more selected papers
Chameleon nanofibers by green Electrospinning.
E. Giebel, C. Mattheis, S. Agarwal, A. Greiner, Adv. Funct. Mater. 2013, 23, 3156-3163.
Electrospun ionic nonwovens are obtained by green electrospinning of aqueous dispersions. The resulting nonwovens are termed as chameleon nonwovens since their surface properties can be tailored in a large variety by coating of different functionalities following the protocol of the layer-by-layer process (LBL). The dimensional stability of the electrospun fibers in the chameleon nonwovens is achieved by photo-cross-linking after electrospinning and thereby overcoming the repulsive forces of the ionic moieties in the fibers. Depending on the nature of the ionic moieties different materials are coated by LBL including dyes, antibacterial materials, silver, and gold nanoparticles. Enhanced coating efficiency for coating of metal nanoparticles is observed when the chameleon nonwovens were precoated by a polyelectrolyte.
Preparation of Continuous Gold Nanowires by Electrospinning of High-Concentration Aqueous Dispersions of Gold Nanoparticles.
K. Gries, H. Vieker, A. Gölzhäuser, S. Agarwal, A. Greiner, Small 2012, 8, 1436-1441.
Gold nanowires are prepared by the electrospinning of highly concentrated aqueous dispersions of gold nanoparticles (AuNPs) in the presence of poly(vinyl alcohol) and subsequent annealing at higher temperatures. Continuous wires of sintered AuNPs are obtained as a result of this process. The Au wires are characterized by transmission electron microscopy, helium ion microscopy, optical microscopy, and X-ray diffractometry.
Precisely Designed Gold Nanoparticles by Surface Polymerization – Artificial Molecules as Building Blocks for Novel Materials.
S. Bokern, K. Gries, H.-H. Görtz, V. Warzelhan, S. Agarwal, A. Greiner, Adv. Funct. Mater. 2011, 21, 3753-3759.
The synthesis of gold nanoparticles (2–4 nm) carrying a single functional polystyrene chain and its characterization by gel permeation chromatography are reported. This has been achieved by a new type of macromolecular azo-initiator based on telechelic polystyrene with containing α,ω-methylcoumarin endgroups and an azo group in the middle of the polystyrene. The structure and the near-quantitative functionality of the initiator have been verified by performing NMR, GPC, and UV–vis measurements. This macroinitiator has been used to initiate a surface polymerization of 4-vinylthiophenol molecules immobilized on the surface of gold nanoparticles. As a product, gold nanoparticles carrying exactly one polystyrene chain have been synthesized with functionalization degrees of up to 90% (crude yield). Proof and quantification of the functionalization degree have been demonstrated by application of a GPC setup with a diode array detector for online UV–vis spectra.
Electrospun and solution blown three-dimensional carbon fiber nonwovens for application as electrodes in microbial fuel cells
S. Chen, H. Hou, F. Harnisch, S. Patil, A. A. Carmona-Martinez, S. Agarwal, Y. Zhang, S. Sinha-Rey, A. Yarin, A. Greiner, U. Schröder, Energy Environ. Sci. 2011, 4, 1417-1421.
Three-dimensional carbon fiber electrodes prepared by electrospinning and solution blowing are shown to be excellent electrode materials for bioelectrochemical systems such as microbial fuel cells or microbial electrolysis cells. The bioelectrocatalytic anode current density is shown to reach values of up to 30 A m−2, which represents the highest reported values for electroactive microbial biofilms.
Reusable Catalysts Based on Dendrimers Trapped in Poly(p-xylylene) Nanotubes.
J.-P. Lindner, C. Röben, A. Studer, M. Stasiak, R. Ronge, A. Greiner, J. H. Wendorff, Angew. Chem. Int. Ed. 2009, 48, 8874-8877.
Catalysts in a bottle are readily prepared by coelectrospinning of PAMAM dendrimers and poly(ethylene oxide) (PEO). The nanofibers thus obtained can be coated with poly(p-xylylene) by chemical vapor deposition. Removal of the core PEO fibers by extraction with water results in PAMAM dendrimers entrapped in the tubes (see picture). The entrapped dendrimers show a high catalytic activity as reusable organocatalysts.
Polymer tubes with longitudinal composition gradient by face-to-face wetting.
O. Kriha, P. Göring, M. Milbradt, S. Agarwal, M. Steinhart, R. Wehrspohn, J. H. Wendorff, A. Greiner, Chem. Mater. 2008, 20, 1076-1081.
