Stabilization of electrospun poly(vinyl alcohol) nanofibrous mats in aqueous solutions

<正>The stability ofpoly(vinyl alcohol)(PVA) nanofibrous mats in water media was improved by post-electrospinning treatments.Bifunctional glutaraldehyde(GA) in methanol was used as a crosslinking agent to stabilize PVA nanofiber,but fiber twinning was observed frequently,and the highly porous structure of PVA nanofibrous mats was destroyed when the crosslinked fiber was soaked in water.To overcome this shortcoming,chitosan(CS) was introduced into the PVA spinning solution to prepare PVA/CS composite nanofibers.Their treatment in GA/methanol solution could retain the fiber morphology of PVA/CS nanofibers and porous structure of PVA/CS nanofibrous mats even if they were soaked in aqueous solutions for 1 month.Scanning electron microscopy(SEM),X-ray diffraction(XRD),thermal gravimetric analysis(TGA) and differential scanning calorimetry(DSC) were applied to characterize the physicochemical structure and thermal properties of PVA nanofibers.It was found that the water resistance of PVA nanofibrous mats was enhanced because of the improvement of the degree of crosslinking and crystallinity in the electrospun PVA fibers after soaking in GA/methanol solution.     

Blended chitosan and polyvinyl alcohol membranes for the pervaporation dehydration of isopropanol

Homogeneous membranes were prepared by casting the solution of blended chitosan and polyvinyl alcohol (PVA) on a glass plate. The percent weight of chitosan in the membrane was varied from 0 to 100%. The membrane thickness was in the range of 15–30 μm. The membranes were heat treated at 150 °C for an hour. After that the membranes were crosslinked by glutaraldehyde and sulfuric acid in acetone aqueous solution. The membranes were tested at 30–60 °C for dehydration performance of 50–95% isopropanol aqueous solutions. At around 90% of isopropanol in the feed mixture, permeate flux increased whereas the percent of water in permeate tended to decrease when the feed temperature increased for all membranes, except that the water content in permeate from the membrane containing 75 wt.% chitosan remained constant. The swelling degree in water and the total flux increased with increasing chitosan content in membranes. The effect of temperature on permeate flux followed the Arrhenius relationship. The permeate flux decreased when isopropanol in the feed increased for all membranes. However, water content in permeate and isopropanol concentration in the feed formed complex relationship for different chitosan content membranes. Sorption did not appear to have significant effects on separation. The membrane containing chitosan 75% performed the best. For a feed solution containing 90% isopropanol at 60 °C, the permeate flux was 644 g/m² h with water content of nearly 100% in the permeate. At 55% isopropanol in the feed at 60 °C, the permeate flux was 3812 g/m² h. In the range of 55–95% of isopropanol in the feed, the water content in permeate was more than 99.5%. This membrane showed very excellent performance with good mechanical strength. It is promising to develop this membrane for industrial uses.     

A new hydrogen peroxide biosensor based on gold nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film-modified electrode

Gold nanoelectrode ensembles were produced by electrodeposition using multiwalled carbon nanotubes (MWNTs) as template. A new third generation amperometric biosensor for hydrogen peroxide was developed based on adsorption of horseradish peroxidase (HRP) at the glassy carbon (GC) electrode modified with Au nanoelectrode ensembles/multiwalled carbon nanotubes/chitosan film. The resulting HRP biosensor offered an excellent detection for hydrogen peroxide at −0.11 V with a linear response range of 2.08 × 10⁻⁷ to 7.6 × 10⁻³ M with a correlation coefficient of 0.998, and response time <5 s. The detection limit was 1.02 × 10⁻⁷ M at 3σ. The biosensor displays rapid response, expanded linear response range, and excellent repeatability. The simple and fast fabrication of the sensor makes it superior to other techniques.     

