Effect of dimensions of multi-walled carbon nanotubes on its enrichment efficiency of metal ions from environmental waters

The effect of dimensions (length and external diameter) of multi-walled carbon nanotubes (MWCNTs) on its preconcentration efficiency towards some metal ions (Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺ and MnO₄⁻) from environmental waters prior to their analysis by flame atomic absorption spectroscopy (FAAS) was investigated. MWCNTs (as-received from the manufacturer) of various external diameters and lengths were involved. Other variables optimized included effects of pH of water sample, composition and volume of eluent, mass of the MWCNTs, breakthrough volume and coexisting ions. Maximum recovery of metal ions was obtained at pH 9 where it was thought that precipitation of metals as their hydroxides played the major factor in metals uptake by MWCNT. It was suggested that the use of appropriate dimensions of MWCNTs may support the trapping process of the precipitated metal hydroxides by MWCNTs. It was found that long MWCNT of length 5-15 μm and external diameter 10-30 nm gave the highest enrichment efficiency towards almost all the targeted metal ions. It could be used for preconcentration of MnO₄⁻, Cu²⁺, Zn²⁺ and Pb²⁺ with almost full recovery; but not for Cd²⁺ due to its low recovery. The optimized solid phase extraction (SPE) procedure was capable of determining metal ions in the linear range 20-100 ng mL⁻¹ (except for Zn²⁺ from 20 to 150 ng mL⁻¹). Detection limits were 0.709 ng mL⁻¹ for MnO₄⁻, 0.278 ng mL⁻¹ for Pb²⁺, 0.465 ng mL⁻¹ for Cu²⁺, 0.867 ng mL⁻¹ for Zn²⁺. Application of the optimized SPE procedure to environmental waters (tap water, reservoir water and stream water) gave spike recoveries of the metals in the range of 81-100%.     

Critical evaluation and comparison of enrichment efficiency of multi-walled carbon nanotubes, C18 silica and activated carbon towards some pesticides from environmental waters

In this work, optimization of multi-residue solid phase extraction (SPE) procedures coupled with high-performance liquid chromatography for the determination of Propoxur, Atrazine and Methidathion from environmental waters is reported. Three different sorbents were used in this work: multi-walled carbon nanotubes (MWCNTs), C18 silica and activated carbon (AC). The three optimized SPE procedures were compared in terms of analytical performance, application to environmental waters, cartridge re-use, adsorption capacity and cost of adsorbent. Although the adsorption capacity of MWCNT was larger than AC and C18, however, the analytical performance of AC could be made close to the other sorbents by appropriate optimization of the SPE procedures. A sample of AC was then oxidized with various oxidizing agents to show that ACs of various surface properties has different enrichment efficiencies. Thus researchers are advised to try AC of various surface properties in SPE of pollutants prior to using expensive sorbents (such as MWCNT and C18 silica).     

The cytotoxicity of oxidized multi-walled carbon nanotubes on macrophages

Carbon nanotubes (CNTs) have been developed for medical and biotechnological applications in the past decades. Their widespread applications make it important to understand their potential hazards to human and the environment. In this study, the possible toxicological effects of the oxidized multi-walled carbon nanotubes (O-MWCNTs) were assessed on RAW 264.7 macrophages in vitro. Several toxicological endpoints, such as cell viability, the release of LDH and IL-8, GSH/GSSG ratio, intracellular calcium concentration and ultrastructural changes in cell morphology, were carried out. The results showed that O-MWCNTs had very limited effects on oxidative stress, cellular toxicity and apoptosis. Transmission electron microscope clearly demonstrates RAW 264.7 macrophages engulfed plenty of O-MWCNTs, and some of them resided in the cytoplasm, while the morphology was not altered by O-MWCNTs. As the control, the pristine MWCNTs (p-MWCNTs) show higher cytotoxicity than O-MWCNTs, damaging cell viability and inducing cell apoptosis. All these toxicological data are of benefit to more wide applications of O-MWCNTs in the future.     

