A facile and green synthesis method for mesoporous gold sponges has been developed, which involves a simple mixing of a very small amount of thiolated-poly(ethylene glycol) (SH-PEG) and citrate-covered gold nanoparticles (Au NPs) in aqueous solution at room temperature. While SH-PEG molecules have been widely used as biocompatible hydrophilic capping agents for Au NPs for stable dispersion in aqueous solution, here they are used as destabilizing agents. When SH-PEG molecules are mixed with citrate-covered Au NPs at the molar ratio ranging from 3 to 20 (SH-PEG/Au NP), mesoporous gold sponges with randomly interconnected 3D network structures are formed within 2 to 3 h. This is driven by the destabilization of negatively charged citrate molecules on Au NPs by a small number of SH-PEG molecules bonded on the particle surface, which results in the decrease in zeta potential and thus the assembly of Au NPs into porous sponges. The use of very low concentration of SH-PEG (ca. 20-200 nM) in aqueous solution at room temperature makes the method highly eco-friendly as well as results in high-purity as-synthesized gold sponges (98.7 wt %). The mesoporous gold sponges fabricated with the present method exhibit a high SERS activity, making them highly applicable for sensitive SERS detection of molecules.

Nanoparticle supercrystals (NPSCs) are of great interest as materials with emergent properties. Different types of intermolecular forces, such as van der Waals interaction and hydrogen bonding, are present in the NPSCs fabricated to date. However, the limited structural stability of such NPSCs that results from the weakness of these intermolecular forces is a challenge. Here, we report a spontaneous formation of NPSCs driven by covalent bonding interactions, a type of intramolecular force much stronger than the above-mentioned intermolecular forces. A model solutionphase anhydride reaction is used to form covalent bonds between molecules grafted on the surface of gold nanoparticles, resulting in three-dimensional NPSCs. The NPSCs are very stable in different solvents, in dried conditions, and at temperatures as high as 160 °C. In addition to this, the large library of covalent-bond-forming reactions available and the low cost of reactants make the covalent bonding approach highly versatile and economical.

Nanoparticle superlattices (NPSLs) are of great interest as materials with designed emerging properties depending on the lattice symmetry as well as composition. The symmetry transition of NPSLs depending on environmental conditions can be an excellent ground for making new stimuli-responsive functional materials. Here, we report a spherical micelle-assisted method to form exceptionally ordered NPSLs which are inherently sensitive to environmental conditions. Upon mixing functionalized gold nanoparticles (AuNPs) with a nonionic surfactant spherical micellar solution, NPSLs of different symmetries such as NaZn 13 , MgZn 2 , and AlB 2 -type are formed depending on the size ratio between micelles and functionalized AuNPs and composition. The NPSLs formed by the spherical micelle-assisted method show thermally reversible order−order (NaZn 13 −AlB 2 ) and order−disorder (MgZn 2 −isotropic) symmetry transitions, which are consistent with the Gibbs free energy calculations for binary hard-sphere model. This approach may open up new possibilities for NPSLs as stimuli-responsive functional materials.

Highly ordered binary superlattices … … of 1D nano-objects are demonstrated for hydrophilically functionalized single-walled carbon nanotubes (p-SWNTs) and cylindrical surfactant micelles. In their Communication on page 12548 ff., S.-M. Choi et al. show that when p-SWNTs are added into a hexagonally packed cylindrical micellar system, p-SWNTs form a hexagonal array embedded in a honeycomb lattice of surfactant cylinders to maximize the free-volume entropies of both 1D nano-objects.

The nanoparticle supercrystals (NPSCs) have been of great interests for their collective emergent properties. While various NPSCs have been successfully fabricated using intermolecular forces, the limited structural stability of NPSCs due to the weak nature of the intermolecular forces still remains a major hurdle for practical applications. Herein, we report a new method to fabricate highly stable three-dimensional NPSCs by using aldol reaction, a model covalent bond forming reaction, in conjunction with slow solvent evaporation. Gold nanoparticles functionalized with thiol poly-ethylene glycol formyl are linked to each other by carbon-carbon covalent bonds formed by aldol reaction as the particle dispersion in aqueous NaOH solution is slowly evaporated, resulting in highly faceted three-dimensional NPSCs. Assynthesized NPSCs show excellent structural stability in solvents of different polartities as well as the dried condition and at temperature up to 160 °C, which is far superior to NPSCs stabilized by intermolecular forces such as hydrogen bonding and van der Waals interactions. The new covalent bonding appraoch opens up new opportunities in the synthesis of NPSCs and their applications.