Self-assembly of amphiphilic alkyloligosiloxanes within cylindrically and spherically confined spaces

The self-assembly of amphiphilic alkyloligosiloxane molecules within cylindrically and spherically confined spaces has been investigated. Hydrolyzed solutions of the precursors consisting of an alkylsiloxane core and three branching trimethoxysilyl groups (C n H 2 n+1 Si(OSi(OMe) 3) 3, n = 10 and 16) were impregnated into the cylindrical pores of porous anodic alumina membranes (PAAMs), leading to the formation of rod- and tubelike hybrids. A two-dimensional (2D) hexagonal mesostructure with a circular orientation and a lamellar mesostructure with a multitubular orientation were confirmed for n = 10 and 16, respectively. The pore diameters of PAAMs ranging from 30 to 400 nm did not significantly affect the mesostructures of the hybrids. The self-assembly in the spherical droplets was also performed by spray-drying of the hydrolyzed solutions. At high temperature, vesicular lamellar mesostructures were formed, independent of the alkyl chain length of the precursors ( n = 10 or 16). Spherical hybrids with a core-shell structure (a 2D hexagonal core and a lamellar shell) were also prepared by lowering the drying temperature in the case of n = 10. These are the first findings on the confined assembly of single siloxane-based amphiphiles that will lead to the fabrication of novel hierarchically ordered hybrid materials having Si-C covalent bonds at the interfaces.     

Self-assembly of alkyl-substituted cubic siloxane cages into ordered hybrid materials

Siloxane-organic hybrids with well-ordered mesostructures were synthesized through the self-assembly of novel amphiphilic molecules that consist of cubic siloxane heads and hydrophobic alkyl tails. The monoalkyl precursors functionalized with ethoxy groups (C(n)H(2n+1)Si(8)O(12)(OEt)(7), 1 Cn, n=16, 18, and 20) were hydrolyzed under acidic conditions with the retention of the siloxane cages, leading to the formation of two-dimensional hexagonal phases by evaporation-induced self-assembly processes. Analysis of the solid-state (29)Si MAS NMR spectra of these hybrid mesostructures confirmed that the cubic siloxane units were cross-linked to form siloxane networks. Calcination of these hybrids gave mesoporous silica, the pore diameter of which varied depending on the alkyl-chain length. We also found that the precursors that had two alkyl chains formed lamellar phases, thus confirming that the number of alkyl chains per cage had a strong influence on the mesostructures. These results expand the design possibility of novel nanohybrid and nanoporous materials through the self-assembly of well-defined oligosiloxane-based precursors.     

Controlled synthesis of nanostructured silica-based materials from designed alkoxysilanes

Recent progress in the synthesis of nanostructured silica-based materials through the self-assembly process using well-designed alkoxysilane precursors is presented. Alkoxysilanes with covalently attached hydrophobic organic tails become amphiphilic when hydrolyzed to form silanol groups, leading to the formation of various mesostructures upon evaporation of solvents. The precursors having large oligosiloxane heads are particularly important because of their ability to form cylindrical assemblies, providing a direct pathway to ordered porous silica by removal of the organic groups. Our recent research includes (i) templated-synthesis of hierarchically ordered structures and (ii) design of molecules having chemically cleavable bonds to generate pores without calcination.     

Self-assembly of designed oligomeric siloxanes with alkyl chains into silica-based hybrid mesostructures

