Dispersion of multi-wall carbon nanotubes in polyethylenimine: A new alternative for preparing electrochemical sensors

In this work we report on the analytical performance of glassy carbon electrodes modified with a dispersion of multi-wall carbon nanotubes in polyethylenimine (GCE/(PEI/CNT)). The resulting electrodes show an excellent electrocatalytic activity toward different bioanalytes like ascorbic acid, dopamine, 3,4-dihydroxyphenylacetic acid (dopac) and hydrogen peroxide. An important decrease in the overvoltages for the oxidation of ascorbic acid (505 mV) and hydrogen peroxide (350 mV) and for the reduction of hydrogen peroxide (450 mV), as well as a dramatic improvement in the reversibility of the electrochemical behavior of dopamine and dopac is obtained. The currents are higher than those obtained with other dispersant agents like Nafion, concentrated acids or chitosan, evidencing the high efficiency of the dispersion in PEI. The GCE/(PEI/CNT) demonstrated to be highly reproducible, with 3.0% RSD for the sensitivity of hydrogen peroxide for 10 electrodes prepared with five different dispersions. Differences in sensitivity of 10.0% were obtained for hydrogen peroxide with electrodes prepared using the same dispersion even after 14 days preparation. The CNT/PEI layer immobilized on glassy carbon electrodes has been also used as a platform for building supramolecular architectures based on the self-assembling of polyelectrolytes without any pretreatment of the electrode surface, oxidation or derivatization of the carbon nanotubes, just taking advantages of the polycationic nature of the polymer used for dispersing the nanotubes. The self-assembling of glucose oxidase has allowed us to obtain a supramolecular multistructure for glucose biosensing, with detection limits of 11 μM (0.02 g/L). Such an excellent performance of GCE/(PEI/CNT) toward hydrogen peroxide and the effectiveness of the use of CNT/PEI as a platform for obtaining supramolecular multistructures, represents a very good alternative for developing other enzymatic biosensors.     

多壁碳纳米管功能化及其在电化学葡萄糖传感器中的应用

利用硅烷偶联剂的表面修饰技术,将氨基丙基三乙氧基硅烷(APS)共价接枝到酸处理后的碳纳米管的表面.红外光谱数据证实了该反应的可行性.修饰后的碳纳米管在水中具有良好的分散性.利用西夫碱反应将氨基化的碳纳米管与醛基化的葡萄糖氧化酶共价层层自组装到电极表面,获得灵敏度可控的葡萄糖传感器.用电化学交流阻抗法和扫描电镜对成膜过程...     

Ethane/ethylene adsorption on carbon nanotubes: temperature and size effects on separation capacity

We present the results of Monte Carlo simulations of the adsorption of single-component ethane and ethylene and of equimolar mixtures of these two gases on bundles of closed, single-walled carbon nanotubes. Two types of nanotube bundles were used in the simulations: homogeneous (i.e., those in which all the nanotubes have identical diameters) and heterogeneous (those in which nanotubes of different diameters are allowed). We found that at the same pressure and temperature more ethane than ethylene adsorbs on the bundles over the entire range of pressures and temperatures explored. The simulation results for the equimolar mixtures show that the pressure at which maximum separation is attained is a very sensitive function of the diameter of the nanotubes present in the bundles. Simulations using heterogeneous bundles yield better agreement with single-component experimental data for isotherms and isosteric heats than those obtained from simulations using homogeneous bundles. Possible applications of nanotubes in gas separation are discussed. We explored the effect of the diameter of the nanotubes on the separation ability of these sorbents, both for the internal and for the external sites. We found that substrate selectivity is a decreasing function of temperature.     

A general electrochemical approach to deposition of metal hydroxide/oxide nanostructures onto carbon nanotubes

This communication describes a new and relatively general electrochemical approach to the deposition of transition metal hydroxide/oxide nanostructures onto multi-walled carbon nanotubes (MWNTs) based on the precipitation of metal hydroxide/oxide nanostructures onto MWNTs by increasing the local pH values at the electrode/electrolyte interface induced by the proton-consuming electrochemical reduction of hydrogen peroxide (H₂O₂). The results obtained with cyclic voltammetry, scanning electron microscopy, and X-ray photoelectron spectroscopy of the synthetic nanocomposites substantially suggest the deposition of the metal hydroxides/oxides onto MWNTs induced by the electrochemical reduction of H₂O₂. This study essentially offers a facile but effective and relatively general electrochemical approach to the synthesis of the nanocomposites consisting of metal hydroxides/oxides and MWNTs.     

