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. 

Amphiphilic Core-Shell Nanocarriers Based On Hyperbranched Poly(ester amide)-star-PCL: Synthesis, Characterization, and Potential as Efficient Phase Transfer Agent

Amphiphilic biodegradable star-shaped polymer was conveniently prepared by the Sn(Oct)(2)-catalyzed ring opening polymerization of c-caprolactone (CL) with hyperbranched poly(ester amide) (PEA) as a macroinitiator. Various monomer/initiator ratios were employed to vary the length of the PCL arms. H-1 NMR and FTIR characterizations showed the successful synthesis of star polymer with high initiation efficiency. SEC analysis using triple detectors, RI, light scattering, and viscosity confirmed the controlled manner of polymerization and the star architecture.

Core-shell macromolecules with dendritic polyphenylene core and polymer shells

Core-shell macromolecules with dendritic polyphenylene core and polymer shell Zusammenfassung / Abstract Core-shell macromolecular structures have become of great interest in materials science because they gave an opportunity to combine a large variety of chemical and physical properties in the single molecule, by combination of different (in terms of chemistry and physics) cores and shells. The interest in such complex structures was provoked by their potential applications in the coating and painting industry (latexes), as supports for catalysts in polymer industry, or as nano-containers and transporters for genes or drug delivery. The aim of this study was the synthesis, characterization and further application of core-shell macromolecules possessing a hydrophobic stiff core (polyphenylene dendrimers) surrounded with a hydrophilic, soft, covalently bonded polymer shell (poly(ethylene oxide) and its copolymers). The requirements for such complex substances were that they should be well-defined in terms of molecular weight (narrow molecular weight distribution) and in molecular structure. The preparation of core-shell molecules containing dendrimer as a core was possible via two synthetic routs: “grafting-onto” and “grafting-from”. The resulting core-shell macromolecules possessed narrow polydispersity as guaranteed by the excellent structural and functional definition of the dendrimer and the narrow polydispersity of the PEO, PS-b-PEO and PI-b-PEO attached to the dendrimer surface. Additional investigation of the size of the particles indicated a relation between both the length and the number of the polymer chains and the hydrodynamic radius determined by Dynamic Light Scattering and Fluorescent Correlation Spectroscopy. Core-shell nano-particles were applied as metallocene supports in heterogeneous olefin polymerizations. Our results indicate that such catalyst systems, that have a size of at least one order of magnitude smaller than the used by now organic supports, could be very useful as model compounds for investigations on catalyst fragmentation and its influence on the product parameters.

In situ preparation and protein delivery of silicate/alginate composite microspheres with core-shell structure