Simultaneous precursor wetting of porous templates from both opposite surfaces with polymeric solutions yields nanotubes with a longitudinal composition gradient. Mixing of the components infiltrated from the opposite surfaces can be prevented by face-to-face wetting with polymeric melts, leading to the formation of tubular components predominantly consisting of the pure components that are separated by sharp interfaces.
Magnetically Anisotropic Cobalt and Iron Nanofibers via Electrospinning.
M. Graeser, M. Bognitzki, W. Massa, C. Pietzonka, A. Greiner, J. H. Wendorff, Adv. Mater. 2007, 19, 4244-4247.
Magnetically anisotropic cobalt and iron nanofibers are obtained via oriented polymer-supported electrospinning and subsequent reduction and thermal treatment. The cobalt and iron nature of the fibers is confirmed by XRD analysis. Fibers of both metals show ferromagnetic behavior. Parallel aligned iron nanofibers revealed anisotropic hysteresis loops depending on field-fiber orientation.
Connection of Hippocampal Neurons by Magnetically Controlled Movement of Short Electrospun Polymer Fibers – A Route to Magnetic Micromanipulators
O. Kriha, M. Becker, M. Lehmann, D. Kriha, J. Krieglstein, M. Yosef, S. Schlecht, R. B. Wehrspohn, J. H. Wendorff, A. Greiner, Adv. Mater. 2007, 19, 2483-2485.
Controlled movement of short electrospun fluorescent fibers has been accomplished by electrospinning of anthracene-modified PMMA loaded with superparamagnetic cobalt nanoparticles. Subsequently, composite fibers were mechanically cut, dispersed in water and moved under the influence of an external ferromagnet.
Novel Biohybrid Materials by Electrospinning: Nanofibers of Poly(ethylene oxide) and Living Bacteria.
M. Gensheimer, M. Becker, A. Brandis-Heep, J. H. Wendorff, R. K. Thauer, A. Greiner, Adv. Mater. 2007, 19, 2480-2482.
Poly(ethylene oxide) (PEO) fibers with living bacteria were obtained by electrospinning of aqueous dispersions of E. coli and Microccus luteus bacteria with PEO. E. coli as well as Micrococcus luteus bacteria survived electrospinning process. However, E. coli did not survive dry storage in PEO nonwovens at 20 °C for more than one hour. In contrast, Micrococcus luteus survived at least 250 h dry storage at 20 °C in eletrospun PEO nonwovens.
One-Step Production of Polymeric Microtubes by Co-electrospinning.
Y. Dror, W. Salalha, R. Avrahami, E. Zussman, A. L. Yarin, R. Dersch, A. Greiner, J. H. Wendorff, Small 2007, 3, 1064-1073.
Herein we demonstrate the ability to fabricate polymeric microtubes with an inner diameter of approximately 3 μm through co-electrospinning of core and shell polymeric solutions. The mechanism by which the core/shell structure is transformed into hollow fibers (microtubes) is primarily based on the evaporation of the core solution through the shell and is described here in detail. Additionally, we present the filling of these microtubes, thus demonstrating their possible use in microfluidics. We also report the incorporation of a protein (green fluorescent protein) within such fibers, which is of interest for sensorics.
Organic Tube/Rod Hybrid Nanofibers with Adjustable Segment Lengths by Bidirectional Template Wetting.
O. Kriha, L. Zhao, E. Pippel, U. Gösele, R. B. Wehrspohn, J. H. Wendorff, M. Steinhart, A. Greiner, Adv. Funct. Mat. 2007, 17, 1327-1332.
Segmented nanotubes and nanorods exhibiting a variation in their composition along their long axes represent a new and exciting class of nanomaterials. It is shown that bidirectional template wetting enables the integration of functional and complex polymeric materials into segmented nanofibers. First, a template is wetted under conditions in which a solid polymeric thread with adjustable length fills a pore segment starting from one template surface. Subsequently, a second wetting step starting from the opposite template surface yields segmented nanofibers. The exploitation of different wetting mechanisms results in the formation of tube/rod hybrid nanofibers.
Nondestructive Mechanical Release of Ordered Polymer Microfiber Arrays from Porous Templates.
S. Grimm, K. Schwirn, P. Göring, H. Knoll, P. T. Miclea, A. Greiner, J. H. Wendorff, R. B. Wehrspohn, U. Gösele, M. Steinhart, Small 2007, 3, 993-1000.
The fabrication of one-dimensional (1D) nanostructures and microstructures inside the pores of porous templates is intensively investigated. The release of these structures is commonly accomplished by etching and destroying the templates. The 1D nanostructures and microstructures tend to condense because of the occurrence of capillary forces during drying of the specimens. It is shown that highly ordered arrays of polymer microfibers can be easily detached from silanized porous templates by mechanical lift-off. This procedure leaves the templates intact, thus allowing their recycling, and does not involve the use of solutions or solvents, thus circumventing condensation. Therefore, mechanical lift-off may enable the up-scaling of template-based approaches to the fabrication of highly ordered assemblies of 1D nanostructures and microstructures.