Substituted carborane-appended water-soluble single-wall carbon nanotubes: new approach to boron neutron capture therapy drug delivery

Substituted C(2)B(10) carborane cages have been successfully attached to the side walls of single-wall carbon nanotubes (SWCNTs) via nitrene cycloaddition. The decapitations of these C(2)B(10) carborane cages, with the appended SWCNTs intact, were accomplished by the reaction with sodium hydroxide in refluxing ethanol. During base reflux, the three-membered ring formed by the nitrene and SWCNT was opened to produce water-soluble SWCNTs in which the side walls are functionalized by both substituted nido-C(2)B(9) carborane units and ethoxide moieties. All new compounds are characterized by EA, SEM, TEM, UV, NMR, and IR spectra and chemical analyses. Selected tissue distribution studies on one of these nanotubes, {([Na(+)][1-Me-2-((CH(2))(4)NH-)-1,2-C(2)B(9)H(10)][OEt])(n)(SWCNT)} (Va), show that the boron atoms are concentrated more in tumors cells than in blood and other organs, making it an attractive nanovehicle for the delivery of boron to tumor cells for an effective boron neutron capture therapy in the treatment of cancer.     

碳纳米管吸附水溶液中双酚A的热力学

以甲醇和去离子水组成的体系（体积比90∶10）为流动相,建立了以香烟过滤嘴作吸附剂,固相萃取（SPE）与高效液相色谱（HPLC）联用测定水中双酚A(Bisphenol A,BPA)的新方法。研究了水溶液中碳纳米管(CNTs)吸附双酚A的热力学特性,测定了不同温度下的吸附等温线,并探讨了其可能的吸附机理。结果表明,CNTs对BPA 的吸附主要以快速吸附为主,常温下,碳纳米管对于70 mg·L-1的双酚A水溶液的吸附量可达到 24.65 mg g-1,吸附量随初始浓度的增加而增加,随温度的降低而增大,采用Freundlich和Langmuir方程拟合,相关系数均大于0.99,热力学函数ΔG、ΔH及ΔS分别为-39.48 ~ -43.51 KJ·mol-1、-18.06 KJ·mol-1、71.73 J·mol-1·K-1,吸附为放热、熵增的自发过程,降低温度有利于吸附,并且具有物理吸附特征。     

Surface design of carbon nanotubes for optimizing the adsorption and electrochemical response of analytes

Carbon nanotubes (CNTs) from different sources were dissolved in water with high solubility by Congo red (CR) via strong noncovalent pi-stacking interactions. The resulting CNTs were capable of forming uniform, compact, stable films on various substrates. This provided a chance to explore the relationship between the surface property of CNTs and the adsorptive behavior of analytes on CNTs without considering the influence of film structures or free additives. Electrochemical behaviors of several small biomolecules and glucose oxidase (GOD) on various CR-functionalized CNT films were examined. The results showed that both the hydrophobic structural defect sites and the hydrophilic oxygen-containing groups were the electroactive sites of CNTs, which was further proven by UV-vis and FTIR spectra. Moreover, the surface properties of CNTs could be conveniently designed by simple pretreatments for optimizing the adsorption and the electrochemical response of analytes. For instance, the hydrophobic defect sites created during the growth or the workup of CNTs were favorable to the adsorption and the electrochemical response of hydrophobic analytes, whereas the hydrophilic oxygen-containing groups produced by acid treatment facilitated the stable adsorption and the direct electrochemistry of redox proteins.     

A simple approach to covalent functionalization of boron nitride nanotubes

A novel and simple method for the preparation of chemically functionalized boron nitride nanotubes (BNNTs) is presented. Thanks to a strong oxidation followed by the silanization of the surface through 3-aminopropyl-triethoxysilane (APTES), BNNTs exposing amino groups on their surface were successfully obtained. The efficacy of the procedure was assessed with EDS and XPS analyses, which demonstrated a successful functionalization of ~15% boron sites. This approach opens interesting perspectives for further modification of BNNTs with several kinds of molecules. Since, in particular, biomedical applications are envisaged, we also demonstrated in vitro biocompatibility and cellular up-take of the functionalized BNNTs.     

A triptycene-based approach to solubilising carbon nanotubes and C60

We describe herein the synthesis of a triptycene-based surfactant designed with the ability to solubilise single-walled carbon nanotubes (SWNTs) and C(60) in water through non-covalent interactions. Furthermore, an amphiphilic naphthalene-based surfactant with the same ability to solubilise SWNTs and C(60) has also been prepared. The compounds synthesised were designed with either two ionic or non-ionic tails to ensure a large number of supramolecular interactions with the solvent, thereby promoting strong solubilisation. The surfactants produced stable suspensions in which the SWNTs are dispersed and the surfactant/SWNT complexes formed are stable for more than one year. UV/Vis/NIR absorption spectroscopy, TEM and AFM were employed to probe the solubilisation properties of the dispersion of surfactants and SWNTs in water.     

