed as the change in resonance angle or wavelength per refractive index unit (RIU)
• Typical SPR sensitivity ranges from 50-100 degrees per RIU
• LSPR sensitivity varies with nanoparticle properties (100-500 nm per RIU)
• Figure of merit combines sensitivity with resonance peak width for overall performance evaluation
• Detection limits for biomolecules can reach picomolar to femtomolar concentrations

Real-time Monitoring Capabilities

• SPR and LSPR enable continuous observation of binding kinetics
• Association and dissociation rates can be determined from real-time data
• Allows study of biomolecular interactions under physiological conditions
• Concentration-dependent binding studies provide affinity constants
• Temperature control enables thermodynamic analysis of interactions
• Multi-channel systems permit simultaneous monitoring of multiple analytes

Label-free Detection Advantages

• Eliminates need for fluorescent or radioactive labels on target molecules
• Reduces sample preparation time and complexity
• Avoids potential interference from labels on biomolecular interactions
• Enables detection of native, unmodified biomolecules
• Simplifies quantification of binding events
• Allows for regeneration and reuse of sensor surfaces in many cases