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 TiO₂ nanofibers generated via sol–gel electrospinning technique, and the incorporation of as-synthesized CdSe quantum dots (QDs) to different phases of TiO₂ nanofibers (NFs) via bifunctional surface modification. The effect of different phases of TiO₂ on photo-excited electron injection from CdSe QDs to TiO₂ 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 TiO₂ phases in the fibers. At a low calcination temperature of 400 ^oC only anatase TiO₂ nanofiber are obtained; with increasing calcination temperature (up to 500 ^oC) these anatase crystals became larger. Crystal transformation from the anatase to the rutile phase is observed above 500^oC, with most of the crystals transforming into the rutile phase at 800^oC. Bi-functional surface modification of calcined TiO₂ nanofibers with 3-mercaptopropionic acid (3-MPA) is used to incorporate as-synthesized CdSe QD nanoparticles on to TiO₂ nanofibers. Evidence of formation of CdSe/TiO₂ composite nanofibers is obtained from elemental analysis using Energy Dispersive X-ray spectroscopy (EDS) and TEM microscopy that reveal templated quantum dots on TiO₂ nanofibers. Photoluminescence emission intensities increase considerably with the addition of QDs to all TiO₂ 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

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

Alginate–Polyethylene Oxide Blend Nanofibers and the Role of the Carrier Polymer in Electrospinning

We present here a systematic investigation to understand why aqueous sodium alginate can only be electrospun into fibers through a blend with another polymer; specifically, polyethylene oxide (PEO). We seek to examine and understand the role of PEO as the “carrier polymer”. The addition of PEO favorably reduces electrical conductivity and surface tension of the alginate solution, aiding in fiber formation. While PEO has the ability to coordinate through its ether group (−COC−) with metal cation like the sodium cation of sodium alginate, we demonstrate in this study using PEO as well as polyvinyl alcohol (PVA) that coordination may have little effect on electrospinnability. More importantly, we show that PEO as carrier polymer provides molecular entanglement that is required for electrospinning. Since the selected carrier polymer provides the necessary entanglement, this carrier polymer must be electrospinnable, entangled and of a high molecular weight (more than 600 kDa for PEO). On the basis of these requirements, we stipulate that the PEO–PEO interaction of the high molecular-weight entangled PEO is key to “carrying” the alginate from solution to fibers during electrospinning. Further, using the resulting understanding of the role of PEO, we were able to increase the alginate concentration by employing a higher molecular-weight PEO: up to 70 wt % alginate using 2000 kDa PEO and, with, the addition of Triton X-100 surfactant, up to 85 wt % alginate, higher than previously reported