A novel self-assembly route to ordered silica-organic hybrids using well-defined siloxane oligomers with alkoxy functionality and covalently attached alkyl chains has been investigated. Various hybrid mesostructures were obtained by hydrolysis and polycondensation without the use of any structure-directing agents. The oligomers 1(Cn), having an alkylsilane core and three branched trimethoxysilyl groups, formed highly ordered lamellar phases when n = 14-18, while those with shorter alkyl chains formed cylindrical assemblies, slightly distorted two-dimensional (2D) hexagonal structures (n = 6-10), and a novel 2D monoclinic structure (n = 12). Furthermore, the mixtures of 1(Cn) with different chain lengths yielded well-ordered 2D hexagonal phases, possibly due to the better packing of the precursors. The hybrids consisting of cylindrical assemblies were converted to ordered porous silica with tunable pore sizes upon calcination to remove organic groups. The liquid-state 29Si NMR analysis of the hydrolysis and polycondensation processes of 1(Cn) revealed a unique intramolecular reaction yielding primarily the oligomer with a tetrasiloxane ring which is a new class of amphiphilic molecule having both self-assembling ability and high cross-linking ability. We also found that the mesostructure (lamellar or 2D hexagonal) was strictly controlled by varying the number of siloxane units per alkyl chain. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of oligosiloxane precursors.     

Sol–gel nanohybrid materials prepared via supramolecular organization

This article reports on recent progress in the synthesis of sol–gel nanohybrid materials based on the supramolecular organization. A variety of nanohybrid materials has been obtained by molecular design of the precursors and systematical control of synthetic processes. Organoalkoxysilanes with covalently attached hydrophobic tails are hydrolyzed to form amphiphilic molecules containing silanol groups, leading to the formation of vesicular structures. The obtained hybrid has analogous structures of both cell membrane and silica particle and was named “cerasome”. The cerasome can achieve the hierarchical three-dimensional organization of vesicular particles on the substrate. The nanohybrids are developed not only by the hydrophobic interaction of amphiphilic molecules but also by the electrostatic interaction. The layer-by-layer (LbL) assembly of the water-soluble titania precursor with polycation is adopted for nanohybrid coatings containing titania nanoparticles on the substrates. In addition, preparation of hybrid hollow capsules via LbL assembly and sol–gel method with colloid templating is also discussed.     

Nanohybrids from liquid crystalline extended amphiphilic dendrimers

A novel extended amphiphilic dendrimer with linear poly(ethylene oxide) (PEO) attached to a PEO-like dendritic core as hydrophilic fraction and eight docosyl chain branches as hydrophobic fraction has been prepared for the use as structure-directing agent for silica-type materials. The extended dendrimer exhibits a hexagonal columnar liquid crystalline phase in the melt. Organically modified inorganic precursors and the extended dendrimer co-assemble into nanostructured hybrids. Hybrids with 0.44 weight fraction (fw) of aluminosilicate show a lamellar morphology, while hybrids with 0.21 fw exhibit a cylindrical structure. Nanostructures were characterized by a combination of small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and atomic force microscopy (AFM). The results suggest that dendrimer-based amphiphiles may provide an exciting platform for the formation of multifunctional organic-inorganic nanostructured hybrid materials with unique structural characteristics.     

Formation of Lamellar Silica from Lyotropic Liquid Crystals of Dodecyl Benzene Sulfonic Acid

The formation of lamellar mesostructured silica by a neutral route in dodecylbenzene sulfonic acid (DBSA)/aminoalkoxysilane/water systems was investigated by phase study and Small Angle X‐ray Scattering (SAXS). Initially, two lamellar lyotropic phases are found, one corresponding to the DBSA and other corresponding to the DBSA‐aminoalkoxysilane salt. With the hydrolysis and condensation of siloxane groups, the DBSA‐aminoalkoxysilane lyotropic phase disappears and lamellar silica is formed. The lyotropic phase does not act as a true template but as a source of amphiphilic molecules, hence synthesis takes place via a phase separation mechanism. This synthesis route provides an easy way to prepare amino functionalized lamellar silica.     

Preparation of mesostructured siloxane-organic hybrid films with ordered macropores by templated self-assembly

Novel hierarchically ordered siloxane-based hybrid films with well-defined macropores and mesostructured pore walls have been prepared by the self-assembly process using oligomeric siloxane precursors bearing alkyl chains (CnH2n+1Si(OSi(OMe)3)3) in the presence of polystyrene opal films as a template. Either a two-dimensional (2D) hexagonal structure or a lamellar structure was formed depending on the alkyl chain length of the precursors (n = 10 and 16, respectively). In both of the films, the mesostructures were oriented along the spherical surface of the template and were retained after removal of the template. Calcination of the 2D hexagonal hybrid produced ordered porous silica with both macro- and microporosities. The lamellar hybrid film exhibited a unique property of accommodating alkyl alcohols with an expansion of the interlayer spacings. These results provide a new concept for designing hierarchical hybrid materials that are potentially applicable as adsorbents, catalysts, sensors, and photonic crystals.     

