Ordered Two‐ and Three‐Dimensional Arrays Self‐Assembled from Water‐Soluble Nanocrystal–Micelles

Abstract

The synthesis and self-assembly of nanocrystals (NCs) have gained much interest due to the NCs' unique size-dependent optical, electronic, and chemical properties and their potential applications in areas such as optics, electronics, catalysis, magnetic storage, and biological labeling and sensing. [1][2][3][4][5][6] To date, most work has focused on the synthesis and self-assembly of nanocrystals that are stabilized with alkane ligands (CH 3 (CH 2 ) n R, R = SH, NH 2 , PO, etc.). [3,7,8] Such NCs can be made at fairly high quality (narrow size distribution, preferred shapes such as rod, cube, etc., and large production). These NCs are hydrophobic, and their self-assembly is limited to organic solvents. However, there are many applications requiring hydrophilic or aqueous environments, such as biolabeling and surface-enhanced Raman spectroscopy (SERS)-based NC films or arrays for biosensing. [9][10][11][12] In addition, water-soluble NCs and their ordered arrays/films provide a great opportunity for further integration into inorganic ceramic frameworks that offer the chemical and mechanical robustness needed for enhanced device functionality. [13][14][15][16] Although some NC superlattices have been fabricated through aqueous media, [17][18][19] the methods are limited to spherical gold NCs.Here we report a facile route to prepare ordered NC arrays self-assembled from water-soluble NC-micelles. The method is simple, widely applicable to other materials, and can be used to prepare water-soluble NCs with different compositions and shapes, such as sphere, rod, and cube, as well as their ordered arrays. Our approach involves the synthesis of watersoluble gold-NC-micelles using our recently developed surfactant encapsulation techniques, [16,20] which were conducted in an interfacially driven water-in-oil microemulsion process. The whole process takes only ten minutes. Subsequent selfassembly of NC-micelles upon evaporation of a drop of NCmicelle aqueous solution on substrates, such as a transmission electron microscopy (TEM) grid, silicon wafer, glass, etc., leads to ordered two-and three-dimensional (2D and 3D) gold-NC superlattice films (Scheme 1). 1-Dodecanethiol (DT)-stabilized gold NCs were prepared using the method of Brust et al. [21] Heat treatment at 140°Cfor 30 min followed by size-selective precipitation using the solvent/non-solvent pair of toluene/ethanol was used to further narrow the size distribution to ∼ 7 %. The narrow size distribution of NCs is essential for formation of ordered NCmicelle arrays. In a typical NC-micelle synthesis procedure, a concentrated solution of monosized DT-stabilized gold NCs in chloroform was added to an aqueous solution of hexadecyltrimethylammonium bromide (C16TAB) under vigorous stirring to create an oil-in-water microemulsion. Chloroform evaporation during a heat course (40-80°C, ∼ 10 min) transfers the NCs into the aqueous phase (forming NC-micelle stock solution). This interfacial process is driven by the hydrophobic van der Waals interactions between the primary alka...

Synthesis of Inorganic and Organic–Inorganic Hybrid Hollow Particles Using a Cationic Surfactant with a Partially Fluorinated Tail

“…Molecular modeling of 1-(10-perfluorooctyldecyl)pyridinium suggests that this dimension is most consistent with interdigitated fluorocarbons in the micelle layer (Scheme 2). Interdigitation of bilayers is thought to be a key feature leading to stability of bilayers supported on colloidal particles, [42][43][44] and fluorination of lipids has been shown to induce the formation of interdigitated bilayers. [45,46] Optimizing the structure with MOPAC 2000 [23] at the PM3 level [47] suggests a fluorocarbon length of 10.3 Å, and an all-trans hydrocarbon length of 13.8 Å.…”

Three‐Dimensionally Ordered Gold Nanocrystal/Silica Superlattice Thin Films Synthesized via Sol–Gel Self‐Assembly

“…[35] The thermodynamically favorable interdigitation between the n-DT and CTAB layers stabilizes the gold NC micelles in aqueous solution. [30] Tetraethylorthosilicate (TEOS) was used as a precursor and was added to the above solution, followed by hydrolysis and condensation under acidic conditions at room temperature for 1 h. In addition to TEOS, bis(triethoxylsilyl) ethane (BTEE) was used to tune the matrix composition within the superlattice films. Ordered superlattice thin films were prepared by spin-coating the above solution on substrates such as silicon wafers, glass, etc.…”

“…The ease of preparing semiconducting and magnetic NC micelles using an interfacially driven microemulsion process makes it possible to synthesize ordered quantum dot/metal oxide and magnetic NC/metal oxide arrays, and to integrate them into laser and memory devices. [30,31] The robust, 3D NC/silica superlattice films are of interest for the development of collective optical and electronic phenomena, and, importantly, for the integration of NC arrays into device architectures. [8,44] Ultimately, the ordered gold/silica superlattice films are ideal platforms for the fabrication of molecular electronic nanodevices [45] and surface-enhanced Raman spectroscopy (SERS)-based chemical and biosensors.…”

“…[9,11,28] We recently reported a facile synthesis of water-soluble NC micelles [29][30][31] and their self-assembly into ordered, 3D gold-NC/silica superlattice arrays in a sol-gel process. [19] Under basic conditions, fast silica hydrolysis and condensation led to a heterogeneous self-assembly system and ordered arrays in solid/powder form.…”

Fe3O4 octahedral colloidal crystals

“…They are also of major significance for potential technological applications based on their unique collective physicochemical properties [2,3,[8][9][10][11][12][13][14]. The classic strategy to obtain colloidal crystals is similar to solar salt production, namely, slowly evaporating the nanocrystal colloidal solution on a solid substrate [1,3,9,10,[14][15][16]. More recently, other techniques including electrostatic assembly [5], pH-controlled assembly [8,17], polarnonpolar solvent diffusion precipitation [7,13,18,19] and colloidal sphere assembly [20][21][22] have also been developed.…”