Growth of Gold Dendritic Structures under the Coexistence of Hexadecyltrimethylammonium Ions (CTA⁺) and Ethanol

We investigated the growth of gold nano/micro-structures by reducing gold ions with ascorbic acid in a mixed solvent of ethanol and water containing hexadecyltrimethylammonium bromide (CTAB). The geometries of the gold structures were influenced by the ratio of ethanol to water and the concentrations of CTAB and NaBr. We observed gold dendrites exceeding 10 μm in size, which were composed of numerous needles of varying sizes, some as small as a few tens of nanometers. The gold dendrites were grown within 20 min, a shorter duration compared to previous reports. The overall size of the gold dendrites exhibited dependence on the concentration of Br^-, allowing for smaller sizes of less than 10 μm. The micelles of CTA⁺ were essential for the growth of gold dendrites. The ethanol ratio altered the viscosity and surface tension of the solution, resulting in a variety of geometries. We measured the extinction spectra of the gold dendrites, which revealed that they exhibited a broadband optical response. Observations using a dark-field optical microscope showed that their plasmon resonances were predominantly in the red region. The Raman spectrum of <i>para</i>-aminothiophenol (<i>p</i>-ATP) was measured using the gold dendrites under 785 nm, which revealed field enhancement at the surface of the gold dendrites.