Phase-structure effects of electrospun TiO2 nanofiber membranes on As(III) adsorption

TiO2 nanofibers (NFs) with different phases such as amorphous, anatase, mixed anatase?rutile, and rutile have been prepared by combining the electrospinning technique with the subsequent process of heat treatment or acidic-dissolution method. The obtained NFs are characterized by a Fourier transform infrared spectrometer (FT-IR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption?desorption isotherm measurements. Phase structure effects of electrospun TiO2 NFs on As(III) adsorption behaviors have been investigated. The results showed a significant effect of the phase structures of TiO2 NFs on As(III) adsorption rates and capacities. Amorphous TiO2 NFs have the highest As(III) adsorption rate and capacity in the investigated samples, which can be attributed to its higher surface area and porous volume. This research provides a simple and low-cost method for phasecontrolled fabrication of TiO2 NFs and application for effective removal of arsenic from aqueous solution.

Bending behaviour of step-wise graded carbon nanofiber/polymer nanocomposites

In this study, a finite element-based model was developed to investigate the mechanical behavior of step-wise graded carbon nanofibre/phenolic nanocomposites. Four step-wise graded nanocomposites (FGNs), a non-graded nanocomposite (NGN), and a pure phenolic with the same geometry and total carbon nanofiber content were designed, fabricated and analyzed. Flexural tests were conducted to validate the finite element model. Close agreement was obtained between experimental results and numerical predictions. The results showed that flexural modulus was highly influenced by the compositional gradients.

Highly sensitive SnO2 nanofiber chemiresistors with a low optimal operating temperature: synergistic effect of Cu2+/Au co-doping

Metal oxide chemiresistors (MOCs) with a low optimal operating temperature, high sensitivity and fast response/recovery are highly promising for various applications, but remain challenging to realize. Herein, we demonstrate that SnO2 nanofibers after being co-doped with Cu2+ and Au show considerably enhanced sensing performances at an unexpectedly decreased operating temperature. A synergistic effect occurs when the two dopants are introduced together. Co-doping may form a novel strategy to the development of ultrasensitive MOCs working at a low optimal temperature. This journal is © the Partner Organisations 2014.

Recent developments in electrospinning of nanofiber yarns

Nanofibers possess high surface area and excellent porosity. Though nanofibers can be produced by a variety of techniques, electrospinning stands distinct because of its simplicity and flexibility in processing different polymer materials, and ability to control fiber diameter, morphology, orientation, and chemical component. Nonetheless, electrospun nanofibers are predominantly produced in the form of randomly oriented fiber webs, which restrict their wide use. Converting nanofibers into twisted continuous bundles, i.e., nanofiber yarns, can improve their strength and facilitate their subsequent processes, but remains challenging to make. Nanofiber yarns also create enormous opportunities to develop well-defined three-dimensional nanofibrous architectures. This review article gives an overview of the state-of-the-art techniques for electrospinning of nanofiber yarns and control of nanofiber alignment. A detailed account on techniques to produce twisted/non-twisted short bundles and continuous yarns are discussed.

Preparation of cellulose nanofiber from softwood pulp by ball milling

This paper reports the possibility of producing cellulose nanofiber from softwood pulp using a simple ball milling technique under ambient pressure and at room temperature. The effects of milling conditions including the ball-to-cellulose mass ratio, milling time, ball size and alkaline pretreatment were investigated. It was found that milling-ball size should be carefully selected for producing fibrous morphologies instead of particulates. Milling time and ball-to-cellulose mass ratio were also found important to control the fiber morphology. Alkali pre-treatment helped in weakening hydrogen bonds between cellulose fibrils and removing small particles, but with the risks of damaging the fibrous morphology. In a typical run, cellulose nanofiber with an average diameter of 100 nm was obtained using soft mechanical milling conditions using cerium-doped zirconia balls of 0.4–0.6 mm in diameter within 1.5 h without alkaline pretreatment.

