Plasmonic nanoparticles to probe the mechanical properties of synthetic lipid bilayers

In recent years, the combination of inorganic nanoparticles (NPs) and biomimetic lipid interfaces represents a promising approach to investigate relevant events occurring at the nano-bio interface and to build up novel smart hybrid materials, thereby advancing Nanomedicine. In this context, recent reports have focused on the interaction of gold nanoparticles (AuNPs) with synthetic lipid vesicles. Moreover, the spectral shift of AuNPs localized surface plasmon resonance (LSPR) was shown to depend on some ensemble-averaged properties of lipid assemblies, which can be determined exploiting the spontaneous clustering of citrate-capped AuNPs on the lipid membrane. This contribution explores the plasmon-based relation between AuNPs aggregation and the mechanical response of lipid membranes, aiming at establishing novel plasmonic descriptors for AuNPs-lipid bilayers interactions. By performing UV-Vis measurements, we monitored the evolution of the LSPR profile of AuNPs when challenged with synthetic unilamellar liposomes with different rigidities at varying vesicles/AuNPs molar ratios. The analysis of UV-Vis spectra identified a specific concentration range where an isosbestic point emerges, whose wavelength is unique to each lipid composition and dependent on liposomes’ stiffness. Combining UV-Vis data with structural and morphological information, we introduce the isosbestic wavelength as a new analytical tool to precisely evaluate the stiffness of membrane-enveloped nano-objects. Overall, these results contribute to a comprehensive understanding of AuNPs self-assembly onto lipid vesicles, disentangling the effect of membrane rigidity from concentration effects on AuNPs plasmonics. This description will enable the design of smart hybrids and provide nanosized analytical probes to sample ensemble-averaged properties of membrane-enveloped particles.