Hybrid carbon silica nanofibers through sol-gel electrospinning.

A controlled sol-gel synthesis incorporated with electrospinning is employed to produce polyacrylonitrile-silica (PAN-silica) fibers. Hybrid fibers are obtained with varying amounts of silica precursor (TEOS in DMF catalyzed by HCl) and PAN. Solution viscosity, conductivity, and surface tension are found to relate strongly to the electrospinnability of PAN-silica solutions. TGA and DSC analyses of the hybrids indicate strong intermolecular interactions, possibly between the -OH group of silica and -CN of PAN. Thermal stabilization of the hybrids at 280 °C followed by carbonization at 800 °C transforms fibers to carbon-silica hybrid nanofibers with smooth morphology and diameter ranging from 400 to 700 nm. FTIR analysis of the fibers confirms the presence of silica in the as-spun as well as the carbonized material, where the extent of carbonization is also estimated by confirming the presence of -C═C and -C═O peaks in the carbonized hybrids. The graphitic character of the carbon-silica fibers is confirmed through Raman studies, and the role of silica in the disorder of the carbon structure is discussed.

Cross-linked polymer nanofibers for hyperthermophilic enzyme immobilization: approaches to improve enzyme performance.

We report an enzyme immobilization method effective at elevated temperatures (up to 105 °C) and sufficiently robust for hyperthermophilic enzymes. Using a model hyperthermophilic enzyme, α-galactosidase from Thermotoga maritima, immobilization within chemically cross-linked poly(vinyl alcohol) (PVA) nanofibers to provide high specific surface area is achieved by (1) electrospinning a blend of a PVA and enzyme and (2) chemically cross-linking the polymer to entrap the enzyme within a water insoluble PVA fiber. The resulting enzyme-loaded nanofibers are water-insoluble at elevated temperatures, and enzyme leaching is not observed, indicating that the cross-linking effectively immobilizes the enzyme within the fibers. Upon immobilization, the enzyme retains its hyperthermophilic nature and shows improved thermal stability indicated by a 5.5-fold increase in apparent half-life at 90 °C, but with a significant decrease in apparent activity. The loss in apparent activity is attributed to enzyme deactivation and mass transfer limitations. Improvements in the apparent activity can be achieved by incorporating a cryoprotectant during immobilization to prevent enzyme deactivation. For example, immobilization in the presence of trehalose improved the apparent activity by 10-fold. Minimizing the mat thickness to reduce interfiber diffusion was a simple and effective method to further improve the performance of the immobilized enzyme.

Templating quantum dot to phase-transformed electrospun TiO2 nanofibers for enhanced photo-excited electron injection.

We report on the microstructural crystal phase transformation of electrospun TiO(2) nanofibers generated via sol-gel electrospinning technique, and the incorporation of as-synthesized CdSe quantum dots (QDs) to different phases of TiO(2) nanofibers (NFs) via bifunctional surface modification. The effect of different phases of TiO(2) on photo-excited electron injection from CdSe QDs to TiO(2) NFs, as measured by photoluminescence spectroscopy (PL) is also discussed. Nanofiber diameter and crystal structures are dramatically affected by different calcination temperatures due to removal of polymer carrier, conversion of ceramic precursor into ceramic nanofibers, and formation of different TiO(2) phases in the fibers. At a low calcination temperature of 400 (o)C only anatase TiO(2) nanofiber are obtained; with increasing calcination temperature (up to 500 (o)C) these anatase crystals became larger. Crystal transformation from the anatase to the rutile phase is observed above 500(o)C, with most of the crystals transforming into the rutile phase at 800(o)C. Bi-functional surface modification of calcined TiO(2) nanofibers with 3-mercaptopropionic acid (3-MPA) is used to incorporate as-synthesized CdSe QD nanoparticles on to TiO(2) nanofibers. Evidence of formation of CdSe/TiO(2) composite nanofibers is obtained from elemental analysis using Energy Dispersive X-ray spectroscopy (EDS) and TEM microscopy that reveal templated quantum dots on TiO(2) nanofibers. Photoluminescence emission intensities increase considerably with the addition of QDs to all TiO(2) nanofiber samples, with fibers containing small amount of rutile crystals with anatase crystals showing the most enhanced effect.

