Robust core-shell supramolecular assemblies based on cationic vesicles and ring-shaped {Mo-154} polyoxomolybdate nanoclusters: Template-directed synthesis and characterizations

Substantial progress has been made recently in extending the supramolecular assembly of biomimetic structures to vesicle-based sophisticated nanocomposites and mesostructures. We report herein the successful preparation of unilamellar surfactant vesicles coated with a monolayer of ring-shaped {Mo-154} polyoxometalate (POM) nanoclusters, (NH4)(28)[Mo-154 (NO)(14)O(448)Hi(4)(H2O)(70)].approximate to 350H(2)O, by coulomb attractions using preformed didodecyldimethylammonium bromide (DDAB) surfactant vesicles as templates. The resultant vesicle-templated supramolecular assemblies are robust (they do not disintegrate upon dehydration) both at room-temperature ambient and vacuum conditions, as characterized by conventional transmission electron microscopy (TEM) and atomic force microscopy (AFM). The flexibility of the complex soft assemblies was also revealed by AFM measurements. The effect of POM-vesicle coulomb attractions on the dimensions of the templating vesicles was also investigated by using dynamic light scattering (DLS).Although origins of the structure stability of the as-prepared supramolecular assemblies are not clear yet, the nanometer scale cavities and the related properties of macroions of the POM clusters may play an important role in it.

Probing the micellization of diblock and triblock copolymers of poly(L-lactide) and poly(ethylene glycol) in aqueous and NaCl salt solutions

The micelle formation of a series of amphiphilic block copolymers in aqueous and NaCl solutions was studied by a fluorescent probe technique using pyrene as a 'model drug'. These copolymers were synthesized from poly (ethylene glycol) (PEG) and L-lactide by a new calcium ammoniate catalyst. They had fixed PEG block lengths (44, 104 or 113 ethylene oxide units) and various poly(L-lactide) (PLLA) block lengths (15-280 lactide units). The critical micelle concentration (cmc) was found to decrease with increasing PLLA content. The distinct dissimilarity of the cmc values of diblock and triblock copolymers based on the same block length of PEG provided evidence for the different configurations of their micelles. It was also observed that the introduction of NaCl salt significantly contributed to a decrease in the cmcs of the copolymers with short PEG and PLLA blocks, while it had less influence on the cmcs of copolymers with long PEG or PLLA blocks. The dependence of partition coefficients ranging from 0.2x10(5) to 1.9x10(5) on the PLLA content in the copolymer and on the micelle configuration was also discussed.

Carboxyl-modified single-walled carbon nanotubes selectively induce human telomeric i-motif formation

As the leading nanodevice candidate, single-walled carbon nano-tubes (SWNTs) have potential therapeutic applications in gene therapy and novel drug delivery. We found that SWNTs can inhibit DNA duplex association and selectively induce human telomeric i-motif DNA formation by binding to the 5'-end major groove under physiological conditions or even at pH 8.0. SWNT binding to telomeric DNA was studied by UV melting, NMR, S1 nuclease cleavage, CD, and competitive FRET methods. These results suggest that SWNTs might have the intriguing potential to modulate human telomeric DNA structures in vivo, like biologically relevant B-A and B-Z DNA transitions, which is of great interest for drug design and cancer therapy.

Micellization and reversible pH-sensitive phase transfer of the hyperbranched multiarm PEI-PBLG copolymer

A novel, hyperbranched, amphiphilic multiarm biodegradable polyethylenimine-poly(gamma-benZyl-L-gluta- mate) (PEI-PBLG) copolymer was prepared by the ring-opening polymerization of gamma-benzyl-L-glutamate-N-car-boxyanhydride (BLG-NCA) with hyperbranched PEI as a macroinitiator. The copolymer could self-assemble into core-shell micelles in aqueous solution with highly hydrophobic micelle cores. As the PBLG content was increased, the size of the micelles increased and the critical micelle concentration (CMC) decreased. The surface of the micelles had a positive potential. The cationic micelles were capable of complexing with plasmid DNA (pDNA), which could be released subsequently by treatment with polyanions. The PEI-PBLG copolymer formed unimolecular micelles in chloroform solution. ne pH-sensitive phase-transfer behavior exhibited two critical pH points for triggering the encapsulation and release of guest molecules. Both the encapsulation and release processes were rapid and reversible. Under strong acidic or alkaline conditions, the release process became partially or completely irreversible.

