Influence of polar groups on the wetting properties of vertically aligned multiwalled carbon nanotube surfaces

In this work, we have studied superhydrophilic and superhydrophobic transitions on the vertically aligned multiwalled carbon nanotube (VACNT) surfaces. As-grown, the VACNT surfaces were superhydrophobic. Pure oxygen plasma etching modified the VACNT surfaces to generate superhydrophilic behavior. Irradiating the superhydrophilic VACNT surfaces with a CO₂ laser (up to 50?kW?cm^?2) restored the superhydrophobicity to a level that depended on the laser intensity. Contact angle and surface energy measurements by the sessile drop method were used to examine the VACNT surface wetting. X-ray photoelectron spectroscopy (XPS) showed heavy grafting of the oxygen groups onto the VACNT surfaces after oxygen plasma etching and their gradual removal, which also depended on the CO₂ laser intensity. These results show the great influence of polar groups on the wetting behavior, with a strong correlation between the polar part of the surface energy and the oxygen content on the VACNT surfaces. In addition, the CO₂ laser treatment created an interesting cage-like structure that may be responsible for the permanent superhydrophobic behavior observed on these samples.     

A quantum dot sensitized solar cell based on vertically aligned carbon nanotube templated ZnO arrays

We report on a quantum dot sensitized solar cell (QDSSC) based on ZnO nanorod coated vertically aligned carbon nanotubes (VACNTs). Electrochemical impedance spectroscopy shows that the electron lifetime for the device based on VACNT/ZnO/CdSe is longer than that for a device based on ZnO/CdSe, indicating that the charge recombination at the interface is reduced by the presence of the VACNTs. Due to the increased surface area and longer electron lifetime, a power conversion efficiency of 1.46% is achieved for the VACNT/ZnO/CdSe devices under an illumination of one Sun (AM 1.5G, 100 mW/cm²).     

Growth of vertically aligned carbon nanotubes on carbon fiber: thermal and electrochemical treatments

Composite electrodes of vertically aligned carbon nanotubes (VACNT) were synthesized on carbon fiber (CF) substrate by pyrolysis of camphor/ferrocene using a SiO₂ interlayer as a barrier against metal diffusion into the substrate. Two treatments were used to remove iron from CF/VACNT structure: thermal annealing at high temperature under inert atmosphere and electrochemical oxidation in H₂SO₄ solution. The composites were characterized by scanning electron microscopy and Raman scattering spectroscopy. Besides, the electrochemical behavior of CF/VACNT was analyzed by cyclic voltammetry and charge/discharge tests. CF/VACNT composite submitted to the electrochemical oxidation showed the best electrochemical performance, with high specific capacitance, which makes it very attractive as electrode for supercapacitors.     

Vertically aligned carbon nanotube based electrodes: Fabrication, characterisation and prospects

We present a methodology to fabricate carbon nanotube based electrodes using plasma enhanced chemical vapour deposition. The metal catalyst nanoparticles used to promote nanotube growth are removed using a water plasma treatment in combination with an acid attack. The final integrated microelectrode-based devices present excellent electrocatalytic properties that make them suitable for electrochemical applications. The presented methodology enables the construction of highly regular and dense vertically aligned carbon nanotube (VACNT) forests that can be confined within the patterned bounds of a desired surface. These VACNT electrodes display very low capacitive currents and are amenable to further chemical modifications.     

Biomineralization of superhydrophilic vertically aligned carbon nanotubes

Vertically aligned carbon nanotubes (VACNT) promise a great role for the study of tissue regeneration. In this paper, we introduce a new biomimetic mineralization routine employing superhydrophilic VACNT films as highly stable template materials. The biomineralization was obtained after VACNT soaking in simulated body fluid solution. Detailed structural analysis reveals that the polycrystalline biological apatites formed due to the -COOH terminations attached to VACNT tips after oxygen plasma etching. Our approach not only provides a novel route for nanostructured materials, but also suggests that COOH termination sites can play a significant role in biomimetic mineralization. These new nanocomposites are very promising as nanobiomaterials due to the excellent human osteoblast adhesion.     