In vitro biocompatibility of electrospun hexanoyl chitosan fibrous scaffolds towards human keratinocytes and fibroblasts

With the ability to form a submicron-sized fibrous structure with interconnected pores mimicking the extracellular matrix (ECM) for tissue formation, electrospinning was used to fabricate ultra-fine fiber mats of hexanoyl chitosan (H-chitosan) for potential use as skin tissue scaffolds. In the present communication, the in vitro biocompatibility of the electrospun fiber mats was evaluated. Indirect cytotoxicity evaluation of the fiber mats with mouse fibroblasts (L929) revealed that the materials were non-toxic and did not release substances harmful to living cells. The potential for use of the fiber mats as skin tissue scaffolds was further assessed in terms of the attachment and the proliferation of human keratinocytes (HaCaT) and human foreskin fibroblasts (HFF) that were seeded or cultured on the scaffolds at different times. The results showed that the electrospun fibrous scaffolds could support the attachment and the proliferation of both types of cells, especially for HaCaT. In addition, the cells cultured on the fibrous scaffolds exhibited normal cell shapes and integrated well with surrounding fibers. The obtained results confirmed the potential for use of the electrospun H-chitosan fiber mats as scaffolds for skin tissue engineering.     

Preparation of electrospun chitosan/poly(vinyl alcohol) membranes

Electrospinning of chitosan from its solutions in 2% aqueous acetic acid was studied by adding poly(vinyl alcohol) (PVA) as a “guest” polymer. Properties of the chitosan/PVA solutions including viscosity, conductivity, and surface tension were measured, and effects of the polymer concentration, chitosan/PVA mass ratio and processing parameters (applied voltage, flow rate, capillary-to-collector distance) on the electrospinnability of chitosan/PVA were investigated. Analyses of scanning electron micrographs and transmission electron micrographs suggested that the chitosan/PVA ultrafine fibers were often obtained along with beads, and chitosan was located in the elctrospun fibers as well as in the beads. Uniform chitosan/PVA fibers with an average diameter of 99 ± 21 nm could be prepared from a 7% chitosan/PVA solution in 40:60 mass ratio. Results of Fourier transform infrared spectroscopy and X-ray diffraction demonstrated that there were possible hydrogen bonds between chitosan and PVA molecules, which could weaken the strong interaction in chitosan itself and facilitate chitosan/PVA electrospinnability. The electrospun chitosan/PVA membranes showed higher water uptake and would have potential applications in wound dressings.     

Chitosan/polyvinyl alcohol/amino functionalized multiwalled carbon nanotube pervaporation membranes: Synthesis,characterization, and performance

In the present study, novel biodegradable nanocomposite membranes were prepared by adding the amino functionalized multiwalled carbon nanotube (NH₂‐MWCNT) to the chitosan/polyvinyl alcohol blend polymers, and the obtained membranes were used for dehydration of isopropyl alcohol through pervaporation process. For this purpose, the membranes were prepared with chitosan/polyvinyl alcohol ratio of 4:1 on the basis of “solution casting” method and then crosslinked using glutaraldehyde, after addition of different amounts of NH₂‐MWCNT. The prepared membranes were characterized using scanning electron microscopy, contact angle, mechanical strength, degree of swelling (DS), and biodegradability. Also, the ability of the prepared membranes in dehydration of isopropyl alcohol was determined using pervaporation experiments. Results indicated that contact angle, mechanical resistance, separation factor (α), and pervaporation separation index were increased with the addition of NH₂‐MWCNT up to 10 wt% (relative to the total amount of polymer) and then decreased in the higher presence of nanotubes (15 wt%). Furthermore, the DS and permeate flux were first decreased and then increased for the same mentioned amounts of additive. In this study, optimized membrane was obtained by the addition of 10 wt% NH₂‐MWCNT. This membrane showed the maximum α (99.5), pervaporation separation index parameter (78.29 kg m⁻² h⁻¹), biodegradability, and mechanical stability as well as minimum DS.     

Facile synthesis of multifunctional multiwalled carbon nanotubes/Fe3O4 nanoparticles/polyaniline composite nanotubes

With an average diameter of 100-150 nm, composite nanotubes of polyaniline (PANI)/multiwalled carbon nanotubes (MWNTs) containing Fe₃O₄ nanoparticles (NPs) were synthesized by a two-step method. First, we synthesized monodispersed Fe₃O₄ NPs (d=17.6 nm, σ=1.92 nm) on the surface of MWNTs and then decorated the nanocomposites with a PANI layer via a self-assembly method. SEM and TEM images indicated that the obtained samples had the morphologies of nanotubes. The molecular structure and composition of MWNTs/Fe₃O₄ NPs/PANI nanotubes were characterized by Fourier transform infrared spectra (FTIR), energy dispersive X-ray spectrometry (EDX), X-ray photoelectron spectra (XPS), X-ray diffraction (XRD) and Raman spectra. UV-vis spectra confirmed the existence of PANI and its response to acid and alkali. As a multifunctional material, the conductivity and magnetic properties of MWNTs/Fe₃O₄ NPs/PANI composites nanotubes were also investigated.     

