Catalytic oxidation of thiols at preheated glassy carbon electrode modified with abrasive immobilization of multiwall carbon nanotubes: applications to amperometric detection of thiocytosine, l-cysteine and glutathione

The performance of preheated glassy carbon electrode modified with carbon nanotubes is described. First glassy carbon electrode is heated for 5 min at 50 °C, then abrasive immobilization of multiwall carbon nanotubes on a preheated glassy carbon electrode was achieved by gentle rubbing of electrode surface on a filter paper supporting carbon nanotubes. Carbon nanotubes (CNTs)-modified glassy carbon electrodes exhibit strong and stable electrocatalytic response toward thiols oxidation in wide pH range. These properties permit an important decrease in over voltage for the oxidation of thiocytosine, glutathione and l-cysteine, as well as a dramatic increase in the peak currents in comparison with bare glassy carbon electrode. Furthermore, the thiols amperometric response of the coated electrodes is extremely stable, with more than 95% of the initial activity after 30 min stirring of 0.1 mM thiols. The electrocatalytic behavior is further exploited as a sensitive detection scheme for thiols detection by hydrodynamic amperometry. The substantial decrease in the overvoltage of the thiols oxidation associated with a stable amperometric response and antifouling properties of nanotubes films allow the development of highly sensitive thiols sensor without using any redox mediator. Such ability of carbon nanotubes to promote the thiols electron transfer reaction, short response time (5 s) and long-term stability, low detection limit, extended linear concentration range, high sensitivity suggest great promise for thiols amperometric sensors and detector for chromatographic analysis of thiol derivatives.     

Synthesis and electrochemical characterization of sol–gel-derived RuO2/carbon nanotube composites

Ruthenium oxide was coated on multiwalled carbon nanotubes (MWCNTs) to obtain nanocomposite electrode which has a good response to the pH. To synthesize this electrode, gold and cobalt were coated on a stainless steel 304 substrates, respectively, and then, vertically aligned carbon nanotubes were grown on the prepared substrates by chemical vapor deposition. Gold reduced activity of the stainless steel, while cobalt served as a catalyst for growth of the carbon nanotube. Ruthenium oxide was then coated on MWCNTs via sol–gel method. At last, different techniques were used to characterize the properties of synthesized electrode including scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction, and cyclic voltammetry. SEM results showed that the length of the carbon nanotubes varied with reaction time, and in this research, it was maintained around 9 μm with a diameter about 100 nm. Electrochemical analysis revealed that optimum sol concentration and heat treatment temperature to meet the best pH sensing response were 0.1 M RuCl₃ sol and 200 °C, respectively. Moreover, the obtained electrode represented a linear and near-Nernstian response (about ?63 mV/pH) throughout the whole pH range (2–12) of Britton–Robinson buffer solutions.     

Electrocatalytic Oxidation of Catechol at Multi‐Walled Carbon Nanotubes Modified Electrode

In 0.05 mol/L phosphate buffer solution (pH 7.0), carbon nanotubes modified electrode exhibits rapid response, strong catalytic activity with high stability toward the electrochemical oxidation of catechol. The electrochemical behavior of catechol on both the multi‐walled and single‐walled carbon nanotubes modified electrode was investigated. The experimental conditions, such as pH of the solution and scan rate were optimized. The currents (measured by constant potential amperometry) increase linearly with the concentrations of catechol in the range of 2.0×10^?5–1.2×10^?3 mol/L. Moreover, at the multi‐walled carbon nanotubes modified electrode the electrochemical responses of catechol and ascorbic acid can be separated clearly.     

减压化学气相沉积法制备规则螺旋状纳米碳管

以硅胶负载的Co纳米颗粒为催化剂,在低流量的减压体系中通过化学气相沉积法制备了规则螺旋状的纳米碳管.通过TEM和HRTEM研究了螺旋碳管的形貌、尺度分布以及管身、曲面和节点处的晶型;讨论了催化剂制备中pH值对催化剂的尺寸、规则程度和存在形态以致对螺旋碳管产量、管径厚度以及螺旋管的相对比例的影响;此外,还分析了反应压力对碳管生长的影响.     