Solvent-induced crystallization of spherical micelles formed by copolymer blends

We have followed the morphological evolution and crystallization process of spherical micelles formed by the mixture of polystyrene-b-poly(acrylic acid) (PS-b-PAA) and polystyrene-b-poly(2-vinylpyridine)b-poly(ethylene oxide) (PS-b-P2VP-b-PEO) (the core of the spherical micelles was made of P2VP and PAA blocks through hydrogen bonding in neutral solvent N,N-dimethylformamide, DMF) via DMF vapor treatment. Different phenomena, such as rupture of the film, formation of cylinder aggregates and regular square lamellae, were observed when the micelle film was treated in DMF for different times. At the early stage of annealing in DMF vapor, the micelle film became unstable and ruptured. Cylinder aggregates, within which the PEO blocks achieved the association and primary chain folding, formed as the mesophases before the nucleation of the PEO single crystals at this stage. Further treatment in DMF vapor resulted in the nucleation of the PEO blocks at the corners of quasi-square lamellae. Then a quite regular "sandwich" lamellar structure, constructed by a PEO single-crystal layer covered by two tethered layers of other amorphous blocks on the top and bottom crystal basal surfaces, formed when the film of micelles was annealed in DMF vapor for sufficient times.

Multiple morphologies and their transformation of a polystyrene-block-poly(4-vinylpyridine) block copolymer

We report the multiple morphologies and their transformation of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) in low-alkanol solvents. In order to improve the solubility of polystyrene block in alcohol solvents, the solution of block copolymer sample was treated at a higher temperature, and then the influence of rate of decreasing temperature on multiple morphologies (including spheres, rods, vesicles, porous vesicles, large compound vesicles, and large compound micelles) was observed. The transformation of spheres to rods, to tyre-shaped large compound micelles, and to sphere-shaped large compound micelles was also realized. The formation mechanisms of the multiple morphologies and their transformation are discussed briefly.

Dibenzyl trithiocarbonate mediated reversible addition-fragmentation chain transfer polymerization of acrylonitrile

Reversible addition-fragmentation chain transfer polymerization has been successfully applied to polymerize acrylonitrile with dibenzyl trithiocarbonate as the chain-transfer agent. The key to success is ascribed to the improvement of the interchange frequency between dormant and active species through the reduction of the activation energy for the fragmentation of the intermediate. The influence of several experimental parameters, such as the molar ratio of the chain-transfer agent to the initiator [azobis(isobutyronitrile)], the molar ratio of the monomer to the chain-transfer agent, and the monomer concentration, on the polymerization kinetics and the molecular weight as well as the polydispersity has been investigated in detail. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and H-1 NMR analyses have confirmed the chain-end functionality of the resultant polymer.

Synthesis and characterization of the biodegradable polycaprolactone-graft-chitosan amphiphilic copolymers

A novel synthetic approach to biodegradable amphiphilic copolymers based on poly (epsilon-caprolactone) (PCL) and chitosan was presented, and the prepared copolymers were used to prepare nanoparticles successfully. The PCL-graft-chitosan copolymers were synthesized by coupling the hydroxyl end-groups on preformed PCL chains and the amino groups present on 6-O-triphenylmethyl chitosan and by removing the protective 6-O-triphenylmethyl groups in acidic aqueous solution. The PCL content in the copolymers can be controlled in the range of 10-90 wt %. The graft copolymers were thoroughly characterized by H-1 NAM, C-13 NMR, FT-IR and DSC. The nanoparticles made from the graft copolymers were investigated by H-1 NMR, DLS, AFM and SEM measurements. It was found that the copolymers could form spherical or elliptic nanoparticles it? water. The amount of available primary amines on the surface of the prepared nanoparticles was evaluated by ninhydrin assail, and it can be controlled by the grafting degree of PCL.

Determination of molecular weight and molecular sizes of polymers by high temperature gel permeation chromatography with a static and dynamic laser light scattering detector

Flow-mode static and dynamic laser light scattering (SLS/DLS) studies of polymers, including polystyrene, polyethylene, polypropylene and poly(dimethylsiloxane) (PDMS), in 1,2,4-trichlorobenzene (TCB) at 150 degreesC were performed on a high temperature gel permeation chromatography (GPC) coupled with a SLS/DLS detector. Both absolute molecular weight (M) and molecular sizes (radius of gyration, R-g and hydrodynamic radius, R-h) of polymers eluting from the GPC columns were obtained simultaneously. The conformation of different polymers in TCB at 150 degreesC were discussed according to the scaling relationships between R-g, R-h and M and the rho-ratio (p = R-g/R-h). Flow-mode DLS results of PDMS were verified by batch-mode DLS study of the same sample. The presented technique was proved to be a convenient and quick method to study the shape and conformation of polymers in solution at high temperature. However, the flow-mode DLS was only applicable for high molecular weight polymers with a higher refractive index increment such as PDMS.

Synthesis and characterization of PCL/PEG/PCL triblock copolymers by using calcium catalyst

Triblock copolymer PCL-PEG-PCL was prepared by ring-opening polymerization of epsilon-caprolactone (CL) in the presence of poly(ethylene glycol) catalyzed by calcium ammoniate at 60 degreesC in xylene solution. The copolymer composition and triblock structure were confirmed by H-1 NMR and C-13 WR measurements. The differential scanning calorimetry and wide-angle X-ray diffraction analyses revealed the micro-domain structure in the copolymer. The melting temperature T-c and crystallization temperature T-c of the PEG domain were influenced by the relative length of the PCL blocks. This was caused by the strong covalent interconnection between the two domains. Aqueous micelles were prepared from the triblock copolymer. The critical micelle concentration was determined to be 0.4-1.2 mg/l by fluorescence technique using pyrene as probe, depending on the length of PCL blocks, and lower than that of corresponding PCL-PEG diblock copolymers. The H-1 NMR spectrum of the micelles in D2O demonstrated only the -CH2CH2O- signal and thus confirmed. the PCL-core/PEG-shell structure of the micelles.