Continuous polyacrylonitrile nanofiber yarns: Preparation and dry-drawing treatment for carbon nanofiber production

Precursor fibers with diameters in nanometer scale and highly aligned polymer chains in fibers are highly promising for the preparation of high-performance carbon nanofibers, but are challenging to make. In this study, we demonstrate for the first time that a carbon nanofiber precursor can be prepared by the electrospinning of polyacrylonitrile into a nanofiber yarn and by the subsequent drawing treatment of the yarn in dry conditions. The yarn shows excellent drawing performance, which can be drawn evenly up to 6 times of its original length without breaking. The drawing treatment improves the yarn and fiber uniformity, polymer chain orientation within the fibers, as well as yarn tension and modules, but shows decreased yarn and fiber diameter and elongation at break. The drawing temperature and force show influences on the drawing behavior. The highest strength and modules (362 ± 37 MPa and 9.2 ± 1.4 GPa, respectively) are found on the yarn drawn by 5 times its length, which increased by 800% and 1800% when compared to the as-spun yarn. Through un-optimized stabilization and carbonization treatments, we further demonstrate that the carbonized nanofiber yarn shows comparable tensile properties as the commercial carbon fibers. Electrospun nanofiber yarns may form next generation precursors for making high performance carbon fibers. This journal is

Effect of electrospinning parameters and polymer concentrations on mechanical-to-electrical energy conversion of randomly-oriented electrospun poly(vinylidene fluoride) nanofiber mats

Poly(vinylidene fluoride) (PVDF) nanofiber mats prepared by an electrospinning technique were used as an active layer for making mechanical-to-electric energy conversion devices. The effects of PVDF concentration and electrospinning parameters (e.g. applied voltage, spinning distance), as well as nanofiber mat thickness on the fiber diameter, PVDF β crystal phase content, and mechanical-to-electrical energy conversion properties of the electrospun PVDF nanofiber mats were examined. It was interesting to find that finer uniform PVDF fibers showed higher β crystal phase content and hence, the energy harvesting devices had higher electrical outputs, regardless of changing the electrospinning parameters and PVDF concentration. The voltage output always changed in the same trend to the change of current output whatever the change trend was caused by the operating parameters or polymer concentration. Both voltage and current output changes followed a similar trend to the change of the β crystal phase content in the nanofibers. The nanofiber mat thickness influenced the device electrical output, and the maximum output was found on the 70 μm thick nanofiber mat. These results suggest that uniform PVDF nanofibers with smaller diameters and high β crystal phase content facilitate mechanical-to-electric energy conversion. The understanding obtained from this study may benefit the development of novel piezoelectric nanofibrous materials and devices for various energy uses.

Optical interface based on a nanofiber atom-trap

In this thesis, I present the realization of a fiber-optical interface using optically trapped cesium atoms, which is an efficient tool for coupling light and atoms. The basic principle of the presented scheme relies on the trapping of neutral cesium atoms in a two-color evanescent field surrounding a nanofiber. The strong confinement of the fiber guided light, which also protrudes outside the nanofiber, provides strong confinement of the atoms as well as efficient coupling to near-resonant light propagating through the fiber. In chapter 1, the necessary physical and mathematical background describing the propagation of light in an optical fiber is presented. The exact solution of Maxwell’s equations allows us to model fiber-guided light fields which give rise to the trapping potentials and the atom-light coupling in the close vicinity of a nanofiber. Chapter 2 gives the theoretical background of light-atom interaction. A quantum mechanical model of the light-induced shifts of the relevant atomic levels is reviewed, which allows us to quantify the perturbation of the atomic states due to the presence of the trapping light-fields. The experimental realization of the fiber-based atom trap is the focus of chapter 3. Here, I analyze the properties of the fiber-based trap in terms of the confinement of the atoms and the impact of several heating mechanisms. Furthermore, I demonstrate the transportation of the trapped atoms, as a first step towards a deterministic delivery of individual atoms. In chapter 4, I present the successful interfacing of the trapped atomic ensemble and fiber-guided light. Three different approaches are discussed, i.e., those involving the measurement of either near-resonant scattering in absorption or the emission into the guided mode of the nanofiber. In the analysis of the spectroscopic properties of the trapped ensemble we find good agreement with the prediction of theoretical model discussed in chapter 2. In addition, I introduce a non-destructive scheme for the interrogation of the atoms states, which is sensitive to phase shifts of far-detuned fiber-guided light interacting with the trapped atoms. The inherent birefringence in our system, induced by the atoms, changes the state of polarization of the probe light and can be thus detected via a Stokes vector measurement.

