LSPR

Localized surface plasmon resonance (LSPR) is the resonant oscillation of conduction electrons at the surface of metallic nanoparticles when they interact with light. This collective electron motion is confined to the nanoparticle and results in strong absorption and scattering, producing a characteristic extinction peak in the visible to near-infrared spectrum. The exact peak position and intensity depend on the particle’s size, shape, composition, and the dielectric environment surrounding it.

Mechanism and properties: When light induces an electric field, the conduction electrons oscillate coherently relative to the ion lattice. In small particles, the resonance condition can be described by the metal’s dielectric function and the surrounding medium. LSPR generates enhanced electromagnetic fields near the particle surface, especially in gaps or sharp features, which underpins sensitive interactions with nearby molecules.

Materials and geometry: Gold and silver are the most common LSPR materials due to favorable plasmon lifetimes and biocompatibility (gold is particularly popular in bioapplications). Particle geometry strongly influences the resonance: nanospheres have a single peak, while nanorods or nanoshells exhibit multiple modes and tunable wavelengths. In nanorods, changing the aspect ratio shifts the longitudinal resonance to longer wavelengths.

Applications: LSPR is widely used in label-free biosensing, including detection of DNA, proteins, and other biomolecules, as well as chemical and environmental sensing and imaging. Measurements typically rely on UV–visible extinction spectroscopy to monitor peak shifts or intensity changes as the local refractive index near the particle surface varies.

Limitations: Signal broadening from size dispersion, damping, and interparticle coupling can reduce sensitivity. Stability and reproducibility depend on surface chemistry and aggregation state.