Self-assembly of oppositely charged polyelectrolyte block copolymers containing short thermoresponsive blocks.

The assembly of oppositely charged block copolymers, containing small thermoresponsive moieties, was investigated as a function of salt concentration and temperature. Aqueous solutions of poly-[N-isopropylacrylamide]-b-poly[dimethylaminoethyl methacrylate] (NIPAM44-b-DMAEMA216) and PNIPAM-b-poly[acrylic acid]-b-PNIPAM (NIPAM35-b-AA200-b-NIPAM35) were mixed in equal charge stoichiometry, and analysed by light scattering (LS), NMR spectroscopy and small angle X-ray scattering (SAXS). At room temperature, two different micelle morphologies were found at different salt concentrations. At NaCl concentrations below 0.75 M, complex coacervate core micelles (C3M) with a PNIPAM corona were formed as a result of interpolyelectrolyte complexation. At NaCl concentrations exceeding 0.75 M, the C3M micelles inverted into PNIPAM cored micelles (PCM), containing a water soluble polyelectrolyte corona. This behavior is ascribed to the salt concentration dependence of both the lower critical solution temperature (LCST) of PNIPAM, and the complex coacervation. Above 0.75 M NaCl, the PNIPAM blocks are insoluble in water at room temperature, while complexation between the polyelectrolytes is prevented because of charge screening by the salt. Upon increasing the temperature, both types of micelles display a cloud point temperature (T cp), despite the small thermoresponsive blocks, and aggregate into hydrogels. These hydrogels consist of a complexed polyelectrolyte matrix with microphase separated PNIPAM domains. Controlling the morphology and aggregation of temperature sensitive polyelectrolytes can be an important tool for drug delivery systems, or the application and hardening of underwater glues.

Dendrimer-encapsulated nanoparticle-core micelles as a modular strategy for particle-in-a-box-in-a-box nanostructures.

The hierarchically controlled synthesis and characterization of self-assembling macromolecules and particles are key to explore and exploit new nanomaterials. Here we present a versatile strategy for constructing particle-in-a-box-in-a-box systems by assembling dendrimer-encapsulated gold nanoparticles (DENs) into dendrimicelles. This is realized by combining positively charged PAMAM dendrimers with a negative-neutral block copolymer. The number of particles per dendrimicelle can be controlled by mixing DENs with empty PAMAM dendrimers. The dendrimicelles are stable in solution for months and provide improved resistance for the nanoparticles against degradation. The dendrimicelle strategy provides a flexible platform with a plethora of options for variation in the type of nanoparticles, dendrimers and block copolymers used, and hence is tunable for applications ranging from nanomedicine to catalysis.

Towards 4th generation biomaterials: a covalent hybrid polymer-ormoglass architecture.

Hybrid materials are being extensively investigated with the aim of mimicking the ECM microenvironment to develop effective solutions for bone tissue engineering. However, the common drawbacks of a hybrid material are the lack of interactions between the scaffold's constituents and the masking of its bioactive phase. Conventional hybrids often degrade in a non-homogeneous manner and the biological response is far from optimal. We have developed a novel material with strong interactions between constituents. The bioactive phase is directly exposed on its surface mimicking the structure of the ECM of bone. Here, polylactic acid electrospun fibers have been successfully and reproducibly coated with a bioactive organically modified glass (ormoglass, Si-Ca-P2 system) covalently. In comparison with the pure polymeric mats, the fibers obtained showed improved hydrophilicity and mechanical properties, bioactive ion release, exhibited a nanoroughness and enabled good cell adhesion and spreading after just one day of culture (rMSCs and rEPCs). The fibers were coated with different ormoglass compositions to tailor their surface properties (roughness, stiffness, and morphology) by modifying the experimental parameters. Knowing that cells modulate their behavior according to the exposed physical and chemical signals, the development of this instructive material is a valuable advance in the design of functional regenerative biomaterials.

Tunable doping of a metal with molecular spins.

The mutual interaction of localized magnetic moments and their interplay with itinerant conduction electrons in a solid are central to many phenomena in condensed-matter physics, including magnetic ordering and related many-body phenomena such as the Kondo effect, the Ruderman-Kittel-Kasuya-Yoshida interaction and carrier-induced ferromagnetism in diluted magnetic semiconductors. The strength and relative importance of these spin phenomena are determined by the magnitude and sign of the exchange interaction between the localized magnetic moments and also by the mean distance between them. Detailed studies of such systems require the ability to tune the mean distance between the localized magnetic moments, which is equivalent to being able to control the concentration of magnetic impurities in the host material. Here, we present a method for doping a gold film with localized magnetic moments that involves depositing a monolayer of a metal terpyridine complex onto the film. The metal ions in the complexes can be cobalt or zinc, and the concentration of magnetic impurities in the gold film can be controlled by varying the relative amounts of cobalt complexes (which carry a spin) and zinc complexes (which have zero spin). Kondo and weak localization measurements demonstrate that the magnetic impurity concentration can be systematically varied up to ∼800 ppm without any sign of inter-impurity interaction. Moreover, we find no evidence for the unwanted clustering that is often produced when using alternative methods.

