Preparation of crystalline gold nanoparticles at the surface of mixed phosphatidylcholine-ionic surfactant vesicles

The formation of gold nanoparticles and the crystal growth at the surface of mixed phosphatidylcholine (PC)-ionic surfactant vesicles was investigated. The PC-bilayer surface was negatively charged by incorporating sodium dodecyl sulfate (SDS) and positively charged by adding hexadecyltrimethylammonium chloride (CTAB). The mass ratio phosphatidylcholine:surfactant was fixed in both cases at 1:1. The gold nanoparticle formation was studied by using transmission electron microscopy (TEM) combined with dynamic light scattering (DLS) and UV-vis absorption spectroscopy. TEM micrographs confirm that the particle formation occurs on the vesicle surface. However, the reduction process depends on the ionic surfactant incorporated into the vesicles, the vesicle size distribution, as well as the temperature used for the reduction process. Thereby, it becomes possible to control the crystal growth of the individual spherical gold nanoparticles in a characteristic way. Red colored colloidal dispersions consisting of monodisperse spherical nanoparticles with an average particle size between 2 and 8 nm (determined by dynamic light scattering) can be obtained by using a monodisperse SDS-modified vesicle phase. When the temperature is increased to 45 degrees C, a crystallization in rod-like or triangular structures is observed. In the CTAB-based template phase in general larger gold particles of about 35 nm are formed. In similarity to the anionic vesicles a temperature increase leads to the crystallization in triangular structures.