UV-Vis and surface photovoltage spectra of Fe2O3/polystyrene composite microspheres

Fe2O3 sol with the particle diameter of 3-5 nm was flocculated by the addition of SDS, and the flocculate formed was redispersed by the further addition of that surfactant. Thus the surfactant bilayer was formed on the surface of Fe2O3. The emulsion polymerization of styrene (St) adsolubilized oil the surfactant adsorbed bilayer was carried out by initiator potassium persulfate (KPS). The UV-Vis and surface photovoltage spectra (SPS) indicate that the Fe2O3 particles were encapsulated in polystyrene(PSt) successfully.

Influence of capping ligands on the self-organizations of gold nanoparticles into superlattice from CTAB reverse micelles

Gold nanoparticles with size 3-10 nm (diameter) were prepared by the reduction of HAuCl4 in a CTAB/octane + 1-butanol/H2O reverse micelle system using NaBH4 as the reducing agent. The as-formed gold nanoparticle colloid was characterized by UV/vis absorption spectrum and transmission electron microscopy(TEM). Various capping ligands, such as alkylthiols with different chain length and shape, trioctylphosphine (TOP), and pyridine are used to passivate the gold nanoparticles for the purpose of self-organization into superstructures. It is shown that the ligands have a great influence on the self-organization of gold nanoparticles into superlattices, and dodecanethiol C12H25SH is confirmed to be the best ligand for the self-organization. Self-organization of C12H25SH-capped gold nanoparticles into 1D, 2D and 3D superlattices has been observed on the carbon-coated copper grid by TEM without using any selective precipitation process.

Preparation and characterization of PSt-b-PHEMA metal hybrids

The block copolymer polystyrene-b-poly[2-(trimethylsilyloxy)ethylene methacrylate] (PSt-b-PTMSEMA) was synthesized using atom-transfer radical polymerization (ATRP). The hydrolysis of PSt-b-PTMSEMA led to the formation of an amphiphilic block copolymer, polystyrene-b-poly(2-hydroxylethyl methacrylate) (PSt-b-PHEMA), which was characterized by GPC and H-1-NMR. TEM showed that the PSt-b-PHEMA formed a micelle, which is PSt as the core and PHEMA as the shell. Under appropriate conditions, the nickel or cobalt ion cause chemical reactions in these micelles and could be reduced easily. ESCA analysis showed that before reduction the metal existed as a hydroxide; after reduction, the metal existed as an oxide, and the metal content of these materials on the surface is more than that on the surface of the copolymer metal ion. XRD analysis showed that the metal existed as a hydroxide before reduction and existed as a metal after reduction.

Study on polymer micelles of hydrophobically modified ethyl hydroxyethyl cellulose using single-molecule force spectroscopy

Individual hydrophobically modified ethyl hydroxyethyl cellulose (HM-EHEC) molecules under different conditions were elongated using a new atomic force microscope (AFM) based technique-single-molecule force spectroscopy (SMFS). The critical concentration of HM-EHEC for micelle-like clusters at a solid/liquid interface was around 0.8 wt %, which is lower than that in solution. The different mechanical properties of HM-EHEC below and above the critical concentration were displayed on force-extension curves. Through a comparison with unmodified hydroxyethyl cellulose, substituent-induced effects on nanomechanical features of HM-EHEC were investigated. Because of hydrophobic interactions and cooperative binding with the polymer, surfactants such as sodium dodecyl sulfate (SDS) dramatically influence the elastic properties of HM-EHEC below the critical concentration, and further addition of SDS reduces the interactions between the hydrophobic groups and the surfactant.

Formation of ordered arrays of gold nanoparticles from CTAB reverse micelles

In this presentation, a reverse micelle technique was described to create colloid gold nanoparticles and their self-organization into superlattices. Gold nanoparticles were prepared by the reduction of HAuCL4 in CTAB/octane + 1-butanol/H2O reverse micelle system using NaBH4 as reducing agent. Dodecanethiol (C12H25SH) was used to passivate the gold nanoparticles immediately after formation of the gold colloid. After re-dispersing in toluene under ultrasonication, a supernatant containing nearly monodispersed dodecanethiol-capped gold nanoparticles was obtained. Self-organization of the gold nanoparticles into 1D, 2D and 3D superlattices was observed on the carbon-coated copper grid by TEM. UV-vis absorption spectra were also used to characterize the gold colloids with and without dodecanethiol capping. (C) 2001 Elsevier Science B.V. All rights reserved.