Hydrophobic silver nanoparticles trapped in lipid bilayers: Size distribution, bilayer phase behavior, and optical properties

The dispersions were stable at 50°C where the bilayers existed in a liquid crystalline state, but phase separated at 25°C where the bilayers were in a gel state, consistent with vesicle aggregation below the lipid melting temperature. Formation of bilayer-embedded nanoparticles was confirmed by differential scanning calorimetry and fluorescence anisotropy, where increasing nanoparticle concentration suppressed the lipid pretransition temperature, reduced the melting temperature, and disrupted gel phase bilayers. The characteristic surface plasmon resonance (SPR) wavelength of the embedded nanoparticles was independent of the bilayer phase; however, the SPR absorbance was dependent on vesicle aggregation.These results suggest that lipid bilayers can distort to accommodate large hydrophobic nanoparticles, relative to the thickness of the bilayer, and may provide insight into nanoparticle/biomembrane interactions and the design of multifunctional liposomal carriers.Hybrid lipid/nanoparticle conjugates provide a biologically inspired means of designing stable agents for biomedical imaging, drug delivery, targeted therapy, and biosensing [1]. An advantage of using lipids as stabilizing or functional ligands is that they mimic the lipidic scaffolding of biological membranes and have well-characterized physicochemical properties and phase behavior. In lipid vesicles, nanoparticle encapsulation can be achieved by trapping particles within the aqueous vesicle core or within the hydrophobic lipid bilayer. Becker et al [2], Kim et al [3], and Zhang et al [4] have shown that iron oxide (Fe3O4), cadmium selenide (CdSe) quantum dots, and gold nanoparticles, respectively, can be trapped within aqueous vesicle cores. To embed nanoparticles within lipid bilayers, the nanoparticle must be small enough to fit within a DPPC bilayer and it must present a hydrophobic surface. Using physisorbed stearylamine, Park et al [5,6] have stabilized 3–4 nm gold and silver particles in 1,2-dipalmit