Heterogeneity of multiwalled carbon nanotubes based on adsorption of simple aromatic compounds from aqueous solutions

The surface heterogeneity of multiwalled carbon nanotubes (MWCNTs) is studied on the basis of adsorption isotherms from dilute aqueous phenol and dopamine solutions at various pH values. The generalized Langmuir–Freundlich isotherm equation was applied to investigate the cooperative effect of the surface heterogeneity and the lateral interactions between the adsorbates. The theoretical isosteric heats of adsorption were obtained assuming that the heat of adsorption profile reveals both the energetic heterogeneity of the adsorption system and the strength of the interactions between the neighboring molecules. The adsorption energy distribution functions were calculated by using algorithm based on a regularization method. The great advantage of this method is that the regularization makes no assumption about the shape of the obtained energy distribution functions. Analysis of the isosteric heats of adsorption for MWCNTs showed that the influence of the surface heterogeneity is much stronger than the role of the lateral interactions. The most typical adsorption heat is 20–22 kJ/mol for both phenol and dopamine. After purification of nanotubes, heat value for phenol dropped to 16–17 kJ/mol. The range of the energy distribution is only slightly influenced by the surface chemistry of the nanotubes in the aqueous conditions.     

Mixed adsorption of ionic and nonionic surfactants on active carbon

Summary Adsorption depends mainly on the relative amounts of anionic and nonionic surfactants present, the equilibrium concentration and the duration of exposure. In the case of similar hydrophobic chain lengths nonionic surfactants will be adsorbed more strongly than anionic compounds, thus displacing the latter from the carbon surface.The difference in the attraction to the carbon surface can be such, that significant adsorption of anionics is only observed where anionics are present in considerable excess.Under such conditions, anionics will diffuse more rapidly into the pore system of the adsorbant than nonionics. Therefore, the surface coverage with anionics will be higher after short exposure than after a longer period of time, when replacement by nonionics has started.At very low equilibrium concentrations (corresponding to low surface coverage), adsorption of anionics will be even increased by the presence of nonionics. This is due to the formation of mixed layers and the fact that in such layers the repulsion between the charged hydrophilic groups of the anionic surfactants will decrease.

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Zusammenfassung Die Adsorption hängt entscheidend von dem Mischungsverhältnis Aniontensid/ nichtionogenes Tensid, der Gleichgewichtskonzentration und der Adsorptionszeit ab. Bei annähernd gleicher hydrophober Kette werden nichtionogene Tenside stärker adsorbiert als Aniontenside und verdrängen diese von der Kohlenstoffoberfläche. Der Unterschied in der Attraktion zur Kohlenstoffoberfläche ist so groß, daß eine signifikante Adsorption von Aniontensiden erst bei hohem Überschuß in der Mischung im Vergleich zum nichtionogenen Tensid beobachtet werden kann. Unter diesen Verhältnissen diffundieren Aniontenside schneller in das Porensystem des Adsorbens, so daß im Bereich kurzer Zeiten, bevor die Verdrängung durch das nichtionogene Tensid einsetzt, an der Oberfläche Aniontenside stärker adsorbiert sind. Im Bereich sehr geringer Gleichgewichtskonzentrationen und dementsprechend geringen Oberflächenbelegungen wird jedoch wegen der Bildung von Mischfilmen beider Tensidarten und Verminderung der gegenseitigen Abstoßung der gleichsinnig geladenen hydrophilen Gruppen des Aniontensides durch das nichtionogene Tensid die Adsorption des Aniontensids sogar gesteigert.

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With 7 figures

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Presented at IUPAC-International Conference on Colloid and Surface Science, Budapest 15–20 September 1975.     

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.     

Joint adsorption of benzene and water on carbon nanotubes

The joint adsorption of water and benzene on nanosized carbon tubes (NCTs) (with a specific surface area of 413 m²/g) synthesized by carbonizing methylene chloride in cylindrical pores of an Al₂O₃ matrix was studied. ¹H NMR spectroscopy with layer-by-layer freezing of the liquid phase was used to characterize the water bound in pores at various contents of benzene and water. Due to its higher energy of interaction with carbon surfaces, benzene was demonstrated to decrease the energy of interaction of water with the surface of the NCT sample from 43 to 15 J/g. It was suggested that, in the presence of benzene, H-bonded water clusters only weakly bound to the surface are formed in the cylindrical cavities of the NCTs.     