Synthesis of new lamellar materials by self-assembly and coordination chemistry in the solids

We report the preparation of a new class of lamellar hybrid organic–inorganic materials obtained by self-assembly of bridged organosilica precursors containing long alkylene chains during the sol–gel process. The self-assembly is induced by lipophilic van der Waals interactions. The introduction of –SS– bonds in the core of the alkylene chains permitted the functionalisation of lamellar materials, which were subsequently transformed into SH and –SO₃H groups. This methodology was extended to the formation of lamellar hybrid materials containing amino groups thanks to CO₂ as bridging groups as well as the formation of lamellar hybrid materials containing carboxylic groups. In this last case, the hydrolysis and polycondensation of cyanoalkyltrialkoxysilanes permitted the one pot synthesis of lamellar hybrid materials thanks to in situ hydrogen bonds formation between carboxylic acids groups. All these functional lamellar materials exhibit a very high chelating capability towards transition metal and lanthanide ions.     

Lamellar bridged silsesquioxanes: self-assembly through a combination of hydrogen bonding and hydrophobic interactions

The synthesis of four bis(trialkoxysilylated) organic molecules capable of self-assembly--(EtO)3Si(CH2)3NHCONH-(CH2)n-NHCONH(CH2)3Si(OEt)3 (n = 9-12)--associating urea functional groups and alkylidene chains of variable length is described. These compounds behave as organogelators, forming supramolecular assemblies thanks to the intermolecular hydrogen bonding of urea groups. Whereas fluoride ion-catalysed hydrolysis in ethanol in the presence of a stoichiometric amount of water produced amorphous hybrids, acid-catalysed hydrolysis in an excess of water gave rise to the formation of crystalline lamellar hybrid materials through a self-organisation process. The structural features of these nanostructured organic/inorganic hybrids were analysed by several techniques: attenuated Fourier transformed infrared (ATR-FTIR), solid-state NMR spectroscopy (13C and 29Si), scanning and transmission electron microscopy (SEM and TEM) and powder X-ray diffraction (PXRD). The reaction conditions, the hydrophobic properties of the long alkylidene chains and the hydrogen-bonding properties of the urea groups are determining factors in the formation of these self-assembled nanostructured hybrid silicas.     

Synthesis of mesostructured silica from monoalkyl-substituted double five-ring units

A mesostructured silica-based material was synthesized by self-assembly of a novel amphiphilic molecule consisting of a well-defined siloxane head with a double five-ring (D5R) structure and a hydrophobic alkyl tail. A precursor functionalized with ethoxy groups, C₂₂H₄₅Si₁₀O₁₅(OEt)₉ (1), was hydrolyzed under an acidic condition with the retention of the D5R units, leading to the formation of two-dimensional (2D) hexagonal phase by evaporation-induced self-assembly of amphiphilic hydrolyzed molecules. Solid-state ²⁹Si MAS NMR analysis of the resulting hybrid solid confirmed that the D5R units were cross-linked to form siloxane networks. Calcination of this hybrid solid gave mesoporous silica with high BET surface area (740 m² g⁻¹). These results expand the design possibility of silica-based materials at both molecular- and meso-scales, leading to the bottom up synthesis of hierarchically ordered materials.     

