Gelation effect on the synthesis of high-aspect-ratio gold nanorods

Effect of the Crystallization Process of Surfactant Bilayer Lamellar Structures on the Elongation of High-Aspect-Ratio Gold Nanorods

The growth mechanism of an "in-gel synthesis method", that is, the effects of composition and structure of the lamellar gel phase below the Krafft temperature of surfactant solutions on the growth of long gold nanorods, was investigated. We changed the alkyl chain length of surfactant molecules to investigate the effect of surfactant self-assembly on the elongation of gold nanorods systematically; eight mixed solutions of alkyltrimethylammonium bromide (C nTAB; n = 2-16; n = even) with C18TAB were used for investigation. The Krafft temperature, self-assembly of surfactant molecules, and the crystallization process of each mixture were observed by differential scanning calorimetry, wide-angle X-ray scattering, visual inspection, and small-angle X-ray scattering. Gold nanorods were synthesized in these eight surfactant mixtures. These observations demonstrated that when the surfactant L_β phase sustains for a long time, the space of the water layer is also kept large enough for the seeds to take up Au ions bound to surfactant micelles. In this case, the seeds can form long nanorods between bilayers. We conclude that not only the stability of the lamellar gel phase but also co-existence of Au-ion carriers, that is, surfactant micelles, is essential for the elongation of long gold nanorods.

Effects of surfactant concentration on formation of high-aspect-ratio gold nanorods

The effects of surfactant concentration in a growth solution on the elongation of gold nanorods were examined. Gold nanorods were synthesized in solutions with different concentrations of hexadecyltrimethylammonium bromide (HTAB): 100, 200, 300, 400, 500, and 600 mM. The nanorods grown in a solution with higher surfactant concentrations were longer (aspect ratio ~30) than those grown in that with lower concentrations (aspect ratio <10). The self-assembled surfactant structures in the solutions were analyzed using viscosity measurement and small-angle X-ray scattering. These results showed a decrease in the inter-micellar distance with increasing surfactant concentration. Taking the chemical equilibrium for the complex formation between Au ions and HTAB micelles into account, we found that the free Au ion concentration decreases accompanied with the increase in the surfactant concentration. This decrease in the free Au ion concentration suppresses undesirable secondary nucleation of gold crystals in a growth solution, resulting in gold nanorod elongation.