Synthesis of organo-silane functionalized nanocrystal micelles and their self-assembly.

Nanocrystal (NC) synthesis and self-assembly have received much attention due to unique size-dependent properties and new collective physical properties derived from ordered NC arrays. However to fully exploit these properties, nanocrystal functionalization is critical and essential. In general, the key to functionalize NCs is maintenance of the original physical properties of the NCs, and in many situations, it is necessary to develop flexible NC surface chemistry, allowing further, subsequent functionalization. To date, most work has focused on alkane chain functionalized/ stabilized NCs prepared using hot soap 1,2 or a two-phase approach. 3 These methods can effectively make high quality NCs (narrow size distribution, preferred shape and composition, large production). Unfortunately, the alkane chains are chemically inert, preventing further chemical functionalization. In addition, these NCs are hydrophobic, precluding their direct use in bioapplications. There is an urgent need for general NC functionalization methods. Here, we report a new synthetic route to functionalize NCs with organo-silane groups through encapsulation of monodisperse, hydrophobic gold nanocrystals within the core of silane precursor micelles. Octadecyldimethyl(3-trimethoxysilylpropyl)ammonium chloride (precursor 1) is an amphiphilic functional methoxysilane that self-assemble into micelles and liquid crystal mesophase. 4 After encapsulation, the nanocrystal micelles are monodisperse, hydrophilic, and can self-assemble into 3-D ordered arrays through silane headgroup cross-linking under acidic conditions. In comparison with the previous method concerning the preparation of hybrid silica with stabilized gold NCs, 5,6 this method is simple, general, and allows extensive further functionalization based on silane coupling chemistry. The synthesis of organo-silane functionalized gold NC micelles was based on our recently developed surfactant encapsulation techniques.7,8 In a typical nanocrystal micelle synthesis procedure, a concentrated monosized 1-dodecanethiol (DT)-stabilized gold NC chloroform solution was added to an aqueous solution of precursor 1 under vigorous stirring to create an oil-in-water microemulsion. The highly volatile chloroform was then evaporated during heating at 30 ° C( 10 min), which transfers the nanocrystals into the aqueous phase, forming NC micelles. To prevent the silane groups from extensive hydrolysis, the heating process was kept as fast as possible and the temperature was kept as low as possible (30 °C). The encapsulation was driven by the hydrophobic van der Waals interactions between the primary alkane of the stabilizing ligand and the secondary alkane of the surfactant, resulting in thermodynamically defined interdigitated “bilayer” structures (Scheme 1c), which have been used to stabilize rod- and cube-shaped nanocrystals. 9-12 The gold NC micelles are monodisperse, as suggested by UVvis spectra. Figure 1a shows the UV-vis spectra of DT-stabilized gold NCs in chloroform. The absorbance at 519 nm corresponds to the gold surface plasmon resonance band. By comparison of Figure 1a-e, we observe no difference in the positions and peak widths of the plasmon resonance band of the DT-stabilized gold NCs in chloroform, those of the corresponding water-soluble NC