Hybrid silica-PVA nanofibers via sol-gel electrospinning.

We report on the synthesis of poly(vinyl alcohol) (PVA)-silica hybrid nanofibers via sol-gel electrospinning. Silica is synthesized through acid catalysis of a silica precursor (tetraethyl orthosilicate (TEOS) in ethanol-water), and fibers are obtained by electrospinning a mixture of the silica precursor solution and aqueous PVA. A systematic investigation on how the amount of TEOS, the silica-PVA ratio, the aging time of the silica precursor mixture, and the solution rheology influence the fiber morphology is undertaken and reveals a composition window in which defect-free hybrid nanofibers with diameters as small as 150 nm are obtained. When soaked overnight in water, the hybrid fibers remain intact, essentially maintaining their morphology, even though PVA is soluble in water. We believe that mixing of the silica precursor and PVA in solution initiates the participation of the silica precursor in cross-linking of PVA so that its -OH group becomes unavailable for hydrogen bonding with water. FTIR analysis of the hybrids confirms the disappearance of the -OH peak typically shown by PVA, while formation of a bond between PVA and silica is indicated by the Si-O-C peak in the spectra of all the hybrids. The ability to form cross-linked nanofibers of PVA using thermally stable and relatively inert silica could broaden the scope of use of these materials in various technologies.

Impact of plastics on fate and transport of organic contaminants in landfills.

Factors controlling organic contaminant sorption to common plastics in municipal solid waste were identified. Consumer plastics [drinking water container, prescription drug bottle, soda bottle, disposable cold cup, computer casing, furniture foam, carpet, vinyl flooring, formica sheet] and model polymers [high-density polyethylene (HDPE), medium-density polyethylene, low-density polyethylene, poly(vinyl chloride) (PVC)] were characterized by X-ray diffractometry, differential scanning calorimetry, and elemental analysis. The material characterization was used to interpret batch isotherm and kinetic data. K(p) values describing toluene sorption to rubbery or "soft" polymers could be normalized by the amorphous polymer fraction (f(amorphous)) but not by the organic carbon fraction (f(oc)). Diffusion coefficients (D) describing the uptake rate of toluene by rubbery plastics (HDPE, drinking water container, prescription drug bottle) were similar (D approximately 10(-10) cm(2)/s), indicating that pure HDPE can be used as a model for rubbery plastics. Toluene diffusivity was similar among glassy or "hard" plastics (PVC, soda bottle, computer casing, disposable cold cup; D approximately 10(-12) cm(2)/s) but lower than for rubbery plastics. Plastics in landfills are potential sinks of hydrophobic organic contaminants (HOCs) because of their higher affinity for HOCs compared to lignocellulosic materials and the slow desorption of HOCs from glassy plastics.

Electrospun nanoparticle-nanofiber composites via a one-step synthesis.

A facile approach to synthesize and incorporate metal nanoparticles (NPs) into electrospun polymer nanofibers (NFs) wherein the electrospinning polymer acts as both a reducing agent for the metal salt precursor, as well as a protecting and templating agent for the ensuing NPs, is reported. Such a true one-step process at ambient conditions and free of organic solvents is demonstrated using a system comprising AgNO(3) and poly(ethylene oxide) (PEO) at electrospinnable molecular weights of 600, 1000, or 2000 kDa. The PEO transforms Ag(+) into AgNPs, a phenomenon that has not been previously possible at PEO molecular weights less than 20 kDa without the addition of a separate reducing agent and stabilizer or the application of heat. Results from X-ray photoelectron spectroscopy and UV-Vis absorption spectrophotometry analyses support the formation of pseudo-crown ethers in high molecular weight PEO as the mechanism in the development of NPs. The AgNPs reduce fiber diameter and enhance fiber quality (reduced beading) due to increased electrical conductivity. Interestingly, several of the NFs exhibit AgNP-localized nanochain formation and protrusion from the NF surface that can be attributed to the combined effect of applied electrical field on the polymer and the differences between the electrical conductivity and polarizability of the polymer and metal NPs.