Competitive adsorption of aqueous metal ions on an oxidized nanoporous activated carbon

Competitive adsorption is the usual situation in real applications, and it is of critical importance in determining the overall performance of an adsorbent. In this study, the competitive adsorption characteristics of all the combinations of binary mixtures of aqueous metal ion species Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) on a functionalized activated carbon were investigated. The porous structure of the functionalized active carbon was characterized using N2 (77 K) and CO2 (273 K) adsorption. The surface group characteristics were examined by temperature-programmed desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, acid/base titrations, and measurement of the point of zero charge (pHpzc). The adsorption of aqueous metal ion species, M2+(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism. The ratios of protons displaced to the amount of M2+(aq) metal species adsorbed have a linear relationship for both single-ion and binary mixtures of these species. Hydrolysis of metal species in solution may affect the adsorption, and this is the case for adsorption of Hg2+(aq) and Pb2+(aq). Competitive adsorption decreases the amounts of individual metal ions adsorbed, but the maximum amounts adsorbed still follow the order Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq) obtained for single metal ion adsorption. The adsorption isotherms for single metal ion species were used to develop a model for competitive adsorption in binary mixtures, involving exchange of ions in solution with surface proton sites and adsorbed metal ions, with the species having different accessibilities to the porous structure. The model was validated against the experimental data.     

The conversion of polyaniline nanotubes to nitrogen-containing carbon nanotubes and their comparison with multi-walled carbon nanotubes

Polyaniline (PANI) nanotubes were prepared by the oxidation of aniline in solutions of acetic or succinic acid, and subsequently carbonized in a nitrogen atmosphere during thermogravimetric analysis running up to 830 °C. The nanotubular morphology of PANI was preserved after carbonization. The molecular structure of the original PANI and of the carbonized products has been analyzed by FTIR and Raman spectroscopies. Carbonized PANI nanotubes contained about 8 wt.% of nitrogen. The molecular structure, thermal stability, and morphology of carbonized PANI nanotubes were compared with the properties of commercial multi-walled carbon nanotubes.     

Removal of oxidation debris from multi-walled carbon nanotubes

Conventional liquid phase oxidation of multiwall carbon nanotubes (MWCNTs) using concentrated acids generates contaminating debris that should be removed using aqueous base before further reaction.     

Surface functionalization of oxidized multi-walled carbon nanotubes: Candida rugosa lipase immobilization

This work examines two approaches for immobilization of lipase from Candida rugosa on oxidized multi-walled carbon nanotubes (o-MWCNTs). One method included the presence of activating agents to promote covalent bonding and the other the adsorption on o-MWCNTs to elucidate if non-specific bonding on the o-MWCNTs surface exists. The influence of the immobilization time and initial enzyme concentration on protein loading and the expressed lypolitic activity of the immobilized preparation were investigated. The results showed that the enzyme adsorbs on o-MWCNTs in a maximal amount of 37 μg mg⁻¹ CNTs, while the attached amount was more than 2-times higher under covalent promoting conditions (80 μg mg⁻¹ CNTs). Furthermore, similar trends were observed for the lypolitic activity, whereby preparations obtained under covalent promoting conditions had almost 3-times higher activity (560 IU g⁻¹ of immobilized enzyme). In addition, immobilization of the enzyme was confirmed by Fourier transformation infrared spectroscopy and thermogravimetric analysis.     

磺胺在碳纳米管修饰玻碳电极上的电化学行为

磺胺类抗菌药是一类允许在饲料中添加的兽用广谱抗菌药.它被广泛用于治疗家畜呼吸道、消化道细菌感染、猪萎缩性鼻炎、禽霍乱、伤寒等疾病[1].停药期用药或用药不当将导致动物食品中抗菌药残留超标.人们长期食用含磺胺类抗菌药残留超标的动物产品,将导致肝肾损伤和体内耐药菌株产生,危害到人们的身体健康和疾病治疗.     

Interaction of multi-walled carbon nanotubes and zinc ions enhances cytotoxicity of zinc ions

More and more nanomaterials enter the environment along with their production, application and deposal. They may alter the biological effect of pollutants already existing in the real environment by different interactions. Therefore efforts should also be paid to investigate the combined toxicity of nanomaterials and pollutants. Herein, we studied the combined toxicity of oxidized multi-walled carbon nanotubes (O-MWCNTs) and zinc ions on cells. It is found that cytotoxicity of the combined O-MWCNTs and zinc ions elevates significantly, compared with O-MWCNTs or zinc ions alone. This result comes from the assays of cell morphology, cell viability and proliferation, cell membrane integrity, mitochondrial membrane potential and cell apoptosis. Mechanism studies indicate that O-MWCNTs absorb zinc ions and form slight aggregation. These enhance remarkably the cellular uptake of O-MWCNTs, and thus induce the death of cells by bringing in more zinc ions into cells. Our study indicates that the existence of nanomaterials could change the bioconsequence of other pollutants and emphasizes the importance of the combined toxicity research in the presence of nanomaterials.     