Covalent modification of multiwalled carbon nanotubes with a low molecular weight chitosan

Covalent modification of shortened multiwalled carbon nanotubes (MWNTs) with a natural low molecular weight chitosan (LMCS) was accomplished by the nucleophilic substitution reaction. The LMCS modified MWNTs (MWNT-LMCS) were characterized by FTIR, solid-state 13C NMR, and XPS spectroscopies, thermogravimetric analysis, and transmission electron microscopy. The results revealed that amino and primary hydroxyl groups of the LMCS participated mainly in the formation of the MWNT-LMCS conjugates. The MWNT-LMCS consists of 58 wt.% LMCS, and about four molecular chains of the LMCS were attached to 1000 carbon atoms of the nanotube sidewalls. As a novel derivative of the MWNTs, the MWNT-LMCS not only solved in DMF, DMAc and DMSO, but also in aqueous acetic acid solution.     

基于多壁碳纳米管/壳聚糖多层膜修饰玻碳电极邻苯二酚的测定

改进了碳纳米管在壳聚糖溶液中的分散方法,制备了多壁碳纳米管/壳聚糖多层膜修饰玻碳电极,对比了不同修饰层数膜电极的循环伏安和电化学阻抗行为,5层多壁碳纳米管/壳聚糖膜修饰玻碳电极的电化学性能优良.在最优实验条件下,该修饰玻碳电极对邻苯二酚(CAT)有灵敏的响应,CAT浓度在3.99×10-6～9.09×10-4mol/L范围内与氧化峰电流呈良好的线性关系,检出限为2.39×10-6mol/L(S/N=3).该修饰玻碳电极性能稳定,测定4×10-5mol/LCAT溶液,RSD(n=10)为2.1%;15周后,该电极的响应值仅降低1.9%.     

Preconcentration of diazinon using multiwalled carbon nanotubes as solid-phase extraction adsorbents

A sensitive and selective column adsorption method is proposed for the preconcentration and determination of diazinon. Diazinon was preconcentrated on multiwalled carbon nanotubes (MWCNTs) as an adsorbent and then determined by high-performance liquid chromatography (HPLC). Several parameters on the recovery of the analyte were investigated. The experimental results showed that it was possible to obtain quantitative analysis when the solution pH was 6 using 200 mL of validation solution containing 2 μg of diazinon and 5 mL of acetonitrile as an eluent. Recovery of diazinon was 95.2 ± 4.2% with a relative standard deviation for seven determinations of 4.9% under optimum conditions. The maximum preconcentration factor was 200 for diazinon when 1000 mL of sample solution volume was used. The linear range of calibration curve was 0.3 to 10,000 ng mL^− 1 with a correlation coefficient of 0.997 and the detection limit (3S/N) was 0.06 ng mL^− 1. The proposed method was successfully applied to the determination of diazinon in tap water with high precision and accuracy.     

A thermal degradation mechanism of polyvinyl alcohol/silica nanocomposites

The thermal degradation mechanism of a novel polyvinyl alcohol/silica (PVA/SiO₂) nanocomposite prepared with self-assembly and solution-compounding techniques is presented. Due to the presence of SiO₂ nanoparticles, the thermal degradation of the nanocomposite, compared to that of pure PVA, occurs at higher temperatures, requires more reaction activation energy (E), and possesses higher reaction order (n). The PVA/SiO₂ nanocomposite, similar to the pure PVA, thermally degrades as a two-step-degradation in the temperature ranges of 300-450 °C and 450-550 °C, respectively. However, the introduction of SiO₂ nanoparticles leads to a remarkable change in the degradation mechanism. The degradation products identified by Fourier transform infrared/thermogravimetric analysis (FTIR/TGA) and pyrolysis-gas chromatography/mass spectrometric analysis (Py-GC/MS) suggests that the first degradation step of the nanocomposite mainly involves the elimination reactions of H₂O and residual acetate groups as well as quite a few chain-scission reactions. The second degradation step is dominated by chain-scission reactions and cyclization reactions, and continual elimination of residual acetate groups is also found in this step.     

