LSPRs

Localized surface plasmon resonances (LSPRs) are resonant oscillations of the free electrons at the surface of metallic nanoparticles excited by incident light. These oscillations are confined to the particle and do not propagate along a surface, hence the term localized. When metals such as gold or silver interact with light at specific frequencies, the conduction electrons collectively oscillate in resonance with the field, producing strong absorption and scattering at a characteristic wavelength that depends on particle size, shape, material, and the surrounding medium.

In small particles, the LSPR condition can be approximated by the dielectric properties of the metal and its environment. For a spherical particle in a homogeneous medium, a common quasi-static criterion is Re[ε(ω)] ≈ -2εm, where ε is the metal’s dielectric function and εm is the ambient dielectric constant. More complete descriptions use Mie theory for spheres or numerical methods for complex shapes, with the resonance shifting as particle geometry and the local refractive index change.

LSPRs yield substantial local electromagnetic field enhancements near the nanoparticle surface, enabling applications in sensing and spectroscopy. The resonance position and linewidth are influenced by material losses, radiative damping, and interband transitions. Silver nanoparticles can offer sharper resonances, while gold provides chemical stability; adjusting aspect ratio or facets tunes the resonance from visible to near-infrared.

Applications include refractive-index and biosensing, where binding events shift the LSPR, as well as imaging, spectroscopy, and chemical sensing. LSPRs underpin surface-enhanced Raman spectroscopy (SERS) and photothermal therapies. Characterization commonly uses UV-Vis–NIR extinction or scattering spectra, dark-field microscopy, and electron energy loss spectroscopy; theory combines Mie theory, quasi-static models, and numerical simulations.