Formation of water-dispersible gold nanoparticles using a technique based on surface-bound interdigitated bilayers

We demonstrate the phase transfer of dodecylamine (DDA)-capped colloidal gold particles dispersed in an organic solvent into water containing the cationic surfactant, cetyltrimethylammonium bromide (CTAB). Vigorous shaking of the biphasic mixture results in the rapid phase transfer of DDA-capped gold nanoparticles from the organic to aqueous phase, the aqueous phase acquiring a pink, foamlike appearance. Drying of the colored aqueous phase results in the formation of a highly stable reddish powder of gold nanoparticles that may be readily redispersed in water. The water-dispersible gold nanoparticles have been investigated by UV-vis spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). These studies indicate the presence of interdigitated bilayers consisting of a DDA primary monolayer directly coordinated to the gold particle surface and a secondary monolayer of CTAB, this secondary monolayer providing sufficient hydrophilicity to facilitate gold nanoparticle phase transfer and render them water-dispersible. The CTAB-DDA stabilized particles may be dispersed in water at very high nanoparticle concentrations with stability even in the presence of high amounts of electrolyte and over a wide range of solution pH.