Poly(ethylene carbonate): A thermoelastic and biodegradable biomaterial for drug eluting stent coatings?
F. Unger, U. Westedt, P. Hanefeld, R. Wombacher, S. Zimmermann, A. Greiner, M. Ausborn, T. Kissel, J. Control. Rel. 2007, 117, 312-321.
A first feasibility study exploring the utility of poly(ethylene carbonate) (PEC) as coating material for drug eluting stents under in vitro conditions is reported. PEC (Mw 242 kDa, Mw/Mn = 1.90) was found to be an amorphous polymer with thermoelastic properties. Tensile testing revealed a stress to strain failure of more than 600%. These properties are thought to be advantageous for expanding coated stents. In vitro cytotoxicity tests showed excellent cytocompatibility of PEC. Based on these findings, a new stenting concept was suggested, pre-coating a bare-metal stent with PPX-N as non-biodegradable basis and applying a secondary PEC coating using an airbrush method. After manual expansion, no delamination or destruction of the coating could be observed using scanning electron microscopy.The surface degradation-controlled release mechanism of PEC may provide the basis for “on demand” drug eluting stent coatings, releasing an incorporated drug predominantly at an inflamed implantation site upon direct contact with superoxide-releasing macrophages. As a release model, metal plates of a defined size and area were coated under the same conditions as the stents with PEC containing radiolabelled paclitaxel. An alkaline KO2− solution served as a superoxide source. Within 12 h, 100% of the incorporated paclitaxel was released, while only 20% of the drug was released in non-superoxide releasing control buffer within 3 weeks.
Preparation of Sub-micrometer Copper Fibers via Electrospinning.
M. Bognitzki, M. Becker, M. Graeser, W. Massa, J. H. Wendorff, A. Schaper, D. Weber, A. Beyer, A. Gölzhäuser, A. Greiner, Adv. Mater. 2006, 18, 2384-2386.
The copper nature of the fibers is confirmed by using energy- dispersive X-ray analysis and wide- angle X-ray analysis (see figure). The copper fibers show metallic conductivity as confirmed by conductance measurements in a low-energy electron point source microscope.
Polyelectrolyte Functionalization of Electrospun Fibers.
K. Müller, J. F. Quinn, A. P. R. Johnston, M. Becker, A. Greiner, F. Caruso, Chem. Mater. 2006, 18, 2397-2403.
Surface functionalization of high aspect ratio polystyrene electrospun fibers with polyelectrolytes (PE), DNA oligonucleotides, and PE/gold nanoparticle composite layers was accomplished using the layer-by-layer process. Hollow fibers were obtained by selective removal of the fiber templates.
Assembly of well-Aligned Multiwalled Carbon Nanotubes in Confined Polyacrylnitrile Environments: Electrospun Composite Nanofiber Sheets.
J. J. Ge, H. Hou, Q. Li, M. J. Graham, A. Greiner, D. H. Reneker, F. W. Harris, S. Z. D. Cheng, J. Am. Chem. Soc. 2004, 126, 15754-15761.
Highly oriented, large area continuous composite nanofiber sheets made from surface-oxidized multiwalled carbon nanotubes (MWNTs) and polyacrylonitrile (PAN) were successfully developed using electrospinning. The preferred orientation of surface-oxidized MWNTs along the fiber axis was determined with transmission electron microscopy and electron diffraction. The surface morphology and height profile of the composite nanofibers were also investigated using an atomic force microscope in tapping mode. For the first time, it was observed that the orientation of the carbon nanotubes within the nanofibers was much higher than that of the PAN polymer crystal matrix as detected by two-dimensional wide-angle X-ray diffraction experiments. This suggests that not only surface tension and jet elongation but also the slow relaxation of the carbon nanotubes in the nanofibers are determining factors in the orientation of carbon nanotubes. The extensive fine absorption structure detected via UV/vis spectroscopy indicated that charge-transfer complexes formed between the surface-oxidized nanotubes and negatively charged ( C⋮N: ) functional groups in PAN during electrospinning, leading to a strong interfacial bonding between the nanotubes and surrounding polymer chains. As a result of the highly anisotropic orientation and the formation of complexes, the composite nanofiber sheets possessed enhanced electrical conductivity, mechanical properties, thermal deformation temperature, thermal stability, and dimensional stability. The electrical conductivity of the PAN/MWNT composite nanofibers containing 20 wt % nanotubes was enhanced to ∼1 S/cm. The tensile modulus values of the compressed composite nanofiber sheets were improved significantly to 10.9 and 14.5 GPa along the fiber winding direction at the MWNT loading of 10 and 20 wt %, respectively. The thermal deformation temperature increased with increased MWNT loading. The thermal expansion coefficient of the composite nanofiber sheets was also reduced by more than an order of magnitude to 13 × 10-6/°C along the axis of aligned nanofibers containing 20 wt % MWNTs.