Effect of Fusarium oxysporum f. sp. lycopersici on the degradation of humic acid associated with Cu, Pb, and Ni: an in vitro study

The intent of this work was to gain further insight on the fungus-assisted degradation/solubilization of humic acid and the related changes in metal-binding profiles. In the experimental design, Aldrich reagent humic acid (HA) or HA enriched with Cu, Pb, and Ni (HA(Me)) was added to Fusarium oxysporum f. sp. lycopersici cultures in vitro. The cultures were supplied by different carbon- and nitrogen-containing nutrients (glucose, Glc, or glutamate, Glu and ammonium, NH₄⁺, or nitrate, NO₃⁻, ions, respectively) in order to examine their possible effect on HA and HA(Me) decomposition. During the first 48 h of fungus growth, gradual acidification to pH 2 was observed in medium containing Glc + NH₄⁺, while for other cultures, alkalinization to pH 9 occurred and then, the above conditions were stable up to at least 200 h. Size exclusion chromatography (SEC) with UV/Vis detection showed progressive degradation and solubilization of both HA and HA(Me) with the increasing time of fungus growth. However, the molecular mass distributions of HA-related soluble species were different in the presence of metals (HA(Me)) as referred to HA and were also influenced by the composition of growth medium. The solubilization of Pb, Cu, and Ni and their association with HA molecular mass fractions were studied using inductively coupled plasma mass spectrometry (ICP-MS) detection. Under acidic conditions, relatively high concentrations of low-molecular-mass metallic species were found in culture supernatants, while in alkaline media, metal solubilization was generally poorer. In contrast to low pH culture, SEC-ICP-MS results obtained in alkaline supernatants indicated metal binding to degradation products of humic substances of MM > 5 kDa. In summary, the results of this study suggest that fungus-assisted degradation of HA and HA(Me) might be controlled using appropriate N- and C- sources required for fungus growth, which in turn would affect molecular mass distribution of soluble metallic species thus potentially influencing their actual bioaccessibility. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Polyelectrolyte mediated formation of hydroxyapatite microspheres of controlled size and hierarchical structure

The understanding of the role of polyelectrolytes in the synthesis of inorganic materials could provide effective routes towards design of advanced materials. In this study, negatively charged poly(styrene sulfonate) (PSS) is employed as a modifier in hydrothermal synthesis of hydroxyapatite (HA). The results indicate that both the morphology and particle size could be well controlled by adjusting the PSS concentration. The presence of PSS (within the range of 0–9.6 wt%) modified the growth pattern of HA crystallites and results in particles from the ribbons to microspheres. The building units of various microspheres change from nanofibers to nanorods or nanoplates. Along with that, the microspheres become smaller and more compact at higher PSS concentrations. The adsorption of PSS onto certain crystal faces as well as the complexing effect of PSS with Ca²⁺ can be considered as the controlling factors which determine the influence of PSS on the growth mode. The drug release study indicates that the flower-like HA microspheres can be possibly used as effective carriers for biomedical applications. The present synthesis method is simple and controllable, and can provide a convenient route to synthesize uniform HA microspheres with different sizes and hierarchical structures.     

The Influence of Mineral Ions on the Microbial Production and Molecular Weight of Hyaluronic Acid

This study aimed to evaluate the influence of the culture medium supplementation with mineral ions, focusing on the growth of Streptococcus zooepidemicus as well as on the production and average molecular weight (MW) of hyaluronic acid (HA). The ions were investigated in terms of individual absence from the totally supplemented medium (C+) or individual presence in the non-supplemented medium (C−), where C+ and C− were used as controls. Differences between the effects were analyzed using the Tukey's test at p < 0.05. The adopted criteria considered required the ions, whose individual absence attained at 80% or less of the C+ and their individual presence was 20% or more than the C−. The supplementation was either inhibitory or acted in synergy with other ions, when the individual absence or presence was 20% higher than C+ or 20% lower than C−, respectively. Results showed that the effects of C+ or C− were equal for both the production of HA and its yield from glucose. However, C+ showed to be beneficial to cell growth while the individual absence of Na⁺ was beneficial to the production of HA. The highest MW of HA (7.4 × 10⁷ Da) was observed in the individual presence of Na⁺ in spite of the lowest HA concentration (0.65 g.L⁻¹). These results suggest that the quality of HA can be modulated through the mineral ion supplementation.     

