Reactive Magnetospinning of Nano- and Microfibers.

Reactive spinning of nano- and microfibers that involves very fast chemical reactions and ion exchange is a challenge for the common methods for nanofiber formation. Herein, we introduce the reactive magnetospinning method. This procedure is based on the magnetic-field-directed collision of ferrofluid droplets with liquid droplets that contain complementary reactants. The collision, start of the chemical reaction, and the fiber drawing are self-synchronized. The method is used to synthesize, cross-link, and chemically modify fiber-forming polymers in the stage of fiber formation. The method provides new opportunities for the fabrication of nanofibers for biomedical applications.

Highly efficient phase boundary biocatalysis with enzymogel nanoparticles.

The enzymogel nanoparticle made of a magnetic core and polymer brush shell demonstrates a novel type of remote controlled phase-boundary biocatalysis that involves remotely directed binding to and engulfing insoluble substrates, high mobility, and stability of the catalytic centers. The mobile enzymes reside in the polymer brush scaffold and shuttle between the enzymogel interior and surface of the engulfed substrate in the bioconversion process. Biocatalytic activity of the mobile enzymes is preserved in the enzymogel while the brush-like architecture favors the efficient interfacial interaction when the enzymogel spreads over the substrate and extends substantially the reaction area as compared with rigid particles.

Conformational transitions of flexible hydrophobic polyelectrolytes in solutions of monovalent and multivalent salts and their mixtures.

Conformations of cationic polyelectrolytes (PEs), a weak poly(2-vinylpyridine) (P2VP) and a strong poly(N-methyl-2-vinylpyridinium iodide) (qP2VP), adsorbed on mica from saline solutions in the presence of counterions of different valences are studied using in situ atomic force microscopy (AFM). Quantitative characteristics of chain conformations are analyzed using AFM images of the adsorbed molecules. The results of the statistical analysis of the chain contour reveal collapse of the PE coils when ionic strength is in a range from tens to hundreds of millimoles per kilogram and re-expansion of the coils with a further increase of ionic strength up to a region of the saturated saline solutions. The competition between monovalent and multivalent counterions simultaneously present in solutions strongly affects conformations of PE chains even at a very small fraction of multivalent counterions. Shrinkage of PE coils is steeper for multivalent counterions than for monovalent counterions. However, the re-expansion is only incremental in the presence of multivalent counterions. Extended adsorbed coils at low salt concentrations and at very high concentrations of monovalent salt exhibit conformation corresponding to a 2D coil with 0.95 fraction of bound segments (segments in "trains") in the regime of diluted surface concentration of the PE. Shrunken coils in the intermediate range of ionic strength resemble 3D-globules with 0.8 fraction of trains. The incrementally re-expanded PE coils at a high ionic strength remain unchanged at higher multivalent salt concentrations up to the solubility limit of the salt. The formation of a strong PE complex with multivalent counterions at high ionic strength is not well understood yet. A speculative explanation of the observed experimental result is based on possible stabilization of the complex due to hydrophobic interactions of the backbone.

Single molecule experiments visualizing adsorbed polyelectrolyte molecules in the full range of mono- and divalent counterion concentrations.

This work provides direct experimental verification (on the level of single molecules) for the behavior of hydrophobic polyelectrolyte chains adsorbed at a solid-liquid interface in the full range of possible salt concentrations: (a) in a dilute salt solution, PE chains possess an extended coil conformation visualized as adsorbed 2D-equilibrated coils; (b) in a moderate salt concentration range, the polymer coil shrinks and approaches the dimensions of a polymer coil under θ-conditions and the chains are visualized as adsorbed 3D-projected coils; (c) at high salt concentrations, the polymer coils reexpand and the molecules are visualized as 2D-equilibrated extended coils; however, (d) reexpansion is limited in the presence of multivalent counterions, presumably due to the bridging of the polymer coils by the counterions.

Reversible "closing" of an electrode interface functionalized with a polymer brush by an electrochemical signal.

The poly(4-vinyl pyridine) (P4VP)-brush-modified indium tin oxide (ITO) electrode was used to switch reversibly the interfacial activity by the electrochemical signal. The application of an external potential (-0.85 V vs Ag|AgCl|KCl, 3M) that electrochemically reduced O(2) resulted in the concomitant consumption of hydrogen ions at the electrode interface, thus yielding a higher pH value and triggering the restructuring of the P4VP brush on the electrode surface. The initial swollen state of the protonated P4VP brush (pH 4.4) was permeable to the anionic [Fe(CN)(6)](4-) redox species, but the electrochemically produced local pH of 9.1 resulted in the deprotonation of the polymer brush. The produced hydrophobic shrunken state of the polymer brush was impermeable to the anionic redox species, thus fully inhibiting its redox process at the electrode surface. The interface's return to the electrochemically active state was achieved by disconnecting the applied potential, followed by stirring the electrolyte solution or by slow diffusional exchange of the electrode-adjacent thin layer with the bulk solution. The developed approach allowed the electrochemically triggered inhibition ("closing") of the electrode interface. The application of this approach to different interfacial systems will allow the use of various switchable electrodes that are useful for biosensors and biofuel cells with externally controlled activity. Further use of this concept was suggested for electrochemically controlled chemical actuators (e.g. operating as electroswitchable drug releasers).

Magnetic field assisted assembly of highly ordered percolated nanostructures and their application for transparent conductive thin films.

Magnetic field assisted assembly is used to fabricate aligned single nanowire mesh-like nanostructured films. Inhomogeneous magnetic field is applied to translocate high aspect ratio silver nanowires from suspensions to the surface of solid supports. The tangential component of the magnetic field vector is rotated in two consecutive steps to arrange the rectangular mesh-like structure of orthogonally oriented nanowires with minimal fractions of loops and bent structures. This work demonstrates highly ordered nanowire films with superior properties to randomly deposited structures- specifically one order of magnitude greater conductivity and more than ten percent higher transparency. This method is simple, scalable and can be used for the directed assembly of magnetic and nonmagnetic highly ordered, percolated structures.

Magnetospinning of Nano- and Microfibers.

Magnetospinning is a new method for spinning of continuous micro- and nano-fibers using a permanent revolving magnet. The method utilizes magnetic forces and the hydrodynamic features of stretched threads to produce highly loaded, fine magnetic nanofibers. The magnetospinning process is independent of the solution dielectric properties and requires no high voltages, in contrast to the more-traditional electrospinning technique.

Touch- and Brush-Spinning of Nanofibers.

Robust, simple, and scalable touch- and brush-spinning methods for the drawing of nanofibers, core-shell nanofibers, and their aligned 2D and 3D meshes using polymer solutions and melts are discussed.