Gold nanoparticles (AuNPs) have been increasingly integrated in biological systems, making it imperative to understand their interactions with cell membranes, the first barriers to be crossed to enter cells. Herein, liposomes composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) as a model membrane system were treated with citrate stabilized AuNPs from 5 to 30 nm at various concentrations. The fluorescence shifts of Laurdan probes reveal that AuNPs in general made liposomes more fluidic. The increased fluidity is expected to result in an increased surface area, and thus liposome shape changes from circular to less circular, which was further confirmed with fluorescence microscopy. The localized stress in lipids induced by electrostatically adsorbed AuNPs was hypothesized to cause the dominant long-range effect of fluidization of unbound lipid membranes. A secondary effect of the AuNP-induced lateral pressure is the membrane rupture or formation of pores, which was probed by AFM under fluid. We found in this study a nanoparticlemediated approach of modulating the stiffness of lipid membranes: by adsorption of AuNPs, lipids at the binding sites are stiffened whereas lipids afar are fluidized. Understanding the factors that modulate lipid packing is important for the discovery of alternative therapeutic methods for diseases linked to membrane integrity such as high blood pressure and cancer metastasis. The size resemblance of nanoparticles (NPs) to biological macromolecules coupled with the NPs' high surface energy leads to intricate interactions between the two entities. Myriads of study have been conducted to gain insights into the bio-nano interface for biomedical applications.

Noble metallic nanoparticles (NPs) such as gold and silver nanoparticles (AuNPs and AgNPs) have been shown to exhibit anti-tumor effect in anti-angiogenesis, photothermal and radio therapeutics. On the other hand, cell membranes are critical locales for specific targeting of cancerous cells. Therefore, NP-membrane interactions need be studied at molecular level to help better understand the underlying physicochemical mechanisms for future applications in cancer nanotechnology. Herein, we report our study on the interactions between citrate stabilized colloidal AuNPs/AgNPs (10 nm in size) and giant unilamellar vesicles (GUVs) using hyperspectral dark-field microscopy. GUVs are large model vesicle systems well established for the study of membrane dynamics. GUVs used in this study were prepared with dimyristoyl phosphatidylcholine (DMPC) and doped with cholesterol at various molar concentrations. Both imaging and spectral results support that AuNPs and AgNPs interact very differently with GUVs, i.e., AuNPs tend to integrate in between the lipid bilayer and form a uniform golden-brown crust on vesicles, whereas AgNPs are bejeweled on the vesicle surface as isolated particles or clusters with much varied configurations. The more disruptive capability of AuNPs is hypothesized to be responsible for the formation of golden brown crusts in AuNP-GUV interaction. GUVs of 20 mol% CHOL:DMPC were found to be a most economical concentration for GUVs to achieve the best integrity and the least permeability, consistent with the finding from other phase studies of lipid mixture that the liquid-ordered domains have the largest area fraction of the entire membrane at around 20 mol% of cholesterol.

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