The Role of Lignocellulosic Composition and Residual Lipids in Empty Fruit Bunches on the Production of Humic Acids in Submerged Fermentations

The aim of this research was to study the production of humic acids (HA) by Trichoderma reesei from empty fruit bunches (EFBs) of palm oil processing, with a focus on the effects of lignocellulosic content and residual lipids. EFBs from two different soils and palm oil producers were previously characterized about their lignocellulosic composition. Submerged fermentations were inoculated with T. reesei spores and set up with or without residual lipids. The results showed that the soil and the processing for removal of the palm fresh fruits were crucial to EFB quality. Thus, EFBs were classified as type 1 (higher lignocellulosic and fatty acids composition similar to the palm oil and palm kernel oil) and type 2 (lower lignocellulosic content and fatty acids composition similar to palm oil). Despite the different profiles, the fungal growth was similar for both EFB types. HA production was associated with fungal growth, and it was higher without lipids for both EFBs. The highest HA productivity was obtained from type 1 EFB (approximately 90 mg L⁻¹ at 48 h). Therefore, the lignocellulosic composition and the nature of the residual lipids in EFBs play an important role in HA production by submerged fermentation.

     

Removal of heavy metal ions by biogenic hydroxyapatite: Morphology influence and mechanism study

Based on the synthesis of hydroxyapatite (HA) with different morphologies, such as nanorod-like, flower-like and sphere-like assembled HA nanorods, a new strategy has been developed for the removal of heavy metal ions such as Pb²⁺, Cu²⁺, Mn²⁺, Zn²⁺. The dependence of removal efficiency on the morphology and the suspended concentration of trapping agent, the removal time and selectivity were evaluated and discussed. The experimental results proved that the removal capacity of flower-like assembled HA nanorods (NAFL-HA) was the best, and the maximum removal ratio for Pb²⁺ ion was 99.97%. The mechanism of Pb²⁺ removal was studied in detail, noting that some metal ions were completely incorporated into hydroxyapatitie to produce Pb-HA. It reveals that the metal ions capture by HA is mainly controlled by sample surface adsorption and co-precipitation, which are directly controlled by sample morphology.     

Continuous Determination of Trace Amounts of Humic Acid in Natural Water by Chemiluminescence Method

A chemiluminescence (CL) method for the determination of humic acid (HA) based on the oxidation of HA with hydrogen peroxide in the presence of formaldehyde in alkaline solution is described. This method is sensitive and selective for the determination of HA in natural water. HA produces strong CL in the oxidation of HA with MnO₄⁻, Br₂, ClO⁻, and Cr₂O₇²⁻, and the H₂O₂. HA-H₂O₂-HCHO system is suitable for the determination of HA because of its high sensitivity and high selectivity. The detection limit was 50 ppb and relative standard deviation for five measurements of 0.5 ppm (w/w) HA was 1.8%. Cations such as Na⁺, K⁺, Mg²⁺, Cu²⁺, and Fe3⁺ and anions such as PO₄³⁻, NO₃⁻, CO₃²⁻, SO₄²⁻, Cl⁻, and Y⁻ (EDTA-Na) did not interfere with the determination of HA. Addition of Mn(II) increased the CL intensity. The concentration of HA in natural water determined with this method is in good agreement with that determined by fluorometric analysis.     

Characterization of MX-80 bentonite and its sorption of radionickel in the presence of humic and fulvic acids

MX-80 bentonite is considered as one of the best backfill materials for high-level radioactive nuclear waste. Herein, the bentonite is characterized by using XRD and FTIR techniques. Sorption of radionickel to MX-80 bentonite in the presence/absence of humic acid (HA) or fulvic acid (FA) as a function of pH is investigated. The results indicate that the presence of HA or FA decreases the sorption of Ni²⁺ obviously. The different experimental processes do not affect the sorption of nickel to FA/HA bound bentonite. The sorption of Ni²⁺ on FA/HA-bound bentonite decreases with the increasing FA/HA content in the systems. The mechanism of nickel sorption is also discussed in detail.     

Binding constants of lead by humic and fulvic acids studied by anodic stripping square wave voltammetry

A better understanding of metal ion binding to soil organic substances is of fundamental importance in geochemical modeling of environments. Fulvic acids (FA) and humic acids (HA) make up an important part of soil organic matter, and their binding capacity affects the fate of metal ions and plays an important role in their mobility. Binding constants of Pb(II) to HA and FA were evaluated by anodic stripping square wave voltammetry (ASSWV) where the binding reaction was performed at pH 4.5 in a medium of 0.020 mol l⁻¹ KNO₃. Results showed that ASSWV technique was well suited for the estimation of the binding capacity of a natural organic matter towards heavy metals. Based on the voltammetric titration curves, binding constants of Pb(II) complexes formed with HA and FA were 0.78 × 10⁶ and 0.15 × 10⁶ mol⁻¹ l, which indicated that complex of Pb²⁺ with HA was more stable than with FA. The average molecular weight of HA and FA prepared from soil samples were also found to be 1821 g mol⁻¹ and 805 g mol⁻¹, respectively.     

