Self-assembly using dendritic building blocks - towards controllable nanomaterials

Dendritic molecules have well defined, three-dimensional branched architectures, and constitute a unique nanoscale toolkit. This review focuses on examples in which individual dendritic molecules are assembled into more complex arrays via non-covalent interactions. In particular, it illustrates how the structural information programmed into the dendritic architecture controls the assembly process, and as a consequence, the properties of the supramolecular structures which are generated. Furthermore, the review emphasises how the use of non-covalent (supramolecular) interactions, provides the assembly process with reversibility, and hence a high degree of control. The review also illustrates how self-assembly offers an ideal approach for amplifying the branching of small, synthetically accessible, relatively inexpensive dendritic systems (e.g. dendrons), into highly branched complex nanoscale assemblies.

The review begins by considering the assembly of dendritic molecules to generate discrete, well-defined supramolecular assemblies. The variety of possible assembled structures is illustrated, and the ability of an assembled structure to encapsulate a templating unit is described. The ability of both organic and inorganic building blocks to direct the assembly process is discussed. The review then describes larger discrete assemblies of dendritic molecules, which do not exist as a single well-defined species, but instead exist as statistical distributions. For example, assembly around nanoparticles, the assembly of amphiphilic dendrons and the assembly of dendritic systems in the presence of DNA will all be discussed. Finally, the review examines dendritic molecules, which assemble or order themselves into extended arrays. Such systems extend beyond the nanoscale into the microscale or even the macroscale domain, exhibiting a wide range of different architectures. The ability of these assemblies to act as gel-phase or liquid crystalline materials will be considered.

Taken as a whole, this review emphasises the control and tunability that underpins the assembly of nanomaterials using dendritic building blocks, and furthermore highlights the potential future applications of these assemblies at the interfaces between chemistry, biology and materials science. 

Exploring molecular recognition pathways within a family of gelators with different hydrogen bonding motifs

We report the synthesis of a family of gelators in which alkyl chains are connected to the amino groups of L-lysine methyl ester using a range of different hydrogen bonding linking groups (carbamate, amide, urea, thiourea and diacylhydrazine) using simple synthetic methodology based on isocyanate or acid chloride chemistry. The ability of these compounds to gelate organic solvents such as toluene or cyclohexane can be directly related to the ability of the linking group to form intermolecular hydrogen bonds. In general terms, the ability to structure solvents can be considered as: thiourea <carbamate <amide <urea similar to diacylhydrazine. This process has been confirmed by thermal measurements, scanning electron microscopy (SEM) and infrared and circular dichroism spectroscopies. By deprotecting the methyl ester group, we have demonstrated that a balance between hydrophobic and hydrophilic groups is essential-if the system has too much hydrophilicity (e. g., diacylhydrazine, urea) it will not form gels due to low solubility in the organic media. However, the less effective gelators based on amide and carbamate linkages are enhanced by converting the methyl ester to a carboxylic acid. Furthermore, subsequent mixing of the acid with a second component (diaminododecane) further enhances the ability to form networks, and, in the case of the amide, generates a two-component gel, which can immobilise a wide range of solvents of industrial interest including petrol and diesel (fuel oils), olive oil and sunflower oil (renewable food oils) and ethyl laurate, isopropyl myristate and isopropyl palmitate (oils used in pharmaceutical formulation). The gels are all thermoreversible, and may therefore be useful in controlled release/formulation applications.

Dendritic receptors designed to bind polyanions in both organic and aqueous media

This paper reports the synthesis of dendrons containing a spermine unit at their focal point. The dendritic branching is based on L-lysine building blocks, and has terminal oligo(ethyleneglycol) units on the surface. As a consequence of the solubilising surface groups, these dendrons have high solubility in solvents with widely different polarities (e.g., dichloromethane and water). The protonated spermine unit at the focal point is an effective anion binding fragment and, as such, these dendrons are able to bind to polyanions. This paper demonstrates that polyanions can be bound in both dichloromethane (using a dye solubilisation assay) and in water (competitive ATP binding assay). In organic media the dendritic branching appears to have a pro-active effect on the solubilisation of the dye, with more dye being solubilised by higher generations of dendron. On the other hand, in water the degree of branching has no impact on the anion binding process. We propose that in this case, the spermine unit is effectively solvated by the bulk solvent and the dendritic branching does not need to play an active role in assisting solubility. Dendritic effects on anion binding have therefore been elucidated in different solvents. The dendritic branching plays a pro-active role in providing the anion binding unit with good solubility in apolar solvent media.