Electrocatalytic oxidation of NADH based on polyluminol and functionalized multi-walled carbon nanotubes

This work presents that the electrocatalytic oxidation of NADH can be enhanced by the hybrid composites of polyluminol and functionalized multi-walled carbon nanotubes (MWCNTs). The hybrid composites can be easily prepared by the electropolymerization of luminol and the adsorption of functionalized MWCNTs. The modified electrode exhibits two redox couples which can show two electrocatalytic peaks at about 0.1 and 0.3 V (vs. Ag/AgCl) to NADH oxidation. The kinetic constant, k(kin), for the electrocatalytic oxidation of NADH, evaluated by chronoamperometry and voltammetry using a rotating disk electrode (RDE), provided values close to 10(5) M(-1) s(-1). At an applied potential of 0.1 V, the sensor provides a linear response range for NADH from 5 × 10(-6) up to 1.5 × 10(-4) M with a sensitivity of 183.9 μA mM(-1) cm(-2), and detection and quantification limits of 0.6 and 5 μM (S/N = 3), respectively.     

Voltammetric oxidation and determination of cinnarizine at glassy carbon electrode modified with multi-walled carbon nanotubes

We discovered a novel method to prepare a protein-based hydrogel, that is, a “Three-Dimensional Nanostructured Protein Hydrogel (3D NPH)”, which is composed of protein–polymer hybrid nanoparticles. In this study, we propose a novel protein microarray whose 3D NPH spots were prepared by dispensing a small volume of the solution of protein–polymer mixture on a substrate. The dispensed solution had a short time for cross-linking before its drying-up and the resulting 3D NPH had loosely cross-linked, thin spongy structure. Therefore, the reaction ratio between ligands and analytes was drastically improved in this system compared with the large volume system for Surface Plasmon Resonance (SPR) protein microarray.     

Electrochemical oxidation of sulfur-containing amino acids on an electrode modified with multi-walled carbon nanotubes

MWNT-modified electrodes are introduced for the voltammetric determination of sulfur-containing amino acids. The morphology of the electrode surface has been characterized by atomic force microscopy. The MWNT layer consists of deeply intertwined vermicular structures with the average diameter of 25 nm. Cysteine, glutathione and methionine are oxidized on the electrode while only cysteine gives signals on the glassy carbon (GC) electrode. The application of such electrodes leads to a decreased overpotential and increase of oxidation currents for cysteine in comparison with a bare GC electrode. The schemes of oxidation are proposed. A decrease of the lower limit of determination and an enlargement of the analytical range for antioxidants were obtained. A simple, fast and accurate procedure for the voltammetric determination of methionine in pharmaceuticals has been developed and can be recommended for quality control purposes.     

Transmission electron microscopy study of multi-walled carbon nanotubes of different morphology oxidized with nitric acid

Oxidation of conical and cylindrical carbon nanotubes (CNTs) was studied by physicochemical methods including high-resolution transmission electron microscopy. Differences in mechanisms of oxidation of these CNTs were revealed. The oxidation of conical CNTs with nitric acid first results in the formation of oxygen-containing groups uniformly distributed over the CNT surface, and then the carbon material undergoes fragmentation and destruction. The treatment of cylindrical CNTs with nitric acid results in oxidation at defect sites followed by a decrease in the tube thickness and a change in the pore structure of the carbon material.     

Preparation of metal wire supported solid-phase microextraction fiber coated with multi-walled carbon nanotubes

Multi-walled carbon nanotubes-coated solid-phase microextraction fiber was prepared by a novel protocol involving mussel-adhesive-protein-inspired polydopamine film. The polydopamine was used as binding agent to immobilize amine-functionalized multi-walled carbon nanotubes onto the surface of the stainless steel wire via Michael addition or Schiff base reaction. Surface properties of the fiber were characterized by field emission scanning electron microscope and X-ray photoelectron spectroscope. Six phenols in aqueous solution were used as model compounds to investigate the extraction performance of the fiber and satisfactory results were obtained. Limit of detection was 0.10 μg/L for 2-methylphenol (2-MP) and 4-methylphenol (4-MP), and 0.02 μg/L for 2-ethylphenol (2-EP), 4-ethylphenol (4-EP), 2-tert-butylphenol (2-t-BuP), and 4-tert-butylphenol (4-t-BuP), which were much lower than commercial fiber and fibers made in laboratory. RSDs for one unique fiber are in the range of 1.92-7.00%. Fiber-to-fiber (n=3) reproducibility ranges from 4.44 to 8.41%. It also showed very high stability and durability to acid, alkali, organic solvent, and high temperature. Real water sample from Yellow river was applied to test the reliability of the established solid-phase microextraction (SPME)-GC method and recoveries with addition level at 5 and 100 μg/L were in the range from 81.5 to 110.0%.     