Microstructuring of a polymer globule in solution in the presence of an amphiphilic substance

Microstructuring in the bulk of a polymer globule in a solution that contains dimeric amphiphilic molecules, in particular, surfactants, is studied in terms of the weak-segregation theory. An inhomogeneous structure can result from a decrease in free energy with the orientation of amphiphilic molecules in the region of inhomogeneity owing to the interaction of hydrophobic and polar parts of the molecules with the solvent. For the sake of simplicity, we discuss the case of identical second virial coefficients of the interaction of monomer units and amphiphilic molecules with different energies of interaction of the hydrophobic and polar parts of the molecule with the solvent. By comparing the free energy for different types of microstructures, we predict that, with deterioration in the quality of the solvent, there is an initial formation of a homogeneous globule followed by formation of a body-centered cubic structure; a hexagonal cylindrical structure; and, finally, a lamellar structure. For a low degree of amphiphilicity, the transition from a homogeneous globule to only a lamellar structure occurs. An increase in the concentration of the amphiphilic substance in the surrounding solution hinders the formation of a globule but facilitates its microstructuring, which is also promoted by an increase in the volume of the amphiphilic molecule and the difference in the interaction energies of its hydrophobic and polar parts with the solvent. Phase diagrams of a globule??s state at different values of model parameters are plotted.     

Design of Amphiphilic Peptide Geometry towards Biomimetic Self‐Assembly of Chiral Mesoporous Silica

In nature, diatoms and sponges are exquisite examples of well‐defined structures produced by silica biomineralisation, in which proteins play an important role. However, the artificial peptide templating route for the silica mesostructure remains a formidable and unsolved challenge. Herein, we report our effort on the design of amphiphilic peptides for synthesising a highly ordered two‐dimensional (2D)‐hexagonal and lamellar chiral silica mesostructure using trimethoxysilylpropyl‐N,N,N‐trimethylammonium chloride as the co‐structure directing agent (CSDA). The geometry of the peptide was designed by adding proline residues into the hydrophobic chain of the peptide to break the β‐sheet conformation by weakening the intermolecular hydrogen bonds; this led to the mesophase transformation from the most general lamellar structure to the 2D hexagonal P6mm mesostructure by increasing the amphiphilic molecules packing parameter g. Enantiomerically pure chiral mesostructures were formed thanks to the intrinsic chirality and relatively strong intermolecular hydrogen bonds of peptides.     

Silica hybrid biomaterials containing gelatin synthesized by sol-gel method

This work reports the sol-gel synthesis of silica hybrids. We determined the effect of the type and quantity of silica precursors and organic compounds on the resulting structure, surface area, nanostructure design and size, and potential applications. The structure of the synthesized hybrids was analyzed using FT-IR, XRD, BET-Analysis, SEM, and AFM. We demonstrate the immovilization of whole living thermophilic bacterial cells with cyanocompound degradation activity in the synthesized silica hybrid biomaterials by entrapment, chemical binding, and adsorption.     

Kinetic Control over Morphology of Nanoporous Graphene on Surface

On-surface synthesis of high-quality nanoporous graphene (NPG) for application in nanotechnology and nanodevices remains challenging. Rational design of molecular precursors and proper kinetic control over the reaction process are the two key factors to tune the synthesis. Herein, we report a detailed study of the coupling reactions of a planar halogen-substituted nanographene molecular precursor, hexaiodo-peri-hexabenzocoronene (I₆-HBC), on the Au(111) surface in the synthesis of NPG. The influence of three basic kinetic processes – molecular adsorption, migration, and coupling – on the synthesis was investigated. The results show that the HBC molecules deposited at low temperature predominantly desorb from the Au(111) surface during the annealing process, whereas depositing the precursor molecules onto a hot surface (700 K) can lead to the formation of NPG. However, at such a high surface temperature, simultaneous intermolecular dehydrogenative coupling between HBC monomers can hinder the ordered growth of NPG through Ullmann coupling. Moreover, the deposition rate of the precursors greatly influences the growth morphology of the NPG nanostructures.     

