Deconvolution of the EPR spectra of vanadium oxide nanotubes

In this work we report results of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of vanadium oxide nanotubes. The observed EPR spectra are composed of a weak well-resolved spectrum of isolated V4+ ions on top of an intense and broad structure-less line shape, attributed to spin-spin exchanged V4+ clusters. With the purpose to deconvolute the structured weak spectrum from the composed broad line, a new approach based on the Krylov basis diagonalization method (KBDM) is introduced. It is based on the discrimination between broad and sharp components with respect to a selectable threshold and can be executed with few adjustable parameters, without the need of a priori information on the shape and structure of the lines. This makes the method advantageous with respect to other procedures and suitable for fast and routine spectral analysis, which, in conjunction with simulation techniques based on the spin Hamiltonian parameters, can provide a full characterization of the EPR spectrum. Results demonstrate and characterize the coexistence of two V4+ species in the nanotubes and show good progress toward the goal of obtaining high fidelity deconvoluted spectra from complex signals with overlapping broader line shapes. (C) 2012 Elsevier Inc. All rights reserved.

Thermal stability and the magnetic properties of hybrid vanadium oxide-tetradecylamine nanotubes

Vanadium oxide nanotubes (NTs) were synthesized by the sol-gel method followed by a long-term hydrothermal treatment. The obtained nanotubes have a multiwall structure, and 70% of vanadium ions are in the V4+ state. This percentage was derived by evaluating three components of the magnetic susceptibility; namely, (i) the paramagnetic Curie-Weiss behavior, (ii) antiferromagnetic dimers, and (iii) magnetic trimers. The as-made NTs were annealed in situ in the cavity of the electron paramagnetic resonance (EPR) spectrometer. The line shape changes irreversibly at 390 K, and the EPR susceptibility presents an anomaly at 425 K. These changes are interpreted as a partial oxidation of the V4+ ions and consequently a decrease in the concentration of the magnetic species. The quantification of the V4+ ions of the annealed NTs reveals a diminution to 39% of V4+, a weakening of the Curie-Weiss and antiferromagnetic dimers contributions, and the suppression of magnetic trimers. Vibrational studies confirm the decrease of V4+ amount. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749417]

Hydrogen peroxide monitoring in Fenton reaction by using a ruthenium oxide hexacyanoferrate/multiwalled carbon nanotubes modified electrode

A novel amperometric sensor based on the incorporation of ruthenium oxide hexacyanoferrate (RuOHCF) into multiwalled carbon nanotubes (MWCNTs) immobilized on a glassy carbon electrode is described. Cyclic voltammetry experiments indicated that the cathodic reduction of hydrogen peroxide at the RuOHCF/MWCNTs100/GC modified electrode is facilitated, occurring at 0.0 V vs. Ag/AgCl/KCl(sat). Following the optimization of the experimental conditions, the proposed sensor presented excellent analytical properties for hydrogen peroxide determination, with a low limit of detection (4.7 mu mol L-1), a large dynamic concentration range (0.1-10 mmol L-1) and a sensitivity of 1280 mu A mmol(-1) L cm(-2). The usefulness of the RuOHCF/MWCNTs100/GC electrochemical sensor was confirmed by monitoring the consumption of hydrogen peroxide during the degradation of phenol by the Fenton reaction. (C) 2012 Elsevier B.V. All rights reserved.

Ab-initio calculations for a realistic sensor: A study of CO sensors based on nitrogen-rich carbon nanotubes

The use of nanoscale low-dimensional systems could boost the sensitivity of gas sensors. In this work we simulate a nanoscopic sensor based on carbon nanotubes with a large number of binding sites using ab initio density functional electronic structure calculations coupled to the Non-Equilibrium Green's Function formalism. We present a recipe where the adsorption process is studied followed by conductance calculations of a single defect system and of more realistic disordered system considering different coverages of molecules as one would expect experimentally. We found that the sensitivity of the disordered system is enhanced by a factor of 5 when compared to the single defect one. Finally, our results from the atomistic electronic transport are used as input to a simple model that connects them to experimental parameters such as temperature and partial gas pressure, providing a procedure for simulating a realistic nanoscopic gas sensor. Using this methodology we show that nitrogen-rich carbon nanotubes could work at room temperature with extremely high sensitivity. Copyright 2012 Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4739280]

Electrochemical detection of carbamate pesticides in fruit and vegetables with a biosensor based on acetylcholinesterase immobilised on a composite of polyaniline-carbon nanotubes

