Plasmonic properties of gold nanoparticles interacting with synthetic lipid bilayers

Despite their revolutionary potential in biomedical applications, the limited number of nanoparticles (NPs) approved for clinical use is attributed to a gap in understanding their biological fate. Biomimetic lipid interfaces represent a promising approach to investigate relevant events occurring at the nano-bio interface, i.e., where NPs meet biological barriers, under simplified and controlled conditions. From a different perspective, the combination of inorganic NPs with lipid membranes is instrumental to build up novel smart hybrid materials, thereby advancing Nanomedicine. In this context, some recent research reports have focused on the interaction of gold nanoparticles (AuNPs) with synthetic lipid vesicles to enhance our knowledge on nanobio interfaces. 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 then 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 AuNPs LSPR in the presence of a series of 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 formulate a unifying hypothesis on the interaction pathway of citrate-coated AuNPs and lipid vesicles and introduce the isosbestic wavelength as a new analytical tool to precisely evaluate the stiffness of membrane-enveloped synthetic and biological nanosized particles. Overall, the recent results contribute to a comprehensive understanding of the association between AuNPs and synthetic and natural 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.