It is predicted that with increased life expectancy in the developed world, there will be a greater demand for synthetic materials to repair or regenerate lost, injured or diseased bone (Hench & Thompson 2010). There are still few synthetic materials having true bone inductivity, which limits their application for bone regeneration, especially in large-size bone defects. To solve this problem, growth factors, such as bone morphogenetic proteins (BMPs), have been incorporated into synthetic materials in order to stimulate de novo bone formation in the center of large-size bone defects. The greatest obstacle with this approach is that the rapid diffusion of the protein from the carrier material, leading to a precipitous loss of bioactivity; the result is often insufficient local induction or failure of bone regeneration (Wei et al. 2007). It is critical that the protein is loaded in the carrier material in conditions which maintains its bioactivity (van de Manakker et al. 2009). For this reason, the efficient loading and controlled release of a protein from a synthetic material has remained a significant challenge. The use of microspheres as protein/drug carriers has received considerable attention in recent years (Lee et al. 2010; Pareta & Edirisinghe 2006; Wu & Zreiqat 2010). Compared to macroporous block scaffolds, the chief advantage of microspheres is their superior protein-delivery properties and ability to fill bone defects with irregular and complex shapes and sizes. Upon implantation, the microspheres are easily conformed to the irregular implant site, and the interstices between the particles provide space for both tissue and vascular ingrowth, which are important for effective and functional bone regeneration (Hsu et al. 1999). Alginates are natural polysaccharides and their production does not have the implicit risk of contamination with allo or xeno-proteins or viruses (Xie et al. 2010). Because alginate is generally cytocompatible, it has been used extensively in medicine, including cell therapy and tissue engineering applications (Tampieri et al. 2005; Xie et al. 2010; Xu et al. 2007). Calcium cross-linked alginate hydrogel is considered a promising material as a delivery matrix for drugs and proteins, since its gel microspheres form readily in aqueous solutions at room temperature, eliminating the need for harsh organic solvents, thereby maintaining the bioactivity of proteins in the process of loading into the microspheres (Jay & Saltzman 2009; Kikuchi et al. 1999). In addition, calcium cross-linked alginate hydrogel is degradable under physiological conditions (Kibat PG et al. 1990; Park K et al. 1993), which makes alginate stand out as an attractive candidate material for the protein carrier and bone regeneration (Hosoya et al. 2004; Matsuno et al. 2008; Turco et al. 2009). However, the major disadvantages of alginate microspheres is their low loading efficiency and also rapid release of proteins due to the mesh-like networks of the gel (Halder et al. 2005). Previous studies have shown that a core-shell structure in drug/protein carriers can overcome the issues of limited loading efficiencies and rapid release of drug or protein (Chang et al. 2010; Molvinger et al. 2004; Soppimath et al. 2007). We therefore hypothesized that introducing a core-shell structure into the alginate microspheres could solve the shortcomings of the pure alginate. Calcium silicate (CS) has been tested as a biodegradable biomaterial for bone tissue regeneration. CS is capable of inducing bone-like apatite formation in simulated body fluid (SBF) and its apatite-formation rate in SBF is faster than that of Bioglass® and A-W glass-ceramics (De Aza et al. 2000; Siriphannon et al. 2002). Titanium alloys plasma-spray coated with CS have excellent in vivo bioactivity (Xue et al. 2005) and porous CS scaffolds have enhanced in vivo bone formation ability compared to porous β-tricalcium phosphate ceramics (Xu et al. 2008). In light of the many advantages of this material, we decided to prepare CS/alginate composite microspheres by combining a CS shell with an alginate core to improve their protein delivery and mineralization for potential protein delivery and bone repair applications

Core-shell microspheres with surface grafted poly(vinyl alcohol) as drug carriers for the treatment of hepatocellular carcinoma

Core(polyvinyl neodecanoate-ethylene glycol dimethacrylate)-shell(polyvinyl alcohol) (core (P(VND-EGDMA))-shell(PVA)) microspheres were developed by seeded polymerization with the use of conventional free radical and RAFT/MADIX mediated polymerization. Poly(vinyl pivalate) PVPi was grafted onto microspheres prepared via suspension polymerization of vinylneodecanoate and ethylene glycol dimethacrylate. The amount of grafted polymer was found to be independent from the technique used with conventional free radical polymerization and MADIX polymerization resulting into similar shell thicknesses. Both systems—grafting via free radical polymerization or the MADIX process—were found to follow slightly different kinetics. While the free radical polymerization resulted in a weight gain linear with the monomer consumption in solution the growth in the MADIX controlled system experienced a delay. The core-shell microspheres were obtained by hydrolysis of the poly(vinyl pivalate) surface grafted brushes to form poly(vinyl alcohol). During hydrolysis the microspheres lost a significant amount of weight, consistent with the hydrolysis of 40–70% of all VPi units. Drug loading was found to be independent of the shell layer thickness, suggesting that the drug loading is governed by the amount of bulk material. The shell layer does not appear to represent an obstacle to the drug ingress. Cell testing using colorectal cancer cell lines HT 29 confirm the biocompatibility of the empty microspheres whereas the clofazimine loaded particles lead to 50% cell death, confirming the release of the drug.