Nanofiber-based spectroscopy of organic molecules

This thesis reports on the experimental realization of nanofiber-based spectroscopy of organic molecules. The light guided by subwavelength diameter optical nanfibers exhibits a pronounced evanescent field surrounding the fiber which yields high excitation and emission collection efficiencies for molecules on or near the fiber surface.rnThe optical nanofibers used for the experiments presented in this thesis are realized as thernsub-wavelength diameter waist of a tapered optical fiber (TOF). The efficient transfer of thernlight from the nanofiber waist to the unprocessed part of the TOF depends critically on therngeometric shape of the TOF transitions which represent a nonuniformity of the TOF. Thisrnnonuniformity can cause losses due to coupling of the fundamental guided mode to otherrnmodes which are not guided by the taper over its whole length. In order to quantify the lossrnfrom the fundamental mode due to tapering, I have solved the coupled local mode equationsrnin the approximation of weak guidance for the three layer system consisting of fiber core andrncladding as well as the surrounding vacuum or air, assuming the taper shape of the TOFsrnused for the experiments presented in this thesis. Moreover, I have empirically studied therninfluence of the TOF geometry on its transmission spectra and, based on the results, I haverndesigned a nanofiber-waist TOF with broadband transmission for experiments with organicrnmolecules.rnAs an experimental demonstration of the high sensitivity of nanofiber-based surface spectroscopy, I have performed various absorption and fluorescence spectroscopy measurements on the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA). The measured homogeneous and inhomogeneous broadening of the spectra due to the interaction of the dielectric surface of the nanofiber with the surface-adsorbed molecules agrees well with the values theoretically expected and typical for molecules on surfaces. Furthermore, the self-absorption effects due to reasorption of the emitted fluorescence light by circumjacent surface-adsorbed molecules distributed along the fiber waist have been analyzed and quantified. With time-resolved measurements, the reorganization of PTCDA molecules to crystalline films and excimers can be observed and shown to be strongly catalyzed by the presence of water on the nanofiber surface. Moreover, the formation of charge-transfer complexes due to the interaction with localized surface defects has been studied. The collection efficiency of the molecular emission by the guided fiber mode has been determined by interlaced measurements of absorption and fluorescence spectra to be about 10% in one direction of the fiber.rnThe high emission collection efficiency makes optical nanofibers a well-suited tool for experiments with dye molecules embedded in small organic crystals. As a first experimental realization of this approach, terrylene-doped para-terphenyl crystals attached to the nanofiber-waist of a TOF have been studied at cryogenic temperatures via fluorescence and fluorescence excitation spectroscopy. The statistical fine structure of the fluorescence excitation spectrum for a specific sample has been observed and used to give an estimate of down to 9 molecules with center frequencies within one homogeneous width of the laser wavelength on average for large detunings from resonance. The homogeneous linewidth of the transition could be estimated to be about 190MHz at 4.5K.

Electrospun PLLA nanofiber scaffolds and their use in combination with BMP-2 for reconstruction of bone defects

Introduction Adequate migration and differentiation of mesenchymal stem cells is essential for regeneration of large bone defects. To achieve this, modern graft materials are becoming increasingly important. Among them, electrospun nanofiber scaffolds are a promising approach, because of their high physical porosity and potential to mimic the extracellular matrix (ECM). Materials and Methods The objective of the present study was to examine the impact of electrospun PLLA nanofiber scaffolds on bone formation in vivo, using a critical size rat calvarial defect model. In addition we analyzed whether direct incorporation of bone morphogenetic protein 2 (BMP-2) into nanofibers could enhance the osteoinductivity of the scaffolds. Two critical size calvarial defects (5 mm) were created in the parietal bones of adult male Sprague-Dawley rats. Defects were either (1) left unfilled, or treated with (2) bovine spongiosa, (3) PLLA scaffolds alone or (4) PLLA/BMP-2 scaffolds. Cranial CT-scans were taken at fixed intervals in vivo. Specimens obtained after euthanasia were processed for histology, histomorphometry and immunostaining (Osteocalcin, BMP-2 and Smad5). Results PLLA scaffolds were well colonized with cells after implantation, but only showed marginal ossification. PLLA/BMP-2 scaffolds showed much better bone regeneration and several ossification foci were observed throughout the defect. PLLA/BMP-2 scaffolds also stimulated significantly faster bone regeneration during the first eight weeks compared to bovine spongiosa. However, no significant differences between these two scaffolds could be observed after twelve weeks. Expression of osteogenic marker proteins in PLLA/BMP-2 scaffolds continuously increased throughout the observation period. After twelve weeks osteocalcin, BMP-2 and Smad5 were all significantly higher in the PLLA/BMP-2 group than in all other groups. Conclusion Electrospun PLLA nanofibers facilitate colonization of bone defects, while their use in combination with BMP-2 also increases bone regeneration in vivo and thus combines osteoconductivity of the scaffold with the ability to maintain an adequate osteogenic stimulus.