A sensitive electrochemical acetylcholinesterase (AChE) biosensor was successfully developed on polyaniline (PANI) and multi-walled carbon nanotubes (MWCNTs) core-shell modified glassy carbon electrode (GC), and used to detect carbamate pesticides in fruit and vegetables (apple, broccoli and cabbage). The pesticide biosensors were applied in the detection of carbaryl and methomyl pesticides in food samples using chronoamperometry (CA). The GC/MWCNT/PANI/AChE biosensor exhibited detection limits of 1.4 and 0.95 mu mol L-1, respectively, for carbaryl and methomyl. These detection limits were below the allowable concentrations set by Brazilian regulation standards for the samples in which these pesticides were analysed. Reproducibility and repeatability values of 2.6% and 3.2%, respectively, were obtained in the conventional procedure. The proposed biosensor was successfully applied in the determination of carbamate pesticides in cabbage, broccoli and apple samples without any spiking procedure. The obtained results were in full agreement with those from the HPLC procedure. (C) 2012 Elsevier Ltd. All rights reserved.

EVALUATION OF THE POTENTIALITIES OF A CARBON NANOTUBES/SILICONE RUBBER COMPOSITE ELECTRODE IN THE DETERMINATION OF HYDROCHLOROTHIAZIDE

A multiwall carbon nanotube/silicone rubber (MWCNT/SR) composite electrode has been used for the determination of hydrochlorothiazide (HCTZ) in pharmaceutical formulations by differential pulse voltammetry (DPV). The electro-oxidation process was evaluated by cyclic voltammetry, from which it was observed that HCTZ presents an irreversible oxidation peak at 0.82 V vs. saturated calomel electrode (SCE) in the potential range from 0.5 to 1.1 V, in Britton-Robinson buffer pH 7.0 at MWCNT/SR. HCTZ was determined by DPV using a MWCNT/SR 70% (MWCNT, m/m) composite electrode after the optimization of the experimental parameters. The linear range was from 5.0 to 70.0 mu mol L-1, with a limit of detection (LOD) of 2.6 mu mol L-1. The HCTZ was determined in pharmaceutical formulations using the proposed composite electrode and the results agreed with those from the official high performance liquid chromatography (HPLC) method within 95% confidence level, according to the t-Student test.

Carbon nanotubes modified with antimony nanoparticles: A novel material for electrochemical sensing

In this study, a novel material for the electrochemical determination of bisphenol A using a nanocomposite based on multi-walled carbon nanotubes modified with antimony nanoparticles has been investigated. The morphology, structure, and electrochemical performance of the nanocomposite electrodes were characterised by field emission gun scanning electron microscopy, energy-dispersive X-ray spectroscopy, and cyclic voltammetry. A scan rate study and electrochemical impedance spectroscopy showed that the bisphenol A oxidation product is adsorbed on nanocomposite electrode surface. Differential pulse voltammetry in phosphate buffer solution at pH 6, allowed the development of a method to determine bisphenol A levels in the range of 0.5-5.0 mu mol L-1, with a detection limit of 5.24 nmol L-1 (1.19 mu g L-1). (C) 2012 Elsevier Ltd. All rights reserved.

Silver-gold nanotubes containing hot spots on their surface: facile synthesis and surface-enhanced Raman scattering investigations

We report the synthesis of silver-gold nanotubes containing hot spots along their surface. The Ag-Au nanotubes exhibited exceptional SERS properties compared to silver nanowires, enabling the detection of crystal violet in the 10(-10) M regime, as well as 9-nitroanthracene and benzo[a] pyrene at 3.3 x 10(-7) M.

The local environment of Co2+ ions intercalated in vanadium oxide/hexadecylamine nanotubes

Vanadium oxide nanotubes constitute promising materials for applications in nanoelectronics as cathode materials, in sensor technology and in catalysis. In this work we present a study on hybrid vanadium oxide/hexadecylamine multiwall nanotubes doped with Co ions using state of the art x-ray diffraction and absorption techniques, to address the issue of the dopant location within the nanotubes' structure. The x-ray absorption near-edge structure analysis shows that the Co ions in the nanotubes are in the 2+ oxidation state, while extended x-ray absorption fine structure spectroscopy reveals the local environment of the Co2+ ions. Results indicate that Co atoms are exchanged at the interface between the vanadium oxide's layers and the hexadecylamines, reducing the amount of amine chains and therefore the interlayer distance, but preserving the tubular shape. The findings in this work are important for describing Co2+ interaction with vanadium oxide nanotubes at the molecular level and will help to improve the understanding of their physicochemical behavior, which is desired in view of their promising applications.

