Bilayers as phase transfer agents for nanocrystals prepared in nonpolar solvents.

The effective water dispersion of highly uniform nanoparticles synthesized in organic solvents is a major issue for their broad applications. In an effort to overcome this problem, iron oxide and cadmium selenide nanocrystals were surrounded by lipid bilayers to create stable, aqueous dispersions. The core inorganic particles were originally generated in oleic acid and 1-octadecene. When these organic solutions were mixed with water and a sparing amount of excess fatty acid, up to 70% of the nanoparticles transferred into the aqueous phase. This simple approach was applied to two different nanocrystal types, and nanocrystal diameters ranging from 5 to 15 nm. In all cases, the resulting materials were stable, nonaggregated suspensions that retained their original magnetic and optical properties. The phase transfer efficiency is maximum when very little oleic acid is added (e.g. 0.2 w/w %). At higher concentrations, above the critical micelle concentration, the formation of micelles begins to compete with bilayer generation leading to less effective phase transfer. Unlike other approaches for water dispersion that rely on amphiphiles with significant water solubility, the fatty acids used in this work are only sparingly soluble in water. As a result, there is minimal dynamic exchange between free and bound surface agents and the resulting aqueous solutions contain little residual free organic carbon. Thermogravimetric analysis (TGA) confirmed the presence of bilayers around the nanocrystal cores. The particle size, size distribution, process yield, and colloidal stability were found using a suite of methods including transmission electron microscopy, small angle X-ray scattering, dynamic light scattering, inductively coupled plasma-optical emission spectroscopy, and ultraviolet-visible spectroscopy. Bilayer-nanocrystal complexes possess many of the same size-dependent features as the original materials, and as such offer new avenues for exploring and exploiting the interface between nanocrystals and biology.