Nanoporous Low-K Polyimide Films via Poly(amic acid)s with Grafted Poly(ethylene glycol) Side Chains from a Reversible Addition-Fragmentation Chain-Transfer-Mediated Process.
Y. Chen, W. Wang, W. Yu, Z. Yuan, E.-T. Kang, K.-G. Neoh, B. Krauter, A. Greiner, Adv. Funct. Mater. 2004, 14, 471-478.
Thermally-initiated living radical graft polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA) with ozone-pretreated poly[N,N′-(1,4-phenylene)-3,3′,4,4′-benzophenonetetra-carboxylic amic acid] (PAmA) via a reversible addition–fragmentation chain-transfer (RAFT)-mediated process was carried out. The chemical compositions and structures of the copolymers were characterized by nuclear magnetic resonance (NMR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and molecular weight measurements. The “living” character of the grafted PEGMA side chains was ascertained in the subsequent extension of the PEGMA side chains. Nanoporous low-dielectric-constant (low-κ) polyimide (PI) films were prepared by thermal imidization of the PAmA graft copolymers under reduced argon pressure, followed by thermal decomposition of the side chains in air. The nanoporous PI films obtained from the RAFT-mediated graft copolymers had well-preserved PI backbones, porosity in the range of 5–17 %, and pore size in the range of 30–50 nm. The pores were smaller and the pore-size distribution more uniform than those of the corresponding nanoporous PI films obtained via graft copolymers from conventional free-radical processes. Dielectric constants approaching 2 were obtained for the nanoporous PI films prepared from the RAFT-mediated graft copolymers.
Compound Core-Shell Polymer Nanofibers by Co-Electrospinning.
Z. Sun, E. Zussman, A. L. Yarin, J. H. Wendorff, A. Greiner, Adv. Mater. 2003, 15, 1929-1932.
Co-electrospinning of core–shell polymer nanofibers is introduced. This process can be used for manufacturing of coaxial nanofibers made of pairs of different materials. Non-spinnable materials can be forced into 1D arrangements by co-electrospinning using a spinnable shell polymer. The method results in a novel two-stage approach for fabrication of nanotubes instead of the previously used three-stage process.
Polymer nanotubes via wetting of ordered porous templates.
M. Steinhart, J. H. Wendorff, A. Greiner, R. B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, U. Gösele, Science 2002, 296, 1997.
We have developed a simple technique for the fabrication of polymer nanotubes with a monodisperse size distribution and uniform orientation. When either a polymer melt or solution is placed on a substrate with high surface energy, it will spread to form a thin film, known as a precursor film, similar to the behavior of low molar mass liquids.
Titanium dioxide tubes from sol-gel coating of electrospun polymer fibers.
R. A. Caruso, J. H. Schattka, A. Greiner, Adv. Mater. 2001, 13, 1577-1579.
Electrospun polymer fibers can be coated with amorphous titanium dioxide using a sol–gel coating technique. After removal of the organic material by thermal treatment hollow titania fibers are produced. The sol–gel coating is able to reproduce the finer details of the fiber, e.g., the Figure shows how oval nodules that were indentations on the initial polymer are mimicked in the final inorganic structure of the titania fibers (see also images on the front cover).
Nanostructured Fibers via Electrospinning.
M. Bognitzki, W. Czado, T. Frese, A. Schaper, M. Hellwig, M. Steinhart, A. Greiner, J. H. Wendorff, Adv. Mater. 2001, 13, 70-72.
Structured polymer fibers with diameters down to tens of nanometers are of interest for applications in filters, in composite reinforcement, or as templates for the preparation of functional nanotubes. The Figure shows fibers of poly-L-lactide produced by electrospinning from a dichloromethane solution exhibiting regular pores or pits in the 100 nm range.
Homoleptic Phosphoraneiminato Complexes of Rare Earth Elements as Initiators for Ring-Opening Polymerization of Lactones
T. Gröb, G. Seybert, W. Massa, F. Weller, R. Palaniswami, A. Greiner, K. Dehnicke, Angew. Chem. 2000, 39, 4373-4375.
As cyclopentadienide equivalents, the first homoleptic phosphoraneiminato complexes of rare earth elements with the very small coordination number four (see, for example, the ytterbium compound shown) are excellent initiators for the ring-opening polymerization of lactones.
