Spontaneous Formation of Highly Stable Nanoparticle Supercrystals Driven by a Covalent Bonding Interaction.

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 solution-phase 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.

Micelle-Assisted Formation of Nanoparticle Superlattices and Thermally Reversible Symmetry Transitions.

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 NaZn13, MgZn2, and AlB2-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 (NaZn13-AlB2) and order-disorder (MgZn2-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.

Nanoparticle Superlattices Driven by Linker-Mediated Covalent Bonding Interaction.

The stability of the nanoparticle superlattice (NPSL) is essential for realizing its broad spectrum of potential applications. Here, we report a linker-mediated covalent bonding interaction method for the synthesis of highly stable NPSLs. Adipic acid is used as a linker molecule which connects two Au NPs functionalized with 6-mercaptohexanol through esterification reactions in the presence of H2SO4. As-prepared NPSLs are mostly fcc Wulff polyhedra with a fairly narrow size distribution and are highly stable in solvents of different polarities and pHs (0-14) as well as in dry conditions and at temperatures as high as 175 °C. The formation of NPSLs involves random homogeneous nucleation simultaneously accompanied by growth, a gradual change of the growth mode from reaction-controlled to diffusion-controlled with time, and the oriented attachments of small crystals. The size of the NPSL can be easily tuned by the concentration of linker molecules and the reaction temperature.

Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability.

A method to fabricate porous silica-coated Au sponges that show high thermal and catalytic stability has been developed for the first time. The method involves dense surface functionalization of Au sponges (made by self-assembly of Au nanoparticles) with thiolated poly(ethylene glycol) (SH-PEG), which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its morphology at temperature as high as 700 °C. The calcination removes all organic molecules, resulting in porous silica-coated Au sponges, which contain hierarchically connected micro- and mesopores. The hierarchical pore structures provide an efficient pathway for reactant molecules to access the surface of Au sponges. The porous silica-coated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH4 to 4-aminophenol as high as 93% even after 10 catalytic cycles. The method may be applicable for other porous metals, which are of great interests for catalyst, fuel cell, and sensor applications.

Aggregation of sodium dodecylsulfate in aqueous nitric acid medium.

Nitric acid medium is invariably used for nitration of organic molecules. Although surfactants are known to influence reaction rates, little is known about the aggregation behavior of surfactants in nitric acid medium. Micellization characteristics of sodium dodecylsulfate (SDS) in aqueous nitric acid are investigated in this work by using the conductance method. The critical micelle concentration (cmc) and the aggregation number were also determined by the surface tension and the steady-state fluorescence methods, respectively. This study reveals that in acidic medium SDS exhibits both normal and unusual conductivity behaviors. Equations developed on the basis of the mixed electrolyte model, Debye-Hückel-Onsager approach, and the pseudophase ion-exchange model successfully simulate the conductivity data. The exchange of sodium and hydrogen counterions at the micellar surface has no significant effect on the cmc of SDS. Acid concentration, surfactant concentration, and cmc control the competitive binding of sodium and hydrogen counterions. Analysis of conductivity data revealed hydrolysis of about 12% SDS when [HNO(3)]≥0.02 mol dm(-3). Hydrolysis of SDS has been confirmed by nitrating some of the substituted phenols. It has been predicted that SDS+aqueous HNO(3) medium with [HNO(3)]≥0.02 mol dm(-3) may be used as a green medium for nitration.

Effect of sodium salicylate, sodium oxalate, and sodium chloride on the micellization and adsorption of sodium deoxycholate in aqueous solutions.

The salicylate ion increases the rate of bile flow (choleretic effect) and bile salts are known to affect the colonic absorption of oxalate. Owing to this physiological relevance of salicylate and oxalate ions, critical micelle concentration (cmc) values of sodium deoxycholate (NaDC) were determined in aqueous sodium oxalate, sodium salicylate, and sodium chloride solutions by using surface tension, fluorescence, and EMF methods. The results indicate, besides a counterion effect, the influence of coanions on the cmc. In the range from 25 to 40 °C, cmc increases almost linearly with temperature. In the temperature range from 30 to 40 °C, the counterion binding constant ß of NaDC micelles has the same value (0.17±0.01) in the presence of sodium chloride and sodium salicylate. On the other hand, in sodium oxalate solution ß=0.05±0.02 when oxalate concentration is less than or equal to c* and ß=0.48±0.04 above c*, where c*≈0.038 mol kg(-1). EMF measurements also supported this type of counterion binding to NaDC micelles in sodium oxalate solutions. In sodium oxalate solution, at c* a change in the shape of deoxycholate micelles is expected to take place. Salicylate, oxalate, and chloride coanions have a similar effect on the adsorption of NaDC. This study reveals that the choleretic effect of salicylate is not due to the influence of salicylate ions on the micellization of NaDC.