Abrasively immobilised multiwalled carbon nanotube agglomerates: a novel electrode material approach for the analytical sensing of pH.

We demonstrate for the first time that agglomerates of multiwalled carbon nanotubes (MWCNTs) can be formed in which the binder in the agglomerate is itself a redox-active molecular solid. Two separate agglomerates were formed by dissolving 9,10-phenanthraquinone (PAQ) or 1,2-napthaquinone (NQ) in acetone together with MWCNTs and adding an excess of aqueous solution to cause precipitation of agglomerates, approximately 10 microns in dimension, which consist of bundles of nanotubes running into and throughout the amorphous molecular solid that binds the agglomerate together. The nature of this structure, when immobilised on a substrate electrode and in contact with aqueous electrolyte solutions, gives rise to many three-phase boundaries, electrolyte|agglomerate|conductor, which is advantageous to the solid-state analytical electrochemistry of such a material as it imparts a larger electroactive surface area than other modified carbon electrodes. The two agglomerates each gave a voltammetrically measurable response to changes in pH; when abrasively immobilised on a basal plane pyrolitic graphite electrode a plot of peak potential against pH produced a linear response for both MWCNT-PAQ and MWCNT-NQ agglomerates over the pH range pH 1-12 and over the temperature range 20-70 degrees C.     

碳纳米管电极电催化氧化降解活性艳红X-3B研究

将催化裂解法制得的碳纳米管,先经稀酸分散和空气热氧化处理,再按一定比例加入酚醛树脂,高速球磨处理后,将碳纳米管电极片热压成型.其表面形貌由扫描电镜(SEM)表征.以碳纳米管电催化电极作阳极,不锈钢片为阴极,降解处理含活性艳红X-3B模拟印染废水.考察了电流密度、电解质浓度、体系反应温度及pH值对废水降解效率的影响.结果表明:酸性条件下有利于染料有机物降解反应的进行,于常温下即可作高效降解处理.     

Electrocatalytic reduction of NAD+ at glassy carbon electrode modified with single-walled carbon nanotubes and Ru(III) complexes

A simple procedure was developed to prepare a glassy carbon electrode modified with carbon nanotubes and Ruthenium (III) complexes. First, 25 μl of dimethyl sulfoxide–carbon nanotubes solutions (0.4 mg/ml) was cast on the surface of the glassy carbon electrode and dried in air to form a carbon nanotube film at the electrode surface. Then, the glassy carbon/carbon nanotube-modified electrode was immersed into a Ruthenium (III) complex solution (direct deposition) for a short period of time (10–20 s for multiwalled carbon nanotubes and 20–40 s for single-walled carbon nanotubes). The cyclic voltammograms of the modified electrode in aqueous solution shows a pair of well-defined, stable, and nearly reversible redox couple, Ru(III)/Ru(II), with surface-confined characteristics. The attractive mechanical and electrical characteristics of carbon nanostructures and unique properties and reactivity of Ru complexes are combined. The transfer coefficient (α), heterogeneous electron transfer rate constants (k _s), and surface concentrations (Γ) for the glassy carbon/single-walled carbon nanotubes/Ru(III) complex-, glassy carbon/multiwalled carbon nanotubes/Ru(III) complex-, and glassy carbon/Ru(III) complex-modified electrodes were calculated using the cyclic voltammetry technique. The modified electrodes showed excellent catalytic activity, fast response time, and high sensitivity toward the reduction of nicotinamide adenine dinucleotide in phosphate buffer solutions at a pH range of 4–8. The catalytic cathodic current depends on the nicotinamide adenine dinucleotide concentration. In the presence of alcohol dehydrogenase, the modified electrode exhibited a response to addition of acetaldehyde. Therefore, the main product of nicotinamide adenine dinucleotide electroreduction at the Ru(III) complex/carbon nanotube-modified electrode was the enzymatically active NADH. The purposed sensor can be used for acetaldehyde determination.     