Multi-Walled Carbon Nanotubes and Poly(lactic acid) Nanocomposite Fibrous Membranes Prepared by Solution Blow Spinning

Nanocomposite fibers based on multi-walled carbon nanotubes (MWCNT) and poly(lactic acid) (PLA) were prepared by solution blow spinning (SBS). Fiber morphology was characterized by scanning electron microscopy (SEM) and optical microscopy (OM). Electrical, thermal, surface and crystalline properties of the spun fibers were evaluated, respectively, by conductivity measurements (4-point probe), thermogravimetric analyses (TGA), differential scanning calorimetry (DSC), contact angle and X-ray diffraction (XRD). OM analysis of the spun mats showed a poor dispersion of MWCNT in the matrix, however dispersion in solution was increased during spinning where droplets of PLA in solution loaded with MWCNT were pulled by the pressure drop at the nozzle, producing PLA fibers filled with MWCNT. Good electrical conductivity and hydrophobicity can be achieved at low carbon nanotube contents. When only 1 wt% MWCNT was added to low-crystalline PLA, surface conductivity of the composites increased from 5 x 10(-8) to 0.46 S/cm. Addition of MWCNT can slightly influence the degree of crystallinity of PLA fibers as studied by XRD and DSC. Thermogravimetric analyses showed that MWCNT loading can decrease the onset degradation temperature of the composites which was attributed to the catalytic effect of metallic residues in MWCNT. Moreover, it was demonstrated that hydrophilicity slightly increased with an increase in MWCNT content. These results show that solution blow spinning can also be used to produce nanocomposite fibers with many potential applications such as in sensors and biosensors.

X-ray absorption spectroscopy study on La0.6Sr0.4CoO3 and La0.6Sr0.4Co1¡yFeyO3 nanotubes and nanorods for IT-SOFC cathodes.

In the last years, extensive research has been devoted to develop novel materials and structures with high electrochemical performance for intermediate-temperatures solid-oxide fuel cells (IT-SOFCs) electrodes. In recent works, we have investigated the structural and electrochemical properties of La0:6Sr0:4CoO3 (LSCO) and La0:6Sr0:4Co1¡yFeyO3 (LSCFO) nanostructured cathodes, finding that they exhibit excellent electrocatalytic properties for the oxygen reduction reaction [1,2]. These materials were prepared by a pore-wetting technique using polycarbonate porous membranes as templates. Two average pore sizes were used: 200 nm and 800 nm. Our scanning electronic microscopy (SEM) study showed that the lower pore size yielded nanorods, while nanotubes were obtained with the bigger pore size. All the samples were calcined at 1000oC in order to produce materials with the desired perovskite-type crystal structure. In this work, we analyze the oxidation states of Co and Fe and the local atomic order of LSCO and LSCFO nanotubes and nanowires for various compositions. For this pur- pose we performed XANES and EXAFS studies on both Co and Fe K edges. These measurements were carried out at the D08B-XAFS2 beamline of the Brazilian Synchrotron Light Laboratory (LNLS). XANES spectroscopy showed that Co and Fe only change slightly their oxidation state upon Fe addition. Surprisingly, XANES results indicated that the content of oxygen vacancies is low, even though it is well-known that these materials are mixed ionic-electronic conductors. EXAFS results were consistent with those expected according to the rhombohedral crystal structure determined in previous X-ray powder dffraction investigations. [1] M.G. Bellino et al, J. Am. Chem. Soc. 129 (2007) 3066 [2] J.G. Sacanell et al., J. Power Sources 195 (2010) 1786

Electrogeneration of platinum nanoparticles in a matrix of dendrimer-carbon nanotubes

Hybrid materials with enhanced properties can now be obtained by combining nanomaterials such as carbon nanotubes and metallic nanoparticles, where the main challenge is to control fabrication conditions. In this study, we demonstrate that platinum nanoparticles (PtNps) can be electrogenerated within layer-by-layer (LbL) films of polyamidoamine (PAMAM) dendrimers and single-walled carbon nanotubes (SWCNTs), which serve as stabilizing matrices. The advantages of the possible control through electrogeneration were demonstrated with a homogeneous distribution of PtNps over the entire surface of the PAMAM/SWCNT LbL films, whose electroactive sites could be mapped using magnetic force microscopy. The Pt-containing films were used as catalysts for hydrogen peroxide reduction, with a decrease in the reduction potential of 60 mV compared to a Pt film deposited onto bare ITO. By analyzing the mechanisms responsible for hydrogen peroxide reduction, we ascribed the enhanced catalytic activity to synergistic effects between platinum and carbon in the LbL films, which are promising for sensing and fuel cell applications.