Gemini-surfactant-directed self-assembly of monodisperse gold nanorods into standing superlattices.

Fabrication of ordered nanoparticle assemblies over extended areas and volumes is still a major challenge in nanomaterials research. The current limitations in the production of such ordered assemblies dramatically hinder the application of nanoparticles in fields such as negative refractive index metamaterials or information technologies. In the particular case of metal nanocrystal assemblies, nanoscale organization of readily accessible spherical gold nanoparticles 4] has been manipulated to produce a diverse range of topologies with interesting optical and electrical properties. 7] However, the use of isotropic nanoparticles strongly limits potential applications that require the formation of lattices with vectorial properties. A recent report demonstrated the formation of 3D gold nanorod (NR) superstructures from liquid-crystalline phases with a limited degree of control over the dimensionality and directionality of the assembly. A major advance is demonstrated herein through the use of a gemini surfactant. Replacement of cetyltrimethylammonium bromide (CTAB) by this unconventional surfactant during nanorod synthesis leads to production of monodisperse NRs that can undergo directional self-assembly into highly ordered 2D and 3D standing superlattices with anisotropic optical properties. The synthesis of highly monodisperse gold NRs is especially appealing because of their strong, polarizationdependent suface-plasmon-based optical properties, which render their assemblies ideal candidates for the preparation of optically anisotropic lattices that allow manipulation of light in the nanoscale. Nowadays, tuning of the longitudinal and transverse localized plasmon resonances of gold NRs by synthetic manipulation is a mature field of research. In particular, the seeded growth method in aqueous solution, based on the reduction by a weak reducing agent of a gold salt on premade small seeds in the presence of CTAB and silver ions provides sufficient flexibility to synthesize nanorods (CTAB-NRs) with diverse sizes and shapes. Besides being a shape-inducing agent, CTAB efficiently prevents aggregation through dynamic adsorption onto the gold NRs surface in a bilayer fashion. This nanoparticle shielding of CTAB in water, together with the intense capillary forces generated at the solvent–air interfaces in aqueous solution and the typical Brownian motion of nanoparticles, brings colloidal stability face-to-face with controlled self-assembly of NRs in water and demands a rational search for new and simple strategies. To date, the construction of assemblies of standing gold NRs has mostly relied on postsynthesis surface functionalization with thiol and silane capping agents and subsequent transfer into organic solvents. However, the degree of order in the self-assembly has still been limited to 2D sub-micrometer areas. Among the different capping agents that have been proposed for the preparation of metal nanoparticle arrays in organic solvents, thiol-functionalized phospholipid derivatives have proven excellent binders in nanoparticle–lipid– nanoparticle assemblies, 21] as they can be efficiently supported as bilayers on different substrates. In this respect, gemini surfactants, made of two hydrophobic tails and two hydrophilic headgroups linked by a spacer chain, are currently available as excellent scaffolds for structurally mimicking the bilayer formation of lipids in water. Moreover, gemini surfactants display exceptional amphiphilic properties, such as high adsorptivities on solid surfaces, which make them ideal candidates to influence the growth 25] and assembly of gold nanorods. We present herein the first study in which cationic gemini surfactants, (oligooxa)alkanediyla,w-bis(dimethyldodecylammonium bromide) (12-EOx-12, see the Supporting Information), were used for the reproducible and controlled synthesis of monodisperse gold NRs, focusing on the role of the chemical structure of these surfactants on the self-assembly of highly ordered, robust 2D and 3D gold NR superlattices with directional optical properties. We synthesized gold nanorods using the gemini surfactant that contains a spacer with one ethylene oxide group, 12-EO112 (Gem1-NRs), as a shape-directing agent by Ag-assisted seeded growth on preformed CTAB-capped gold seeds at 27 8C (see details in the Experimental Section). Controlling the molar ratio between seed and gold salt, the aspect ratio and longitudinal surface plasmon (LSP) resonance of the NRs could be readily tuned (see the Experimental Section). Significantly narrower LSP bands were measured for Gem1NRs than for CTAB-NRs, with typical full width at half LSP maximum (FWHM) reduction of 20–30 % (see the Support[*] Dr. A. Guerrero-Mart nez, Dr. J. P rez-Juste, E. Carb -Argibay, Prof. L. M. Liz-Marz n Departamento de Quimica Fisica and Unidad Asociada CSIC Universidade de Vigo, 36310 Vigo (Spain) Fax: (+ 34)986-812556 E-mail: aguerrero@uvigo.es lmarzan@uvigo.es