Studies on self-assembly hydrothermal fabrication and thermal stability of Chromium oxyhydroxide nanomaterials synthesised from Chromium oxide colloids

Chromium oxyhydroxide nanomaterials with narrow size-distribution were synthesised through a simple hydrothermal method. Experimental conditions, such as reaction duration and pH values of the precipitation process and hydrothermal treatment played important roles in determining the nature of the final product chromium oxyhydroxide nanomaterials. The effect of these synthesis parameters were studied with the assistance of X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and thermogravimetric analyses. This research has developed a controllable synthesis of Chromium oxyhydroxide nanomaterials from Chromium oxide colloids.

Charge-controlled switchable CO2 capture on boron nitride nanomaterials

Increasing concerns about the atmospheric CO2 concentration and its impact on the environment are motivating researchers to discover new materials and technologies for efficient CO2 capture and conversion. Here, we report a study of the adsorption of CO2, CH4, and H2 on boron nitride (BN) nanosheets and nanotubes (NTs) with different charge states. The results show that the process of CO2 capture/release can be simply controlled by switching on/off the charges carried by BN nanomaterials. CO2 molecules form weak interactions with uncharged BN nanomaterials and are weakly adsorbed. When extra electrons are introduced to these nanomaterials (i.e., when they are negatively charged), CO2 molecules become tightly bound and strongly adsorbed. Once the electrons are removed, CO2 molecules spontaneously desorb from BN absorbents. In addition, these negatively charged BN nanosorbents show high selectivity for separating CO2 from its mixtures with CH4 and/or H2. Our study demonstrates that BN nanomaterials are excellent absorbents for controllable, highly selective, and reversible capture and release of CO2. In addition, the charge density applied in this study is of the order of 1013 cm–2 of BN nanomaterials and can be easily realized experimentally.

Magnet-assisted assembly of 1-dimensional hollow PtCo nanomaterials on an electrode surface

In this work, rapid and controllable confinement of one-dimensional (1D) hollow PtCo nanomaterials on an indium tin oxide (ITO) electrode surface was simply realized via magnetic attraction. The successful assembly was verified by scanning electron microscopy (SEM) and cyclic voltammetry, which showed that a longer exposure time of the electrode to the suspension of these 1D hollow nanomaterials (magnetic suspension) led to a larger amount of attached 1D hollow PtCo nanomaterials.

Ag2S morphology controllable via simple template-free solution route

Different morphologies of Ag2S nano- and micro-materials, including spokewise micrometer bars, microfibers, nanowires, worm-like nanoparticles and nanopolyhedrons have been obtained controllably by a facile one-step method at room temperature. Powder X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS) and scanning electron microscope (SEM) were employed to characterize the structure and compositions of those nanomaterials. Furthermore, ultraviolet visible (UV-vis) spectra of Ag2S with different morphologies show different spectral features.

Morphology-controllable synthesis and characterization of single-crystal molybdenum trioxide

Molybdenum trioxide nanobelts and prism-like particles with good crystallinity and high surface areas have been prepared by a facile hydrothermal method, and the morphology could be controlled by using different inorganic salts, such as KNO3, Ca(NO3)(2), La(NO3)(3), etc. The possible growth mechanism of molybdenum trioxide prism-like particles is discussed on the basis of the presence of HI and the modification of metal cations. The as-prepared nanomaterials are characterized by means of powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and ultraviolet and visible spectroscopy (UV-vis). TEM and HRTEM micrographs show that the molybdenum trioxide nanobelts and prism-like particles have a relatively high degree of crystallinity and uniformity. BET specific surface areas of the as-prepared molybdenum trioxide nanocrystals are 67-79 m(2)g(-1). XPS analysis indicates that the hexavalent molybdenum is predominant in the nanocrystals. UV-vis spectra reveal that the direct band gap energy of the annealed molybdenum trioxide prism-like particles shows a pronounced blue shift compared to that of bulk MoO3 powder.

