Aspect-ratio-controlled synthesis of high-aspect-ratio gold nanorods in high-yield

We describe a modified seed-mediated synthesis of high-aspect-ratio gold nanorods controlling the aspect ratio with variation of pH in the growth solution. By adding various amounts of sodium hydroxide, pH in the growth solution was controlled from 1.29 to 7.06 in the presence of nitrate anions. At various pH of the growth solution, the yield of high-aspect-ratio gold nanorods was significantly improved by the role of nitrate anions, which reduce the formation of spherical nanoparticles and triangular nanoplates. The synthesized high-aspect-ratio gold nanorods have diameters of 23–25 nm at all pH conditions and the mean lengths are from 589 to 469 nm with variation of pH. On increasing pH, the mean aspect ratio of gold nanorods decreased from 25.6 to 20.0. This result shows that the aspect ratio of gold nanorods follows a trend as a function of pH in the growth solution. The intensity of the plasmon band of wavelength 780 nm, which resulted from triangular nanoplates, gradually decreased with increasing pH. This observation indicates that pH of the growth solution also influences the formation of triangular nanoplates. A transverse plasmon band of gold nanorods appeared at a wavelength of 510 nm, and a longitudinal plasmon band was observed over 2000 nm in UV–vis–NIR absorption spectra.