Shape homogenization and long-range arrangement of gold nanorods using a pH-responsive multiamine surfactant

A relatively new and efficient method is reported here for the purification and arrangement of high-aspect-ratio gold nanorods (AuNRs) using a multiamine surfactant, bis[[(amidoethyl)carbamoyl]ethyl]octadecylamine (C18N3), which strongly adsorbs to the surface of AuNRs. The adsorbed layers of the multiamine surfactant on AuNRs exhibit the ability to deaggregate gold nanoparticles at low pH in an aqueous medium and to promote their aggregation at high pH. Through regulation of the pH of the dispersion medium, a well-ordered arrangement of 99% monodisperse AuNRs was obtained, having dimensions of approximately 18 nm diameter and 353 nm length and an area of several dozens of square micrometers, which is much larger than what has been reported in the literature. A very strong optical absorption in the near-infrared region of as-prepared AuNRs was shown. This strategy of using pH-responsive multiamine surfactant to mediate both the homogenization in shape and the arrangement of nanoparticles provides a new methodology for the formation of nanoparticle assemblies.