Fabrication and invitro evaluation of electrospun gum ghatti-polyvinyl alcohol polymeric blend green nanofibre mat (GG-PVA NFM) as a novel material for polymeric scaffolds in wound healing

The present research is mainly based on the fabrication of biodegradable nanofiber mats (NFM) through the process of electrospinning using a novel combination of Gum Ghatti (GG) and Poly vinyl alcohol (PVA). The prepared NFM was crosslinked (CL-1) using Glutaraldeyde-HCl vapours and was characterized for its tensile strength along other analytical characterizations using FTIR, TGA, DSC and XRD. The mechanical strength of the NFM was found to be sufficiently high than in comparison to noncrosslinked sample and PVA NFM. The internal architecture of the CL-1 by use of atomic force microscopy (AFM) revealed that there was very well formed crosslinks suitable for drug loading as well as cell proliferation. The wound healing properties of the CL-1 in mice animal model indicated the healing within 5 days as compared to the control wound. Moreover, the sample was also analysed for its ability as polymeric scaffold and no toxicity was found onto the locally applied tissue on histological investigations.     

Electropolymerized multiwalled carbon nanotubes/polypyrrole fiber for solid-phase microextraction and its applications in the determination of pyrethroids

A novel solid-phase microextraction (SPME) fiber coated with multiwalled carbon nanotubes/polypyrrole (MWCNTs/Ppy) was prepared with an electrochemical method and used for the extraction of pyrethroids in natural water samples. The results showed that the MWCNTs/Ppy coated fiber had high organic stability, and remarkable acid and alkali resistance. In addition, the MWCNTs/Ppy coated fiber was more effective and superior to commercial PDMS and PDMS/DVD fibers in extracting pyrethroids in natural water samples. Under optimized conditions, the calibration curves were found to be linear from 0.001 to 10 μg mL⁻¹ for five of the six pyrethroids studied, the exception being fenvalerate (which was from 0.005 to 10 μg mL⁻¹), and detection limits were within the range 0.12-0.43 ng mL⁻¹. The recoveries of the pyrethroids spiked in water samples at 10 ng mL⁻¹ ranged from 83 to 112%.     

Electrospinning and characterization of poly(vinyl alcohol)/chitosan oligosaccharide/clay nanocomposite nanofibers in aqueous solutions

Submicron fibers of the composite of poly(vinyl alcohol) (PVA), chitosan oligosaccharide [COS, (1→4)2-amino-2-deoxy-β-d-glucose], and montmorillonite clay (MMT) were prepared using electrospinning method with aqueous solutions. Scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), thermal gravimetric analyzer, and tensile strength testing machine (Zwick) were utilized to characterize the PVA/COS/MMT nanofiber mats morphology and properties. The PVA/COS ratio and MMT concentration play important roles in nanofiber mat properties. XRD and TEM data demonstrated that exfoliated MMT layers were well-distributed within nanofiber. It was also found that the mechanical property and thermal stability were increased with COS and MMT contents.     

Determination of fluoroquinolones in bovine milk samples using a pipette‐tip SPE step based on multiwalled carbon nanotubes prior to CE separation

A simple CE–UV method was developed for the simultaneous determination of ciprofloxacin, norfloxacin, and ofloxacin in milk samples. The optimum separation was obtained using a 20 mM ammonium dihydrogenphosphate solution with 2 mM cetyltrimethylammonium bromide at pH 3.0 as the BGE. Satisfactory resolution for structurally very similar analytes, like norfloxacin and ciprofloxacin, was achieved without including any organic solvent. Milk samples were prepared using a simple/extraction procedure based on acidic protein precipitation followed by an SPE step using only 5 mg of multiwalled carbon nanotubes as the sorbent material. The LODs for the three compounds were between 7.5 and 11.6 μg/L and the RSDs for the peak areas were between 2.6 and 4.9%. The complete method was applied to spiked real milk samples with satisfactory recoveries for all analytes (84–106%).     

Application of multiwalled carbon nanotubes as solid phase extraction sorbent for preconcentration of trace copper in water samples

The adsorption behavior of multiwalled carbon nanotubes (MWNTs) toward copper has been investigated systemically, and a new method has been developed for the determination of trace copper in water samples based on preconcentration with a microcolumn packed with MWNTs prior to its determination by flame atomic absorption spectrometry. The optimum experimental parameters for preconcentration of copper, such as pH of the sample, sample flow rate and volume, elution solution and interfering ions, have been investigated. Copper can be quantitatively retained by MWNTs in the pH range 5-8, and then eluted completely with 0.5 M HNO3. The detection limit of this method for Cu was 0.42 ng/mL, and the RSD was 3.5% at the 10 ng/mL Cu level. The method was validated using a certified reference material, and has been successfully applied for the determination of trace copper in water samples.     

