Synthesis of gold nanorods and their dimers

Abstract

Gold Nanorods (NR’s) have received substantial attention due to their unique optical and electronic properties (which are mainly dependent on their shape, size and aspect ratio) and have promising applications in the domain of sensing and enhancing electromagnetic fields in the vicinity of the ends of the nanorods. The enhancement of electromagnetic fields can be increased to a larger extent by aligning two nanorods in end-to-end manner with small nanogaps. Molecules adsorbed to the surface of gold nanorod dimers demonstrate large surfaceenhanced Raman or fluorescence spectroscopy as compared to single nanorods. We performed several modified versions of the experiment to understand the role of silver nitrate and the effect of concentration of seed solution in the formation of gold nanorods. We increased the concentration of silver nitrate and seed solution which resulted in red shift in the LSPR. The gold nanorods were synthesized via seed mediated method by using two different types of surfactants namely, CTAB (Cetyl trimethyl ammonium bromide) and Sodium Salicylate. The former was found to produce lesser number of spherical gold nano particles as compared to the latter. This was followed by synthesis of gold nanorods dimers via dithiol coupling with controlling nanogaps. The gap between two rods should preferably be between 5 to 10 nm for larger field enhancement. Allipahtic dithiol was used for assembly. Experiments were conducted to study the dependence of the dimer formation on pH of the solution. We could conclude that pH did not induce any major change in the experiment. The synthesis of dimers required critical control of several factors like chemical concentration, temperature, and stability of gold nanorods. We could thereby successfully synthesize gold nanorods and their dimers (end-to-end assembly) however; the count of dimers was less than expected. Three type of dimers were formed which differed in terms of orientation of the two rods.