Amperometric Detection of Morphine at Preheated Glassy Carbon Electrode Modified with Multiwall Carbon Nanotubes

A highly sensitive and fast responding sensor for the determination of morphine is described. The multiwall carbon nanotubes immobilize on preheated glassy carbon electrode (5 min at 50 °C) by gently rubbing of electrode surface on a filter paper supporting the carbon nanotubes.The results indicated that carbon nanotubes(CNTs) modified glassy carbon electrode exhibited efficiently electrocatalytic oxidation for morphine with relatively high sensitivity, stability and long life. Under conditions of cyclic voltammetry, the potential for oxidation of morphine is lowered by approximately 100 mV and the current is enhanced significantly (10 times) in comparison to the bare glassy carbon electrode at wide pH range (2–9). The electrocatalytic behavior is further exploited as a sensitive detection scheme for morphine determination by hydrodynamic amperometry. Under the optimized conditions the calibration plots are linear in the concentration range 0.5–150 μM with the calculated detection limit (S/N=3) of 0.2 μM and sensitivity of 10 nA/μM and a relative standard deviation (RSD) of 2.5% (n=10). The amperometric response is extremely stable, with no loss in sensitivity over a continual 30 min operation. Such attractive ability of multiwall carbon nanotubes (MWCNTs) modified GC electrode, suggests great promise for a morphine amperometric sensor. Finally the ability of the modified electrode was evaluated for simultaneous determination of morphine and codeine.     

pH-dependent electron-transport properties of carbon nanotubes

Carbon nanotube electrochemical transistors integrated with microfluidic channels are utilized to examine the effects of aqueous electrolyte solutions on the electron-transport properties of single isolated carbon nanotubes. In particular, pH and concentration of supporting inert electrolytes are examined. A systematic threshold voltage shift with pH is observed while the transconductance and subthreshold swing remain independent of pH and concentration. Decreasing pH leads to a negative shift of the threshold voltage, indicating that protonation does not lead to hole doping. Changing the type of contact metal does not alter the observed pH response. The pH-dependent charging of SiO2 substrate is ruled out as the origin based on measurements with suspended nanotube transistors. Increasing the ionic strength leads to reduced pH response. Contributions from possible surface chargeable chemical groups are considered.     

Coating Multi-walled Carbon Nanotubes with Uniform Silica Shells:Independent of Surface Chemistry

A facile and general method was described to coat six types of multi-walled carbon nanotubes, functionalized by either noncovalent or covalent way, with smooth silica shells. 3-Aminopropyltriethoxysilane(APTES) and pH value play important roles in the coating process and the thickness of silica shell could be controlled by the added amount of silicon alkoxides. After the removal of multi-walled carbon nanotubes by calcination, the silica nanotubes were successfully prepared.     

Designing novel materials through functionalization of carbon nanotubes for application in nuclear waste management: speciation of uranyl

Understanding the behavior of radioactive nuclide elements in different environmental conditions is an active area of research. In this work, we have investigated the possible interaction mechanism between carbon nanotubes and uranyl using density functional theory. It is shown that functionalized carbon nanotubes can be used to bind uranyl ions much more efficiently as compared to their unfunctionalized counterpart. The uranyl binding energies are sensitive to the nature of the functional groups rather than the carbon nanotube itself. The binding takes place preferably at the functionalized sites, although pH could determine the strength of uranyl binding. Our predicted results correlate well with the recent experimental uranyl sorption studies on carbon nanotubes. These finding are new and can open up a new era for actinide speciation and separation chemistry using carbon nanotubes.     

Solid-state reference electrodes based on carbon nanotubes and polyacrylate membranes

A novel potentiometric solid-state reference electrode containing single-walled carbon nanotubes as the transducer layer between a polyacrylate membrane and the conductor is reported here. Single-walled carbon nanotubes act as an efficient transducer of the constant potentiometric signal originating from the reference membrane containing the Ag/AgCl/Cl⁻ ions system, and they are needed to obtain a stable reference potentiometric signal. Furthermore, we have taken advantage of the light insensitivity of single-walled carbon nanotubes to improve the analytical performance characteristics of previously reported solid-state reference electrodes. Four different polyacrylate polymers have been selected in order to identify the most efficient reservoir for the Ag/AgCl system. Finally, two different arrangements have been assessed: (1) a solid-state reference electrode using photo-polymerised n-butyl acrylate polymer and (2) a thermo-polymerised methyl methacrylate:n-butyl acrylate (1:10) polymer. The sensitivity to various salts, pH and light, as well as time of response and stability, has been tested: the best results were obtained using single-walled carbon nanotubes and photo-polymerised n-butyl acrylate polymer. Water transport plays an important role in the potentiometric performance of acrylate membranes, so a new screening test method has been developed to qualitatively assess the difference in water percolation between the polyacrylic membranes studied. The results presented here open the way for the true miniaturisation of potentiometric systems using the excellent properties of single-walled carbon nanotubes.     