Synthesis, characterization, and crystal structure of alpha-[Ru(azpy)2(NO3)2] (azpy = 2-(phenylazo)pyridine) and the products of its reactions with guanine derivatives.

The synthesis and characterization of alpha-[Ru(azpy)2(NO3)2], 1, are reported (azpy is 2-(phenylazo)pyridine; alpha indicates the isomer in which the coordinating pairs ONO2, N(py), and N(azo) are cis, trans, and cis, respectively). The solid-state structure of 1 has been determined by X-ray crystallography. Crystal data: orthorhombic a = 15.423(5) A, b = 14.034(5) A, c = 10.970(5) A, V = 2374(2) A3, space group P2(1)2(1)2(1) (No. 19), Z = 4, Dcalc = 1.655 g cm-3. The structure refinement converged at R1 = 0.042 and wR2 = 0.118 for 3615 unique reflections and 337 parameters. The octahedral complex shows monodentate coordination of the two nitrate ligands. The Ru-N(azo) bond distances (2.014(4) and 1.960(4) A), slightly shorter than the Ru-N(py) bonds (2.031(4) and 2.059(4) A), agree well with the pi-back-bonding ability of the azo groups. The binding of the DNA-model bases 9-ethylguanine (9egua) and guanosine (guo) to 1 has been studied and compared with previously obtained results for the binding of model bases to the bis(bipyridyl)ruthenium(II) complex. The ligands 9egua and guo appear to form monofunctional adducts, which have been isolated as alpha-[Ru(azpy)2(9egua)Cl]PF6, 2, alpha-[Ru(azpy)2(9egua)(H2O)]-(PF6)2, 3, alpha-[Ru(azpy)2(guo)(H2O)](PF6)2, 4, and alpha-[Ru(azpy)2(guo)Cl]Cl, 5. The orientations of 9egua and guo in these complexes have been determined in detail with the use of 2D NOESY NMR spectroscopy. In 2 and 5, H8 is directly pointed toward the coordinated Cl, whereas, in 3 and 4, H8 is wedged between the pyridine and phenyl rings. The guanine derivatives in the azpy complexes can have more orientations than found for related cis-[Ru(bpy)2Cl2] species. This fluxionality is considered to be important in the binding of the alpha-bis(2-(phenylazo)pyridine)ruthenium(II) complex to DNA. In complex 1, ruthenium is the chiral center and in the binding to guanosine, two diastereoisomers each of adducts 4 and 5 have been clearly identified by NMR spectroscopy.

Tuning the rotational behavior of lopsided heterocyclic nitrogen ligands (L) in octahedral cis-[Ru(bpy)2(L)2](PF6)2 complexes. A variable-temperature 1H NMR study.

In this paper are presented the syntheses, characterizations, and dynamic solution behaviors of three cis-[Ru(bpy)2(L)2] (bpy = 2,2'-bipyridine) complexes, 1-3, in which L represents the monodentate ligands 1-methylimidazole (MeIm), 1,2-dimethylimidazole (Me2Im), and 1-methylbenzimidazole (MeBim), respectively. Because of their different steric properties, these three monodentate ligands yield complexes that show quite different fluxional behaviors in solution. These behaviors are studied with several 1H NMR techniques at various temperatures between -95 and degrees C. The 1H NMR spectra of 1, which has the smallest monodentate ligand of the three used, indicate the complex to be in fast exchange (i.e., the imidazoles rotate around their Ru-N axes) at all recording temperatures. The sterically more demanding ligands, Me2Im and MeBim, in 2 and 3, respectively, are in fast exchange at 55 degrees C and in slow exchange at low temperatures, showing three different atropisomers: two head-to-tail (HT) isomers and one head-to-head (HH) isomer. The newly synthesized bidentate ligand 1,2-bis-(1-methyl-2-benzimidazolyl)ethane (mdbz) forms the complex cis-[Ru(bpy)2(mdbz)](PF6)2 (4), in which the two benzimidazole moieties are constrained and relatively fixed. The two tethered benzimidazoles in 4 cannot rotate around their Ru-N axes, and therefore 4 is a good model for the main HT isomer of 3.