Prediction of the binding sites of huperzine A in acetylcholinesterase by docking studies

Summary We have performed docking studies with the SYSDOC program on acetylcholinesterase (AChE) to predict the binding sites in AChE of huperzine A (HA), which is a potent and selective, reversible inhibitor of AChE. The unique aspects of our docking studies include the following: (i) Molecular flexibility of the guest and the host is taken into account, which permits both to change their conformations upon binding. (ii) The binding energy is evaluated by a sum of energies of steric, electrostatic and hydrogen bonding interactions. In the energy calculation no grid approximation is used, and all hydrogen atoms of the system are treated explicitly. (iii) The energy of cation- interactions between the guest and the host, which is important in the binding of AChE, is included in the calculated binding energy. (iv) Docking is performed in all regions of the host's binding cavity. Based on our docking studies and the pharmacological results reported for HA and its analogs, we predict that HA binds to the bottom of the binding cavity of AChE (the gorge) with its ammonium group interacting with Trp⁸⁴, Phe³³⁰, Glu¹⁹⁹ and Asp⁷² (catalytic site). At the the opening of the gorge with its ammonium group partially interacting with Trp²⁷⁹ (peripheral site). At the catalytic site, three partially overlapping subsites of HA were identified which might provide a dynamic view of binding of HA to the catalytic site.     

Sorption behavior of Co(II) on γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> in the presence of humic acid

The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The extent of sorption of divalent metal cations is controlled by a number of factors including cosorbing or complexing. In this work, the effects of pH, humic acid HA/Co(II) addition orders, ionic strength, concentration of HA, and foreign cations on the Co(II) sorption on γ-Al₂O₃ in the presence of HA were investigated. The sorption isotherms of Co(II) on γ-Al₂O₃ in the absence and presence HA were also studied and described by using S-type sorption model. The experimental results showed that the Co(II) sorption is strongly dependent on the pH values, concentration of HA, but independent of HA/Co(II) addition orders, ionic strength, and foreign cations in the presence of HA under our experimental conditions. The results also indicated that HA enhanced the Co(II) sorption at low pH, but reduced the Co(II) sorption at high pH. It was hypothesized that the significantly positive influence of HA at low pH on the Co(II) sorption on γ-Al₂O₃ was attributed to strong surface binding of HA on γ-Al₂O₃ and subsequently the formation of ternary surface complexes such as ≡S-OOC-R-(COO⁻) _x Co^2−x . Chemi-complexation may be the main mechanism of the Co(II) sorption on γ-Al₂O₃ in the presence of HA.     

Highly sensitive determination of hydroxylamine using fused gold nanoparticles immobilized on sol-gel film modified gold electrode

We are reporting the highly sensitive determination of hydroxylamine (HA) using 2-mercapto-4-methyl-5-thiazoleacetic acid (TAA) capped fused spherical gold nanoparticles (AuNPs) modified Au electrode. The fused TAA-AuNPs were immobilized on (3-mercaptopropyl)-trimethoxysilane (MPTS) sol-gel film, which was pre-assembled on Au electrode. The immobilization of fused TAA-AuNPs on MPTS sol-gel film was confirmed by UV-vis absorption spectroscopy and atomic force microscopy (AFM). The AFM image showed that the AuNPs retained the fused spherical morphology after immobilized on sol-gel film. The fused TAA-AuNPs on MPTS modified Au electrode were used for the determination of HA in phosphate buffer (PB) solution (pH = 7.2). When compared to bare Au electrode, the fused AuNPs modified electrode not only shifted the oxidation potential of HA towards less positive potential but also enhanced its oxidation peak current. Further, the oxidation of HA was highly stable at fused AuNPs modified electrode. Using amperometric method, determination of 17.5 nM HA was achieved for the first time. Further, the current response of HA increases linearly while increasing its concentration from 17.5 nM to 22 mM and a detection limit was found to be 0.39 nM (S/N = 3). The present modified electrode was also successfully used for the determination of 17.5 nM HA in the presence of 200-fold excess of common interferents such as urea, NO₂⁻, NH₄⁺, oxalate, Mn²⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺ and Cu²⁺. The practical application of the present modified electrode was demonstrated by measuring the concentration of HA in ground water samples.     