Improved kinetics of ethanol oxidation on Pd catalysts supported on tungsten carbides/carbon nanotubes

Pd nanoparticles on tungsten carbides modified multiwalled carbon nanotubes (Pd-WC/MWCNT) catalysts have been prepared by an intermittent microwave heating (IMH) technique for the first time. The Pd-WC/MWCNT catalysts are evaluated and show an improved kinetics for the ethanol oxidation. It is recognized that the significant increase in the catalytic activity for ethanol oxidation on Pd-WC/MWCNT is attributed to both the synergistic effect between Pd nanoparticles and the WC support and the structure effect of the MWCNT. This type catalyst can be universally used as the oxygen reduction catalyst in fuel cells and sensors both in alkaline and acidic solutions.     

Separation of single-stranded oligonucleotides on polyvinyl alcohol gel column

Summary Separation mechanisms for single-stranded oligodeoxyribo-or oligoribonucleic acid fragments were explored on an Asahipak polyvinyl alcohol gel column (GS-320) by use of sequential isomers of such molecules. Substrates having different base numbers were found to be separated by size-exclusion chromatography while those having the same numbers with different base sequences were isolated by use of the reversed-phase mode. By using those dual modes, a limit for the separation of the samples was found to arise because one mode shifted the peaks of the substrates in the sense opposite to the shift resulting from the other mode and it was found that when substrates had less than nine bases, the solutes eluted separately.     

Nucleation, structure and lamellar morphology of isotactic polypropylene filled with polypropylene-grafted multiwalled carbon nanotubes

Polypropylene-based nanocomposites filled with polypropylene-grafted multiwalled carbon nanotubes (PP-g-MWNT) were compared to PP samples filled with pristine MWNT. The effect of such additives on the structure and morphology of the polymer matrix was studied by small angle X-ray scattering (SAXS), wide angle X-ray diffraction (WAXD), polarized light optical microscopy (PLOM) and differential scanning calorimetry (DSC). PP-g-MWNT allowed a more efficient and unhindered crystallization at a lamellar level, while MWNT disrupted the order of lamellar stacks, probably because of their tendency to aggregate. A common trend of tensile properties and lamellar morphology as a function of filler content was noted in the series filled with functionalized carbon nanotubes.     

Synergistic effects and mechanism of multiwalled carbon nanotubes with magnesium hydroxide in halogen-free flame retardant EVA/MH/MWNT nanocomposites

The synergistic effects and mechanism of multiwalled carbon nanotubes (MWNTs) with magnesium hydroxide (MH) in halogen-free flame retardant EVA/MH/MWNT nanocomposites have been studied by cone calorimeter test (CCT), limiting oxygen index (LOI), thermogravimetric analysis (TGA), torque test, morphological evolution experiment, and scanning electron microscopy (SEM). The data obtained from the CCT, LOI, and TGA show that suitable amount of MWNTs has synergistic effects with MH in the EVA/MH/MWNT nanocomposites. The MWNTs can considerably decrease the heat release rates and mass loss rate by about 50-60%, prolongate the combustion time to near two times, and increase the LOI values by 5% when 2 wt% MWNTs substitute for the MH in the EVA/MH/MWNT samples. The TGA data also show that the synergistic effects of MWNTs with MH apparently increase the thermal degradation temperatures and final charred residues of the EVA/MH/MWNT samples. The experimental observations from the torque, morphological evolution tests, and SEM give positive evidences that the synergistic mechanism of MWNTs with MH can be described to: (i) the increase of melt viscosity because of network structure formation of MWNTs in the EVA/MH matrix; (ii) the enhancement of thermo-oxidation stability due to the MWNTs' mechanical strength and integrity of the charred layers in the EVA/MH/MWNT nanocomposites; (iii) the formation of compact charred layers promoted by MWNTs acted as heat barrier and thermal insulation. All the above-mentioned factors efficiently enhance thermal and flame retardant properties and protect the EVA/MH/MWNT nanocomposite materials to be burning.