Simple immobilization of pyrroloquinoline quinone on few-walled carbon nanotubes

Pyrroloquinoline quinone (PQQ) was immobilized on glassy-carbon electrodes (GCE) modified with single-walled carbon nanotubes (SWCNT), few-walled carbon nanotubes (FWCNT) and carbon black (Vulcan XC72R). Modified electrodes were prepared by drop-casting. Immobilization was achieved with an extremely simple dipping procedure and without any further modification to the electrodes. Electrochemical performance of the electrodes was studied by cyclic voltammetry and spectroelectrochemistry. FWCNT adsorbed 30 times more PQQ than the other carbon materials. Compared to more complicated immobilization methods, PQQ/FWCNT/GCE showed well-defined electrochemistry in a considerably wide pH area from 2 to 12. The dipping process is affected by pH and electrostatic forces. At dipping pH 9.5, where both FWCNTs and PQQ have strong negative charge, the adsorption was halved compared to dipping pH 2, where the charges are smaller.     

玻璃基片上纳米碳管电极的集成

利用微波等离子体化学气相沉积法在玻璃孔穴中定位生长纳米碳管电极, 分析了负偏压对纳米碳管电极生长的影响. 该电极对铜离子的电化学检测性能分析结果表明, 所制备的纳米碳管电极具有良好的电化学检测性能, 位于－0.0100 V附近的铜离子的还原峰峰形良好, 其电流在铜离子浓度为0.01~0.30 mmol•L^-1时, 与Cu²⁺浓度呈良好的线性关系, 相关系数为0.9975, 且具有较好的长期稳定性和重现性.     

Interactions between single-walled carbon nanotubes and lysozyme

Dispersions of single-walled and non-associated carbon nanotubes in aqueous lysozyme solution were investigated by analyzing the stabilizing effect of both protein concentration and pH. It was inferred that the medium pH, which significantly modifies the protein net charge and (presumably) conformation, modulates the mutual interactions with carbon nanotubes. At fixed pH, in addition, the formation of protein/nanotube complexes scales with increasing lysozyme concentration. Electrophoretic mobility, dielectric relaxation and circular dichroism were used to determine the above features. According to circular dichroism, lysozyme adsorbed onto nanotubes could essentially retain its native conformation, but the significant amount of free protein does not allow drawing definitive conclusions on this regard. The state of charge and charge distribution around nanotubes was inferred by combining electrophoretic mobility and dielectric relaxation methods. The former gives information on changes in the surface charge density of the complexes, the latter on modifications in the electrical double layer thickness around them. Such results are complementary each other and univocally indicate that some LYS molecules take part to binding. Above a critical protein/nanotube mass ratio, depletion phenomena were observed. They counteract the stabilization mechanism, with subsequent nanotube/nanotube aggregation and phase separation. Protein-based depletion phenomena are similar to formerly reported effects, observed in aqueous surfactant systems containing carbon nanotubes.     

Synthesis of polydopamine‐functionalized magnetic graphene and carbon nanotubes hybrid nanocomposites as an adsorbent for the fast determination of 16 priority polycyclic aromatic hydrocarbons in aqueous samples