Enhancing photoactivity of TiO2(B)/anatase core-shell nanofibers by selectively doping cerium ions into the TiO2(B) Core

Cerium ions (Ce3+) can beselectively doped into the TiO2(B) core of TiO2(B)/anatase core–shell nanofibers by means of a simple one-pot hydrothermal treatment of a starting material of hydrogen trititanate (H2Ti3O7) nanofibers. These Ce3+ ions (≈0.202 nm) are located on the (110) lattice planes of the TiO2(B) core in tunnels (width≈0.297 nm). The introduction of Ce3+ ions reduces the defects of the TiO2(B) core by inhibiting the faster growth of (110) lattice planes. More importantly, the redox potential of the Ce3+/Ce4+ couple (E0(Ce3+/Ce4+)=1.715 V versus the normal hydrogen electrode) is more negative than the valence band of TiO2(B). Therefore, once the Ce3+-doped nanofibers are irradiated by UV light, the doped Ce3+ ions in close vicinity to the interface between the TiO2(B) core and anatase nanoshell can efficiently trap the photogenerated holes. This facilitates the migration of holes from the anatase shell and leaves more photogenerated electrons in the anatase nanoshell, which results in a highly efficient separation of photogenerated charges in the anatase nanoshell. Hence, this enhanced charge-separation mechanism accelerates dye degradation and alcohol oxidation processes. The one-pot treatment doping strategy is also used to selectively dope other metal ions with variable oxidation states such as Co2+/3+ and Cu+/2+ ions. The doping substantially improves the photocatalytic activity of the mixed-phase nanofibers. In contrast, the doping of ions with an invariable oxidation state, such as Zn2+, Ca2+, or Mg2+, does not enhance the photoactivity of the mixed-phase nanofibers as the ions could not trap the photogenerated holes.

Physisorption-induced electron scattering on the surface of carbon-metal core-shell nanowire arrays for hydrogen sensing

Palladium is sputtered on multi-walled carbon nanotube forests to form carbon-metal core-shell nanowire arrays. These hybrid nanostructures exhibited resistive responses when exposed to hydrogen with an excellent baseline recovery at room temperature. The magnitude of the response is shown to be tuneable by an applied voltage. Unlike the charge-transfer mechanism commonly attributed to Pd nanoparticle-decorated carbon nanotubes, this demonstrates that the hydrogen response mechanism of the multi-walled carbon nanotube-Pd core-shell nanostructure is due to the increase in electron scattering induced by physisorption of hydrogen. These hybrid core-shell nanostructures are promising for gas detection in hydrogen storage applications.

Control of core-shell structure and elemental composition of binary quantum dots

The possibility of initial stage control of the elemental composition and core/shell structure of binary SiC quantum dots by optimizing temporal variation of Si and C incoming fluxes and surface temperatures is shown via hybrid numerical simulations. Higher temperatures and influxes encourage the formation of a stoichiometric outer shell over a small carbon-enriched core, whereas lower temperatures result in a larger carbon-enriched core, Si-enriched undershell, and then a stoichiometric SiC outer shell. This approach is generic and is applicable to a broad range of semiconductor materials and nanofabrication techniques. © 2007 American Institute of Physics.

Structural and optical investigation of semiconductor CdSe/CdS core-shell quantum dot thin films

Highly luminescent CdSe/CdS core-shell nanocrystals have been assembled on indium tin oxide (ITO) coated glass substrates using a wet synthesis route. The physical properties of the quantum dots (QD) have been investigated using X-ray diffraction, transmission electron microscopy and optical absorption spectroscopy techniques. These quantum dots showed a strong enhancement in the near band edge absorption. The in situ luminescence behavior has been interpreted in the light of the quantum confinement effect and induced strain in the core-shell structure.

