Preparation of Hierarchically Assembled Silver Nanostructures based on the Morphologies of Crystalline Peptide-Silver(I) Complexes.

The preparation of a hierarchically assembled Ag nanostructures based on a nanocrystalline assembly was demonstrated using an Ag(I) complex of a dipeptide (AspDap). By heating under N2 gas, a spherical assembly of a nanocrystalline dipeptide-Ag(I) complex (diameter 4-5 µm), which has a morphology similar to the assembled structure of the dipeptide, was transformed to an assembly of Ag nanostructures, where the micrometre-order crystalline morphology was maintained. In addition, detailed scanning electron microscopy studies revealed that Ag nanoparticles (diameter ca. 10 nm) were formed on the surface of the Ag nanostructure. When the Ag(I) ions were reduced to Ag(0), this phenomenon exhibited surface dependence due to the anisotropic two-dimensional Ag(I) arrangement in the crystals. Thermogravimetric measurements and X-ray photoelectron spectroscopy revealed that the reduction proceeds in a stepwise manner around 200-250 °C, together with the removal of primary and secondary carboxylic groups in the dipeptide. Comparison with the heating process of the crystalline Ag(I) complex of ß-alanine indicated that stepwise reduction is key for maintaining the original micrometre-order morphology.

Conical Ionic Amphiphiles Endowed with Micellization Ability but Lacking Air-Water and Oil-Water Interfacial Activity.

Micellization in water and reduction of the surface tension at water interfaces with air and oil are two archetypical properties of surfactants, caused by self-aggregation and Gibbs monolayer formation at the interfaces, respectively. We present here a new type of amphiphiles that possess a conical shape consisting of a hydrophobic apex and five ionic termini at the base of the cone. The conical shape and the high charge density cooperatively impede monolayer formation at the interfaces, hence preventing foaming and emulsification. On the other hand, the conical shape strongly assists micelle formation in water and hemimicelle formation on a solid surface to promote dissolution of nanoparticles such as magnetic nanoparticles and nanocarbons in water. The well-defined shape and charge locations distinguish the new amphiphiles from known polymer amphiphiles that show similar surface activity.