Functionalized surfaces and surface functionalization of nanomaterials

Nature leads, we follow. But nanotechnologists are in hot pursuit, in designing controllable structures that can mimic naturally occurring and artificially synthesized materials on a common platform. The supramolecular chemistry concerns the investigation of nature principles to produce fascinating complexed and functional molecular assemblies, as well as the utilization of these principles to generate novel devices and materials, potentially useful for sensing, catalysis, transport and other applications in medical or engineering science. The work presented in this thesis is a compilation of different synthetic methods to achieve inorganic-organic hybrid nanomaterials. Silicatein, a protein enzyme, which acts both as a catalyst and template for the formation of silica needles in marine sponges, has been used for the biosynthesis of semiconductor metal oxides on surfaces. Silicatein was immobilized on gold (111) surfaces using alkane thiol, as well as on a novel self-assembly of NTA on top of a “cushion” of reactive ester polymer has been successfully employed to make functionalised surfaces. The immobilization of silicatein on surfaces was monitored by surface plasmon spectroscopy, atomic force microscopy and confocal laser scanning microscopy. Surface bound silicatein retains its biocatalytic activity, which was demonstrated by monitoring its hydrocatalytic activity to catalyse the synthesis of biosilica, biotitania, and biozirconia. The synthesis of semiconductor metal oxides was characterized using scanning electron microscopy. This hydrolytic biocatalyst is used to synthesize the gold nanoparticles. The gold nanoparticles are formed by reduction of tetrachloroaurate, AuCl4-, by the action of sulfhydryl groups hidden below the surface groups of the protein. The resulting gold nanoparticles which are stabilized by surface bound silicatein further aggregate to form Au nanocrystals. The shape of the nanocrystals obtained by using recombinant silicatein is controlled through chiral induction by the protein during the nucleation of the nanocrystals. As an extension of this work, TiO2 nanowires were functionalized using polymeric ligand which incorporates the nitrilotriacetic acid (NTA) linker in the back bone to immobilize His-tagged silicatein onto the TiO2 nanowires. The surface bound protein not only retains its original hydrolytic properties, but also acts as a reductant for AuCl4- in the synthesis of hybrid TiO2/silicatein/Au nanocomposites. Functionalized, monocrystalline rutile TiO2 nanorods were prepared from TiCl4 in aqueous solution in the presence of dopamine. The surface bound organic ligand controls the morphology as well as the crystallinity and the phase selection of TiO2. The surface amine groups can be tailored further with functional molecules such as dyes. As an example, this surface functionality is used for the covalent binding of a fluorescent dye,4-chloro-7- nitrobenzylurazene (NBD) to the TiO2 nanorods. The polymeric ligands have been used successfully for the in-situ and post-functionalization of TiO2 nanoparticles. Besides to chelating dopamine anchor group the multifunctional ligand system presented here incorporates a modifier molecule which allows the binding of functional molecules (here the dyes pyrene, NBD, and Texas Red) as well as additional entities which allow tailoring the solubility of inorganic nanocrystals in different solvents. A novel method for the surface functionalization of fullerene-type MoS2 nanoparticles and subsequently binding these nanoparticles onto TiO2 nanowires has been reported using polymeric ligands. The procedure involves the complexation of IF-MoS2 with a combination of Ni2+ via an umbrella-type nitrilotriacetic acid (NTA) and anchoring them to the sidewalls of TiO2 nanowires utilizing the hydroxyl groups of dopamine present in the main contents of polymeric ligand. A convenient method for the synthesis of Au/CdS nanocomposites has been presented, which were achieved through the novel method of thiol functionalization of gold colloids. The thermodynamically most stable phase of ZrO2 (cubic) has been obtained at much lower temperature (180°C). These nanoparticles are highly blue fluorescent, with a high surface area.

