Colloidal bag of marbles: The structure and properties of lipid-coated silica nanoclusters

Abstract

Silica particles find applications in drug delivery and bio-sensing, where their large specific surface area can accumulate or release biologically active substances. Such applications would benefit from the ability to control diffusion access to the surfaces of a larger number of silica particles simultaneously. In the present work, we show that a phospholipid bilayer can contain clusters of silica nanoparticles ranging in number from a few to several tens, forming structures resembling a colloidal version of a bag of marbles. The lipidic membrane serves as a diffusion gate that prevents premature leakage of encapsulated substances from the nanoparticles to the bulk during storage, but allows diffusion when the phase transition temperature is exceeded. Using 30 nm SiO2 nanoparticles with or without surface amino groups, combined with lipid membranes containing DPPC, DPPG and cholesterol at molar ratios ranging from 25:60:15–85:0:15, we show that the electrostatic interactions between the nanoparticle surface and the lipid bilayer are crucial for the existence and stability of lipid-coated nanoclusters. We demonstrate the gating mechanism by measuring temperature-dependent diffusion of a model substance 5-(6)-carboxyfluorescein from the clusters. A shift in the phase transition temperature of approx. 20 K has been observed in the lipid-coated silica nanoclusters compared to plain liposomes of identical lipid bilayer composition.