Polymer, metal, and hybrid nano- and mesotubes by coating degradable polymer templates fibers(TUFT process).
M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte, A. Schaper, J. H. Wendorff, A. Greiner, Adv. Mater. 2000, 12, 637-640.
Coating extremely thin degradable template polymer fibers with the desired wall materials forms the basis of the straightforward and highly versatile method described here for the production of polymer, metal, and hybrid nano- and mesotubes. Tubes with highly structured inner surfaces may be fabricated, which is of great interest for applications requiring large surface/volume ratios.
Columnar Discotics for Light Emitting Diodes.
T. Christ, B. Glüsen, A. Greiner, A. Kettner, R. Sander, V. Stümpflen, V. Tsukruk, J. H. Wendorff, Adv. Mater. 1997, 9, 48-52.
The suitability fo columnar discotics for applications in light emitting diodes (LEDs) has been investigated following the demonstration of their unusually large charge carrier mobilities. Results of X-ray diffraction, computer simulations, and electroluminescence measurements are presented and discussed for discotic systems such as that shown in the Figure. Their use in LEDs is expected to the advantageous.
Preparation of Palladium Colloids in Block Copolymer Micelles and Their Use for the Catalysis of the Heck Reaction.
S. Klingelhöfer, W. Heitz, A. Greiner, S. Oestreich, S. Förster, M. Antonietti, J. Am. Chem. Soc. 1997, 119, 10116-10120.
Colloidal dispersions of nanometer sized palladium colloids with very high stability were prepared in block copolymer micelles of polystyrene-b-poly-4-vinylpyridine and analyzed by electron microscopy and X-ray analysis. The resulting polymer/metal hybrids can easily be dissolved and handled in standard organic solvents such as toluene, tetrahydrofuran, and cyclohexane. They were successfully used for the Pd-catalyzed carbon−carbon coupling of aryl halides with alkenes (Heck reaction). Such block copolymer stabilized palladium colloids exhibit about the same reactivity as low molecular weight Pd complexes classically used for the Heck reaction, but show a much higher stability: in most reactions, the hybrids remain catalytically active even after 50000 turn-over cycles. Reaction rates were significantly controlled by the reactivity of the educts, but also respond to micelle architecture and dispersity of the palladium. Other advantages of the block copolymer stabilizer are that they are more simple and readily accessible than the phosphor-containing chelates, and that they dissolve even in “simple” solvents such as toluene (instead of amidic solvents).
Multicoloured Chromophore for white-light-emitting diodes.
T. Christ, A. Greiner, R. Sander, V. Stümpflen, J. H. Wendorff, Adv. Mater. 1997, 9, 219, 222.
Light-emitting diodes that emit white light require active materials that are able to emit throughout the entire visible spectrum. A simple approach is reported here, based on hist-guest systems in which the guest is a single chromophore and the host a polymer blend. It is demonstrated that the emission of the chromophore may depend strongly on the choice of the matrix polymer. The absorption, photoluminescence, and electroluminescence spectra of the chromophore PSA in matrices of polynorbornene, Durel (a polyarylate), polyvinylpyridine, and blends are presented.
Polarized Photoluminescence of Liquid Crystalline Polymers with Isolated Arylenevinylene Segments in the Main Chain.
G. Lüssem, R. Festag, A. Greiner, C. Schmidt, C. Unterlechner, W. Heitz, J.-H. Wendorff, M. Hopmeier, J. Feldmann, Adv. Mater. 1995, 7, 923-925.
Anisotropic charge transport, or polarized photo- and electroluminescence are possible applications for materials combining the optical properties of arylenevinylene polymers with the characteristic orientational order of the liquid-crystalline state. Thermally stale, soluble materials with a broad mesophase, which can be oriented macroscopically using conventional processing techniques and when exhibit highly anisotropic luminescence behavior of use in polarized LEDs are presented.
A blue light emitting polymer with phenylenevinylene segments in the side-chains.
P. Hesemann, H. Vestweber, J. Pommerehne, R. F. Mahrt, A. Greiner, Adv. Mater. 1995, 7, 388-390.
Polymer-based light emitting diodes constructed from amorphous materials with well-defined conjugated segments and high glass transition temperatures are reproted. It is shown that grafting 4-vinyl-trans-stilbene onto polystyrene results in side-chain polymers with conjugated segments in the side chains which have amorphous morphologies whereas blends between a conjugated unit and a matrix polymer exhibit unwanted crystallization.