Bioactive mesopore-glass microspheres with controllable protein-delivery properties by biomimetic surface modification

Microsphere systems with the ideal properties for bone regeneration need to be bioactive, and at the same time possess the capacity for controlled protein/drug-delivery; however, the current crop of microsphere system fails to fulfill these properties. The aim of this study was to develop a novel protein-delivery system of bioactive mesoporous glass (MBG) microspheres by a biomimetic method through controlling the density of apatite on the surface of microspheres, for potential bone tissue regeneration. MBG microspheres were prepared by using the method of alginate cross-linking with Ca2+ ions. The cellular bioactivity of MBG microspheres was evaluated by investigating the proliferation and attachment of bone marrow stromal cell (BMSC). The loading efficiency and release kinetics of bovine serum albumin (BSA) on MBG microspheres were investigated after coprecipitating with biomimetic apatite in simulated body fluids (SBF). The results showed that MBG microspheres supported BMSC attachment and the Si containing ionic products from MBG microspheres stimulated BMSCs proliferation. The density of apatite on MBG microspheres increased with the length of soaking time in SBF. BSA-loading efficiency of MBG was significantly enhanced by co-precipitating with apatite. Furthermore, the loading efficiency and release kinetics of BSA could be controlled by controlling the density of apatite formed on MBG microspheres. Our results suggest that MBG microspheres are a promising protein-delivery system as a filling material for bone defect healing and regeneration.

Bioactive inorganic materials/alginate composite microspheres with controllable drug-delivery ability

Alginate microspheres are considered a promising material as a drug carrier in bone repair due to excellent biocompatibility, but their main disadvantage is low drug entrapment efficiency and non-controllable release. The aim of this study was to investigate the effect of incorporating mesoporous bioglass (MBG), non-mesoporous bioglass (BG) or hydroxyapatite (HAp) into alginate microspheres on their drug-loading and release properties. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and atomic emission spectroscopy (AES) were used to analyse the composition, structure and dissolution of bioactive inorganic materials and their microspheres. Dexamethasone (DEX)-loading and release ability of four microspheres were tested in phosphate buffered saline with varying pHs. Results showed that the drug-loading capacity was enhanced with the incorporation of bioactive inorganic materials into alginate microspheres. The MBG/Alginate microspheres had the highest drug loading ability. DEX release from alginate microspheres correlated to the dissolution of MBG, BG and HAp in PBS, and that the pH was an efficient factor in controlling the DEX release; a high pH resulted in greater DEX release, whereas a low pH delayed DEX release. In addition, MBG/alginate, BG/alginate and HAp/alginate microspheres had varying apatite-formation and dissolution abilities, which indicate that the composites would behave differently with respect to bioactivity. The study suggests that microspheres made of a composite of bioactive inorganic materials and alginate have a bioactivity and degradation profile which greatly improves their drug delivery capacity, thus enhancing their potential applications as bioactive filler materials for bone tissue regeneration.

Transition of chromium oxyhydroxide nanomaterials to chromium oxide : a hot stage Raman spectroscopic study

The transition of disc-like chromium hydroxide nanomaterials to chromium oxide nanomaterials has been studied by hot stage Raman spectroscopy. The structure and morphology of α-CrO(OH) synthesised using hydrothermal treatment was confirmed by X-ray diffraction and transmission electron microscopy. The Raman spectrum of α-CrO(OH) is characterised by two intense bands at 823 and 630 cm-1 attributed to ν1 CrIII-O symmetric stretching mode, bands at 1179 cm-1 attributed to CrIII-OH δ deformation modes. No bands are observed above 3000 cm-1. The absence of characteristic OH vibrational bands may be due to short hydrogen bonds in the α-CrO(OH) structure. Upon thermal treatment of α-CrO(OH), new Raman bands are observed at 599, 542, 513, 396, 344 and 304 cm-1, which are attributed to Cr2O3. This hot-stage Raman study shows that the transition of α-CrO(OH) to Cr2O3 occurs before 350 °C.