Electrogenerated Chemiluminescence of Tris(2,2′‐bipyridyl)ruthenium(II) Immobilized in Humic Acid‐Silica‐Poly(vinyl alcohol) Composite Films

In this paper, we report the first attempt to use humic acid (HA) as modifiers to prepare the organic‐inorganic hybrid modified glassy carbon electrodes based on HA‐silica‐PVA (poly(vinyl alcohol)) sol‐gel composite. Electroactive species of tris(2,2′‐bipyridyl)ruthenium(II) (Ru(bpy) ) can easily incorporate into the HA‐silica‐PVA films to form Ru(bpy) modified electrodes. The amount of Ru(bpy) incorporated in the composite films strongly depends on the amount of HA in the hybrid sol. Electrochemical and electrogenerated chemiluminescence (ECL) of Ru(bpy) immobilized in HA‐silica composite films coated on a glassy carbon electrode have been studied with tripropylamine (TPA) as the coreactant. The analytical performance of this modified electrode was evaluated in a flow injection analysis (FIA) system with a homemade flow cell. The as‐prepared electrode showed good stability and high sensitivity. The detection limits (S/N=3) were 0.050 μmol L^?1 for TPA and 0.20 μmol L^?1 for oxalate, and the linear ranges were from 0.10 μmol L^?1 to 1.0 mmol L^?1 for TPA and from 1.0 μmol L^?1 to 1.0 mmol L^?1 for oxalate, respectively. The resulting electrodes were stable over two months.     

Self‐diffusion in a hyaluronic acid–albumin–water system as studied by NMR

Hyaluronic acid (HA) is an anionic biopolymer that is present in many tissues and can be involved in cancerous neoformations. HA can form complexes with proteins (particularly, serum albumin) in the body. However, HA structures and processes involving HA have not been extensively studied by NMR because the molecule's rigid structure makes these studies problematic. In the current work, self‐diffusion of HA and bovine serum albumin (BSA), and water in solutions was measured by ¹H pulsed field gradient NMR (PFG NMR) with a focus on the HA‐BSA‐D₂O systems at various concentrations of BSA and HA. It was shown that in the presence of even a small amount of HA, the self‐diffusion coefficient (SDC) of BSA decreases. To explain this fact, three hypotheses were proposed and analyzed. The first one was based on the effect of slowing down of water mobility in the presence of HA. The second hypothesis suggested an effect of mechanical collisions of BSA with HA molecules. The third hypothesized that BSA and HA molecules form a complex where BSA molecules reduced in mobility. It was shown that the third mechanism is the most likely. The state of the BSA molecules in the BSA‐HA‐D₂O system corresponds to a ‘fast exchange’ condition from the NMR point of view: BSA molecules reside in the ‘free’ and ‘bound’ (with HA) states for much shorter time than the diffusion time of the PFG NMR experiment, 7 ms. The fractions of ‘bound’ BSA molecules in the BSA‐HA complex were estimated. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of Charge and Structure of Hyaluronic Acid on the Luminescence Quenching in Aqueous Solution

The quenching of electronically excited Ru(bpy)₃²⁺ (bpy = Tris-2,2′-bipyridine) by methylviologen (MV) and ferricyanide (FC) in aqueous solutions of hyaluronic acid (HA) was studied. The structural and viscosity changes occuring with increasing HA concentration were found to influence the photophysical and photochemical properties of the sensitizer. Different kinetic models had to be used for the quenchers studied. The kinetics of the quenching of *Ru(bpy)₃²⁺ by MV can be described by the pseudophase model, which indicates that the rate for the exchange of the quencher between the microdroplets is higher than that for the excited state decay of the Ruthenium complex. In contrast, the quenching by the negatively charged quencher, FC, can be described by the Infelta-Tachiya equation, which indicates that the distribution of this quencher on the aqueous microdroplets is of the Poisson type and there is no exchange of quencher molecules during the lifetime of the sensitizer. The lifetimes of the excited Ruthenium complex, the unimolecular constants for its quenching by FC and the average concentration of the aqueous microdroplets increase with increasing HA concentration, reflecting the change in the solution structure during the transition from semidilute to concentrated regions. For MV no significant dependence of the quenching constant on the HA content of the solution was found. The reaction behavior of charged reactants in HA solution depends strongly on the sign of the charge.