No abstract

Polyhedral gold nanoparticles are of great current interest because of their unique optical and chemical properties which are attributable to their well-defined facets, corners, and size. While various polyhedral gold nanoparticles of different sizes mostly synthesized by the seed-mediated method have been reported, synthesis of gold cuboctahedra with tunable sizes still remains challenging. Here, we report for the first time a seedless method of synthesizing monodisperse gold cuboctahedra with finely tunable sizes ranging from 40 to 80 nm using cetyltrimethylammonium 4-vinylbenzoate (CTAVB) as a selective capping and mild reducing agent in the presence of a high concentration of HCl in aqueous solution. The HCl provides strong oxidative etching power to remove structural defects, resulting in single-crystal seeds, and significantly reduces the particle growth rate. This slow particle growth provides an easy and reliable way of tuning the particle size by stopping the reaction at different times and allowing sufficient time for the surface self-diffusion of Au atoms. Combined with the selective capping of {100} facets with CTA+, the surface self-diffusion of Au atoms from {111} to {100} facets is considered to be the key mechanism for the formation of Au cuboctahedra and their stable growth without morphological deformation.

The aggregation behavior of oppositely charged gold nanorods (GNRs) in aqueous solution has been investigated by zeta potential, UV−vis−NIR, DLS, and TEM measurements. The positively charged GNRs (p-GNRs) and negatively charged GNRs (n-GNRs) were prepared by layer-by-layer deposition of oppositely charged electrolytes on GNR surface. As p-GNRs are added into n-GNR solution (before reaching the isoelectric point), p-GNR/n-GNR aggregations of fairly constant size are formed with the minor component (p-GNRs) mostly at the center and the majority component (n-GNRs) at the outside. While the overall packing of the GNR aggregates is rather random, the local arrangements of GNRs show both side-by-side and end-to-end arrangements, resulting in elongated aggregates due to the anisotropic nature of GNRs. While no precipitation occurs before isoelectric point, the size of aggregates grows rapidly as the isotropic point is approached and rapid precipitation occurs at the isoelectric point, showing ionic-like behavior. Beyond the isoelectric point, partial dissolution of aggregates occurs.

Surface coating with inorganic materials such as SiO 2 , [31][32][33] TiO 2 , [34][35][36] or yttria-stabilized zirconia [37] has been commonly used to achieve thermal stability of various individual nanoparticles including GNRs [38][39][40] or nanoporous gold. [41] Encapsulation of individual GNRs with silica, forming core-shell structure, makes GNRs stable up to 700 and 800 °C [39] while surfactant or polymer-coated GNRs become degraded at temperature as low as 100 °C. [29,42] This encapsulation approach, however, has not been successfully applied for GNR superlattices yet. Recently, an impregnation method has been used to protect the GNR superlattices with porous silica, in which a preformed GNR superlattice on substrate was immersed in a silica precursor solution. [43] While the GNR superlattice made by the impregnation method was quite stable in water, it became significantly degraded at 150 °C because silica layer was formed over the superlattice and between the individual lamellae of GNRs inside the structure without covering the individual GNRs. [43] To achieve the stability of GNR superlattice at high temperature, therefore, a new method to protect individual GNRs in the superlattice should be developed.In this study, we developed a new method to fabricate 2D hexagonal GNR superlattices on substrate in which each GNR is individually encapsulated with silica, providing an excellent stability at high temperature and in solvents of different polarities. In this method, cetyltrimethylammonium bromide (CTAB) bilayer-covered GNRs dispersed in silica precursor solution at a highly acidic condition are self-assembled into the 2D monolayer superlattice upon drop casting on substrate, during which the condensation of silica precursors adsorbed on the CTAB bilayer and the slow solvent evaporation occur simultaneously. The GNR superlattices embedded in the silica matrix are stable at temperature as high as 500 °C and in various solvents of different polarity, well maintaining the cylindrical shape of GNRs and their hexagonal packing. The structural stability makes the GNR superlattice embedded in the silica matrix a highly reusable surface-enhanced Raman scattering (SERS) active substrate for molecular detection, as evidenced by the signal intensities well maintained over 10 cycles. To the best of our knowledge, this is the GNR superlattice with the highest thermal and solvent stability reported so far.A facile method to fabricate 2D hexagonal monolayer superlattices of gold nanorods (GNRs) individually embedded in silica matrix on a substrate is developed. In this method, the cetyltrimethylammonium bromide (CTAB) bilayer-coated GNRs are self-assembled into a hexagonally packed monolayer superlattice on the substrate by slow evaporation in the presence of silica precursors in the solution at a highly acidic condition. The GNR superlattices fabricated by this method show excellent structural stability at high temperature as high as 500 °C and in solvents of a wide range of polarities including water, ethanol, toluene...

Synthesis of uniform polyhedral gold nanoparticles (Au NPs) by the seed-mediated method is often limited by the difficulties in preparing uniform seeds.Here, we report a facile one-pot synthesis of highly monodisperse single-crystalline spherical Au NPs, which can be used as universal seeds. This method only involves simple mixing of cetyltrimethyl ammonium 4-vinylbenzoate, HAuCl 4 , AgNO 3 , and HCl in water at a fixed temperature. The single crystallinity of particles is achieved by the interplay between oxidative etching and controlled surface capping by silver atoms. As-synthesized Au NPs show uniform single crystallinity, shape yield of nearly 100%, and size polydispersity less than 5%. The as-synthesized single-crystalline Au NPs are used as universal seeds to grow monodisperse polyhedral Au NPs of different shapes including octahedra, cubes, rhombic dodecahedra, concave cubes, and concave rhombic dodecahedra with size polydispersity as small as 1.5−4.2% depending on the particle shape, the smallest values for any shape reported so far. This clearly shows the importance of seed uniformity achieved by the present method.