Ordered Hybrids from Template‐Free Organosilane Self‐Assembly

Despite considerable achievements over the last two decades, nonporous organic–inorganic hybrid materials are mostly amorphous, especially in the absence of solvothermal processes. The organosilane self‐assembly approach is one of the few opportunities for creating a regular assembly of organic and inorganic moieties. Additionally, well‐established organosilicon chemistry enables the introduction of numerous organic functionalities. The synthesis of periodically ordered hybrids relies on mono‐, bis‐, or multisilylated organosilane building blocks self‐assembling into hybrid mesostructures or superstructures, subsequently cross‐linked by siloxane Si‐O‐Si condensation. The general synthesis procedure is template‐free and one‐step. However, three concurrent processes underlie the generation of self‐organized hybrid networks: thermodynamics of amphiphilic aggregation, dynamic self‐assembly, and kinetically controlled sol–gel chemistry. Hence, the set of experimental conditions and the precursor structure are of paramount importance in achieving long‐range order. Since the first developments in the mid‐1990s, the subject has seen considerable progress leading to many innovative advanced nanomaterials providing promising applications in membranes, pollutant remediation, catalysis, conductive coatings, and optoelectronics. This work reviews, comprehensively, the primary evolution of this expanding field of research.     

New biosurfactants based on germylated fatty compounds

A simple and efficient method for producing amphiphilic germylated compounds has been developed by hydrogermylation of fatty acid methyl esters. The germylated precursors afford a micelle‐type assembly in various solvents as evident by the formation of nano‐objects, micrometer spheres and lamellar forms. This offers a promising route to synthetically useful monomers for structured materials. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of poly-L-glutamic acid grafted silica nanoparticles and their assembly into macroporous structures

Polypeptide-coated silica nanoparticles represent an interesting class of organic-inorganic hybrids since the ordered secondary structure of the polypeptide grafts imparts functional properties to these nanoparticles. The synthesis of a poly-l-glutamic acid (PLGA) silica nanoparticle hybrid by employing N-carboxyanhydride (NCA) polymerization to synthesize the polypeptide chains and Cu catalyzed azide alkyne cycloaddition reaction to graft these chains onto the silica surface is reported. This methodology enables the synthesis of well-defined polypeptide chains that are attached onto the silica surface at high surface densities. The PLGA-silica conjugate particles are well dispersed in water, and have been thoroughly characterized using multinuclear ((13)C, (29)Si) solid state NMR, thermogravimetric analysis, Fourier transform infrared, dynamic light scattering, and transmission electron microscopy. The pH-dependent reversible aggregation of the PLGA-silica particles, driven by the change in PLGA structure, has also been studied. Preliminary results on the use of aqueous dispersions of silica-PLGA for the preparation of three-dimensional macroporous structures with oriented pores by ice templating methodology are also demonstrated. These macroporous materials, comprising a biocompatible polymer shell covalently attached to rigid inorganic cores, adopts an interesting lamellar structure with fishbone-type architecture.     

Supramolecular architectures assembled from amphiphilic hybrid polyoxometalates

Polyoxometalate (POM)-based inorganic-organic molecular hybrid clusters have been recently recognized as good candidates to design novel multi-functional materials. Tremendous efforts have been invested in synthesizing many interesting hybrid structures with exceptional chemical and physical properties. Grafting organic ligands to the POM clusters render these functional clusters amphiphilic properties. Here we summarize the current progresses and provide some perspectives, from colloidal chemists' point of view, on the self-assembly of the amphiphilic POM-organic hybrids in solution and at interfaces, as well as the related consequent novel features such as enhanced fluorescent properties.     

High-Yield Synthesis of Gold Microplates Using Amphiphilic Block Copolymers: Are Lyotropic Liquid Crystals Required?

Summary : High-yield synthesis of gold microplates is achieved through autoreduction of hydrogen tetrachloroaureate (III) hydrate (HAuCl₄ · 3H₂O) in aqueous solutions of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (Pluronic L64, EO₁₃PO₃₀EO₁₃) at ambient conditions, in the absence of added energy, reductant, or other surfactants. The formation by the amphiphilic block copolymer of lyotropic liquid crystals (e.g., ordered cylindrical/hexagonal or lamellar phases) is not required for templating the formation of such microplates.