Insights on charge transfer doping and intrinsic phonon line shape of carbon nanotubes by simple polymer adsorption

Doping of individual single-walled carbon nanotubes via noncovalent adsorption of polyethylenimine which converts p-type semiconducting nanotubes into n-type is examined by micro-Raman studies. Distinctively different responses are observed in metallic and in semiconducting nanotubes. Very little or no changes in the radial breathing and the disorder modes are observed upon polymer adsorption on semiconducting carbon nanotubes indicating noncovalent nature of this process. Tangential G-band spectral downshift of up to approximately 10 cm(-)(1) without line broadening is observed for semiconducting tubes suggesting similar magnitude of electron transfer as commonly observed in electrochemical doping with alkali metals. Strong diameter dependence is also observed and can be explained by thermal ionization of charge carriers with activation barrier that scales as the energy gap of the semiconducting nanotubes. In contrast, metallic nanotubes exhibit very different behavior with significant line broadening of the G-band and concurrent enhancement of the disorder mode. In certain cases, initially symmetric Lorentzian line shapes of the G-band features with narrow line widths similar to semiconducting tubes are converted to a broad, asymmetric Breit-Wigner-Fano line shape. Implications on the effects of electron injection and the local chemical environment on the intrinsic line shape of isolated carbon nanotubes are discussed.     

Ozone adsorption on carbon nanotubes: the role of Stone-Wales defects

First-principles calculations within the density functional theory have been performed in order to investigate ozone adsorption on carbon nanotubes. Particular emphasis is placed on the effects of Stone-Wales-like defects on the structural and electronic properties of (i) ideal tubes and (ii) tubes in the presence of ozone. Our results show that structural deformations induced on the pure carbon nanotubes by Stone-Wales defects are similar, as expected, to those induced on graphite; for the (10,0) tube, the semiconducting character is kept, though with a small reduction of the band gap. As for the ozone adsorption, the process on ideal nanotubes is most likely physisorption, though slightly stronger if compared to other previously studied molecules and consistent with the strong oxydizing nature of O(3). However, when ozone adsorbs on Stone-Wales defects, a strong chemisorption occurs, leading to relevant structural relaxations and to the formation of a CO covalent bond; this is consistent with experimental observations of CO functional groups, as well as of the liberation of CO gas phase and of the formation of C vacancies, thus explaining the consumption of the nanotube film upon ozone exposure.     

Comparative study of hydrogen adsorption on carbon and BN nanotubes

The physisorption and chemisorption of hydrogen in BN nanotubes, investigated by density functional theory (DFT), were compared with carbon nanotubes. The physisorption of H2 on BN nanotubes is less favorable energetically than on carbon nanotubes; BN nanotubes cannot adsorb hydrogen molecules effectively in this manner. Chemisorption of H2 molecules on pristine BN nanotubes is endothermic. Consequently, perfect BN nanotubes are not good candidates for hydrogen storage by either mechanism. Other strategies must be utilized if BN nanotubes are to be employed as hydrogen storage media such as utilizing them as supporting media for hydrogen-absorbing metal nanoclusters.     

苯酚及取代酚在碳纳米管上的吸附研究

本文研究了水溶液中碳纳米管(CNTs)吸附苯酚、对甲酚和对甲氧基苯酚的热力学特性,测定了不同温度下的吸附等温线,并探讨了其可能的吸附机理。结果表明:在稀溶液中碳纳米管对三种酚类物质的吸附均符合Freund lich和Langmu ir方程,吸附均为放热、熵增的自发过程,并且都具有物理吸附特征;碳纳米管与三种酚类物质分子之间的л—л共轭作用的强弱决定了碳纳米管对三种酚类物质的吸附能力,顺序依次为:对甲氧基苯酚>对甲酚>苯酚。     

A common approach to adsorption and absorption: Polymolecular adsorption

The Brunauer-Emmett-Teller (BET) and Frenkel-Halsey-Hill equations are shown to be special cases of an equation obtained earlier for interphase equilibrium.Institute of Physical Chemistry, Russian Academy of Sciences, Moscow 117915. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 779–785, April, 1992.     

Interaction of water with single-walled carbon nanotubes: reaction and adsorption

The interaction of water vapor with carbon nanotubes at room temperature has been investigated using Fourier transform (FT) IR spectroscopy and density functional theory (DFT) calculations. FTIR data indicate that water molecules adsorb on single-walled carbon nanotubes at room temperature. Comparison to previous studies suggests that the water forms hydrogen-bonded structures inside the nanotubes. Analysis of the FTIR data demonstrates that a small number of water molecules react with the nanotubes, forming C-O bonds, whereas a majority of the water molecules adsorb intact. The DFT calculations show that cleavage of an O-H bond upon adsorption to form adsorbed -H and -OH groups is energetically favorable at defect sites on nanotubes.