Lyotropic liquid crystals of tetradecyldimethylaminoxide in water and the in situ formation of gold nanomaterials

Abstract

Lyotropic liquid crystals (LLCs) produced by the self-assembly of surfactant in water represent an important class of highly ordered soft materials that have a wide range of applications. This study investigates the LLCs formed by a zwitterionic surfactant (tetradecyldimethylaminoxide, C₁₄DMAO) in water. The organization of C₁₄DMAO within the LLCs was determined based on a detailed analysis of small-angle X-ray scattering measurements and polarized microscopy observations of a typical sample. Additional to the singe-phase region, which has a hexagonal organization, several two-phase regions were observed, exhibiting the coexistence of hexagonal/cubic, cubic/lamellar, and hexagonal/lamellar phases. The phase behavior showed an obvious dependence on temperature, with more pronounced two-phase regions at lower temperatures. Using the LLCs as a matrix, Au nanospheres, nanoellipsoids, and nanorods were prepared without requiring additional reducing reagents. These three- and one-dimensional Au nanomaterials could be converted to two-dimensional plates via the introduction of a small amount of cationic surfactant to the LLCs, such as cetyltrimethylammonium bromide (CTAB) and 1-hexadecyl-3-methylimidazolium bromide ([C₁₆MIm]B), which showed pronounced surface-enhanced Raman scattering activity towards solid rhodamine. The LLCs loaded with CTAB (or [C₁₆MIm]B) and HAuCl₄ exhibited slightly different structures and mechanical strength from the original LLCs, thereby forming a new class of highly crowded colloidal materials.