Controlled organization of Au colloids into linear assemblies

A simple and convenient method is described to produce gold nanowires. A controlled organization of Au colloids into linear assemblies was achieved without using any templates. HAuCl4 was reduced by NaBH4 in the presence of lysine to act as capping and bridging agent. At a molar ratio of Au/Lys = 1:0.5 and pH in the range 8.4-9.5, linear aggregates of ALL can be prepared with a diameter of ca. 5 nm and variable lengths in the range of 80-200 nm. At lower or higher pH values, there is no control on the unidirectional growth of linear aggregates. Application of ultrasound during the preparation also impairs linear growth or disrupts the chain structure, yielding discrete colloidal particles of Au as sole product. The key factor for formation of Au linear assemblies is believed to be a subtle balance between the attractive (cross-linking) force, provided by lysine molecules, working against the electrostatic repulsion among Au colloid surfaces bearing like (negative) charges.