Abstract
We report a new development for the direct high-yield synthesis of high aspect ratio gold nanorods. By using a modified seed-mediated synthesis approach for the preparation of high aspect ratio gold nanorods with the addition of an appropriate amount of nitric acid during nanorod growth, uniform and monodisperse gold nanorods were synthesized in large quantity. The formation of triangular nanoplate by-products was substantially reduced by replacing trisodium citrate with CTAB surfactant as the capping agent in the preparation of gold seeds. The nanorods have an average length of 355.3 ± 31.3 nm and an average diameter of 18.7 nm, giving them an average aspect ratio of 19. The ultra-small gold seeds used for nanorod growth are ~1–2 nm in diameter and show a weak absorption band centered at ~480 nm. The nanorods show a transverse surface plasmon resonance (SPR) absorption band at 497 nm and a longitudinal SPR band in the near-infrared (NIR) region. We report a synthesis of branched gold nanocrystals by the addition of a suitable amount of NaOH to an aqueous solution of cetyltrimethylammonium bromide, HAuCl4, and ascorbic acid. NaOH serves as an effective reducing agent to induce the formation of branched gold nanocrystals. The branched nanocrystals were formed within minutes of reaction and showed monopod, bipod, tripod, and tetrapod structures. The branched nanocrytals quickly transformed into spherical nanoparticles within 1 h of reaction, and the process was essentially complete after 2 days of reaction. The morphological transformation has been monitored by both UV–vis absorption spectroscopy and electron microscopy. The appearance of two major absorbance bands for the branched gold nanocrystals eventually became only a single band at 529 nm for the spherical nanoparticles. The resulting nanoparticles are single crystalline with diameters of 20–50 nm and without the presence of twinned boundaries. By keeping the freshly prepared branched nanocrystals in a refrigerator at 4 °C, their multipod structure can be preserved for over a month without significant spectral shifts.