Basics of the LSPR Sensors for Soft Matter at Interfaces

Journal article, 2023

An important class of localized surface plasmon resonance (LSPR)–based sensors implies the fabrication of an array of plasmonic metal nanoparticles on the support in combination with a thin protective dielectric layer. If needed, this layer can be covered, e.g., by a suitable thin biological layer, e.g., a lipid bilayer with receptors. The attachment of analyte (e.g., protein molecules or vesicles) to such interfaces is tracked via its indirect optical effect on the LSPR-related peak extinction wavelength. Such sensors have been commercialized and are now used to study biological soft matter. The length scale of the local field able in probing analyte around plasmonic nanoparticles is in this case on the order of 20 nm. Conceptually, these LSPR sensors are similar to the SPR sensors which were developed much earlier. Herein, the similarities and differences in the formalisms used to interpret SPR and LSPR measurements are discussed in detail. In particular, the exponential and power-law attenuation functions employed in these formalisms to describe the drop of the field are compared from various perspectives. The applicability of the power-law attenuation function in the context of LSPR is illustrated by using a generic model describing spherically shaped plasmonic metal nanoparticles. This model is also employed to illustrate the sensitivity of LSPR sensors with respect to various quantities. Among more specific results, the available expressions for the signal reduction factor for analyte nanoparticles of various shapes are collected and complemented by new ones. In addition, the equation describing the LSPR signal related to analyte attachment to a rough surface is presented.