Nanoplasmonic Isosbestics Uncover Mesoscale Assembly of Gold Nanoparticles on Soft Templates.

Assembly of plasmonic nanoparticles (NPs) generates unique optical properties through coupling of the localized surface plasmon resonance (LSPR) of individual NPs. However, precisely controlling and monitoring how mesoscale assembly dictates final optical properties remain key challenges in designing advanced plasmonic materials. Here, we introduce "nanoplasmonic isosbestics" as optical descriptors of the mesoscale organization of gold nanoparticles (AuNPs) on soft templates. Unlike isosbestic points in molecular spectroscopy, which describe chemical equilibria, our numerical simulations demonstrate that nanoplasmonic isosbestics emerge from the coexistence of individual AuNPs and AuNP clusters, where the interparticle spacing determines the isosbestic wavelength. By templating AuNP assembly onto synthetic free-standing lipid bilayers with tunable membrane rigidity, we experimentally achieve precise control over interparticle spacing and prove that it is mirrored by univocal modulation of the isosbestic wavelength. This provides a fundamental understanding of the structure-function relationship in plasmonic systems, linking, for the first time, nanoplasmonic isosbestics to interparticle spacing and equilibrium structure in plasmonic assemblies. On the analytical perspective, nanoplasmonic isosbestics provide noninvasive optical fingerprints of the templates, opening to appealing applications. As a proof of concept, we apply this approach to profile the stiffness of two extracellular vesicle (EVs) classes─mesenchymal stem cell (MSC)-derived and red blood cell-derived EVs─both recognized for their biological and translational potential.