Effect of oxidation treatment of multi-walled carbon nanotubes on the adsorption of pentachlorophenol from aqueous solution: Kinetics study

Multi-walled carbon nanotubes (MWCNTs) were oxidized using different oxidizing agents and the produced oxidized MWCNTs were characterized using different techniques. IR measurements showed the presence of carboxylic acid function groups especially for the MWCNTs oxidized with nitric acid and hydrogen peroxide. The adsorption of pentachlorophenol (PCP) to pristine and oxidized multi-walled carbon nanotubes (MWCNTs) has been studied. The results showed that the oxidation of the MWCNTs decreased their abilities to adsorb PCP compared with the pristine MWCNTs. The adsorption was studied kinetically and the results showed that the adsorption process occurs in two different steps. The first step involves the transfer of PCP to the surface of the oxidized MWCNTs, which was very fast due to the diffusion of PCP from the liquid phase to the solid phase. This step followed by a second slower step of adsorption could be due to intra-particle diffusion.     

Electrocatalytic oxidation of hydrazine at a glassy carbon electrode modified with nickel ferrite and multi-walled carbon nanotubes

A hydrothermal technique was used to synthesize nickel ferrite nanoparticles (NF-NPs) deposited on multi-walled carbon nanotubes (MWCNTs). The material was characterized by scanning electron microscopy, energy dispersive spectrometry, and X-ray powder diffraction which showed that the NF-NPs are located on the surface of the carboxylated MWCNTs. The material was used to modify a glassy carbon electrode which then was characterized via cyclic voltammetry, electrochemical impedance spectroscopy, and amperometry. The electrode displays strong electrochemical response to hydrazine. A potential hydrazine sensing scheme is suggested.

Electrocatalytic oxidation of ethylene glycol on Pt and Pt-Ru nanoparticles modified multi-walled carbon nanotubes

The synthesis and characterization of catalysts based on nanomaterials, supported on multi-walled carbon nanotubes (CNT) for ethylene glycol (EG) oxidation is investigated. Platinum (Pt) and platinum-ruthenium (Pt-Ru) nanoparticles are deposited on surface-oxidized multi-walled carbon nanotubes [Pt/CNT; Pt-Ru/CNT] by the aqueous solution reduction of the corresponding metal salts with glycerol. The electrocatalytic properties of the modified electrodes for oxidation of ethylene glycol in acidic solution have been studied by cyclic voltammetry (CV), and excellent activity is observed. This may be attributed to the small particle size of the metal nanoparticles, the efficacy of carbon nanotubes acting as good catalyst support and uniform dispersion of nanoparticles on CNT surfaces. The nature of the resulting nanoparticles decorated multiwalled carbon nanotubes are characterized by scanning electron microscopy (SEM) and transmission electron microscopic (TEM) analysis. The cyclic voltammetry response indicates that Pt-Ru/CNT catalyst displays a higher performance than Pt/CNT, which may be due to the efficiency of the nature of Ru species in Pt-Ru systems. The fabricated Pt and Pt-Ru nanoparticles decorated CNT electrodes shows better catalytic performance towards ethylene glycol oxidation than the corresponding nanoparticles decorated carbon electrodes, demonstrating that it is more promising for use in fuel cells.     

Fullerenes inside carbon nanotubes and multi-walled carbon nanotubes: optimum and maximum sizes

The potential energies of interaction between carbon nanotubes and internal fullerenes of spherical and ellipsoidal shape, as well as between nanotubes in multi-walled nanotubes were calculated using the Lennard–Jones (LJ) potential for carbon–carbon interactions. The optimum and maximum size of internal fullerenes and multi-walled nanotubes are determined as a function of the external nanotube radius. It was found that at the potential energy minimum, the van der Waals distance is close to that in graphite for all studied cases. The calculated results agree with available experimental observations and could be used as a guide for future experiments.