Synthesis of Cu@ZnO Core−Shell Nanocomposite through Digestive Ripening of Cu and Zn Nanoparticles

The synthesis of colloids of copper and zinc nanoparticles by solvated metal atom dispersion (SMAD) is described. The as-prepared colloids with a large size distribution of the particles are transformed into colloidal nanoparticles of a narrow size distribution by the digestive ripening process which involves refluxing the colloid at or near the boiling point of the solvent in the presence of a passivating ligand. The copper nanoparticles of 2.1 ± 0.3 nm and zinc nanoparticles of 3.9 ± 0.3 nm diameters have thus been obtained. Digestive ripening of the as-prepared copper and zinc colloids together in the presence of a passivating agent gave Cu@ZnO core−shell nanoparticles, with an average diameter of 3.0 ± 0.7 nm. Particles synthesized in this manner were characterized by UV−visible spectroscopy, high-resolution electron microscopy, energy-filtered electron microscopy, and powder X-ray diffraction methods which confirm the core−shell structure.

Biphenyl ethers conjugated CdSe/ZnS core/shell quantum dots and interpretation of the mechanism of amyloid fibril disruption

The biphenyl ethers (BPEs) are the potent inhibitors of TTR fibril formation and are efficient fibril disrupter. However, the mechanism by which the fibril disruption occurs is yet to be fully elucidated. To gain insight into the mechanism, we synthesized and used a new QD labeled BPE to track the process of fibril disruption. Our studies showed that the new BPE-QDs bind to the fiber uniformly and has affinity and specificity for TTR fiber and disrupted the pre-formed fiber at a relatively slow rate. Based on these studies we put forth the probable mechanism of fiber disruption by BPEs. Also, we show here that the BPE-QDs interact with high affinity to the amyloids of A beta(42), lysozyme and insulin. The potential of BPE-QDs in the detection of senile plaque in the brain of transgenic Alzheimer's mice has also been explored. (C) 2010 Elsevier Ltd. All rights reserved.

Multiferroic and magnetoelectric properties of core-shell CoFe2O4@BaTiO3 nanocomposites

Core-shell CoFe2O4@BaTiO3 nanoparticles and nanotubes have been prepared using a combination of solution processing and high temperature calcination. Both the core-shell nanostructures exhibit magnetic and dielectric hysteresis at room temperature and magnetoelectric effect. The dielectric constant of both the nanocomposites decreases upon application of magnetic field. The core-shell nanoparticles exhibit 1.7% change in magnetocapacitance around 134 K at 1 T, while the core-shell nanotubes show a remarkable 4.5% change in magnetocapacitance around 310 K at 2 T.(C) 2010 American Institute of Physics. [doi:10.1063/1.3478231].

Synthesis and characterization of Pd(0), PdS, and Pd@PdO core-shell nanoparticles by solventless thermolysis of a Pd-thiolate cluster

Colloids of palladium nanoparticles have been prepared by the solvated metal atom dispersion (SMAD) method. The as-prepared Pd colloid consists of particles with an average diameter of 2.8 +/- 0.1 nm. Digestive ripening of the as-prepared Pd colloid, a process involving refluxing the as-prepared colloid at or near the boiling point of the solvent in the presence of a passivating agent, dodecanethiol resulted in a previously reported Pd-thiolate cluster, Pd(SC12H25)(2)](6) but did not render the expected narrowing down of the particle size distribution. Solventless thermolysis of the Pd-thiolate complex resulted in various Pd systems such as Pd(0), PdS, and Pd@PdO core-shell nanoparticles thus demonstrating its versatility. These I'd nanostructures have been characterized using high-resolution electron microscopy and powder X-ray diffraction methods. (C) 2010 Elsevier Inc. All rights reserved.

Ag@Pd core-shell nanoparticles

Colloids of silver and palladium nanoparticles have been prepared by the Solvated Metal Atom Dispersion method. The as-prepared Ag colloid consisting of polydisperse nanoparticles is transformed into a monodisperse colloid by the digestive ripening process which involves refluxing the as-prepared colloid in the presence of a surfactant. In addition to the monodisperse nanoparticles, a small amount of an Ag-thiolate complex is also formed. Refluxing a mixture of the as-prepared Ag and Pd colloids results in Ag@Pd core-shell nanoparticles. The core-shell structure has been established using a combination of techniques such as UV-visible spectroscopy, high resolution electron microscopy, energy filtered electron microscopy, energy dispersive X-ray analysis, high angle annular dark field imaging and powder X-ray diffraction.