A novel adsorbent made of polydopamine‐functionalized magnetic graphene and carbon nanotubes hybrid nanocomposite was synthesized and applied to determine 16 priority polycyclic aromatic hydrocarbons by magnetic solid phase extraction in water samples. FTIR spectroscopy, transmission electron microscopy, scanning electron microscopy, and Raman spectroscopy consistently indicate that the synthesized adsorbents are made of core–shell nanoparticles well dispersed on the surface of graphene and carbon nanotubes. The major factors affecting the extraction efficiency, including the pH value of samples, the amount of adsorbent, adsorption time and desorption time, type and volume of desorption solvent, were systematically optimized. Under the optimum extraction conditions, a linear response was obtained for polycyclic aromatic hydrocarbons between concentrations of 10 and 500 ng/L with the correlation coefficients ranging from 0.9958 to 0.9989, and the limits of detection (S/N = 3) were between 0.1 and 3.0 ng/L. Satisfactory results were also obtained when applying these magnetic graphene/carbon nanotubes/polydopamine hybrid nanocomposites to detect polycyclic aromatic hydrocarbons in several environmental aqueous samples.     

碳纳米管对苯胺的吸附行为

碳纳米管有较大的比表面,有利于用作吸附剂。当前研究主要集中在对气体,尤其是对H2气的吸附。近年来,也有人将碳纳米管应用于环境保护领域,Long等使用碳纳米管除去二恶英,Li等用碳纳米管吸附溶液中的Cd^2 都取得了满意的效果。苯胺类化合物是国家严格控制的一类污染物,使用碳纳米管对苯胺的吸附研究尚未见报道。     

The adsorption kinetics of cadmium by three different types of carbon nanotubes

Oxidized nitrogen-doped multiwall carbon nanotubes (ox-N-MWCNTs), oxidized multiwall carbon nanotubes (ox-MWCNTs), and oxidized single-wall carbon nanotubes (ox-SWCNTs) were evaluated via batch adsorption kinetic experiments to determine the effect of nanotube morphology on the adsorption rate of cadmium. The nanotubes were characterized by HRTEM, XRD and Raman spectroscopy. Cadmium adsorption isotherms were determined at pH 6. Analyses of the kinetic data with an external mass transport model and an intraparticle diffusion model considered two cases: (1) single nanotubes suspended in aqueous solution and (2) agglomerates of nanotubes suspended in aqueous solution. The intraparticle diffusion model produced the best fit to the experimental data. However, only the diffusivity coefficients for single nanotubes suspended in solution were similar to literature values: about 4×10(-9), 1×10(-9) and 2.4×10(-11) cm(2)/s for ox-N-MWCNTs, ox-MWCNTs and ox-SWCNTs, respectively. The morphology of the various carbon nanotubes might determine cadmium diffusivity. The high amount of sidewall pores observed in the single-walled carbon nanotubes could limit cadmium diffusion and account for the slow diffusion rate of 180 min. Conversely, the short length, small surface area and bamboo-type morphology observed with nitrogen-doped multiwall carbon nanotubes may account for the relatively fast adsorption rate of 15 min as this morphology prevents cadmium diffusion through the internal tubular space of these nanotubes.     

Disposable Multiwalled Carbon Nanotube Printed Film Electrochemical Determination of Acetaminophen,Dopamine, and Uric Acid

A novel printed film consisting of multiwalled carbon nanotubes was fabricated on a polyethylene terephthalate substrate by means of a mass flexographic printing process. Potential applications of this film for electrochemical biosensing were examined through the oxidation of acetaminophen, dopamine, and uric acid in phosphate buffer (pH 7.0). The results demonstrate that the printed carbon nanotube film exhibits an enhanced electrochemical response toward these molecules. Dopamine and uric acid did not interfere with each other and, thus, their simultaneous determination may be performed. The results suggest the mass flexographic printing technique has potential application for the construction of low-cost, precise, and disposable multiwalled carbon nanotube films.     

Weak polyelectrolyte control of carbon nanotube dispersion in water

The dispersion of nanotubes by pH-responsive polymers (i.e., weak polyelectrolytes) enables the macroscopic properties of aqueous suspensions to be tuned. Microstructural changes were achieved as a function of pH in aqueous suspensions containing single-walled carbon nanotubes and imaged by cryogenic-TEM. Clear evidence of pH-sensitive nanotube dispersion is shown. We expect that many useful properties of these nanotube-polymer systems could be sensitive to microstructure, making this technique important for aqueous processing of carbon nanotubes and macroscopic tailoring of solid polymer nanocomposite behavior.