Showing papers by "Malcolm L. H. Green published in 2008"

Copper oxide nanoparticle impurities are responsible for the electroanalytical detection of glucose seen using multiwalled carbon nanotubes

Abstract: In this report we demonstrate the second known example whereby multiwalled carbon nanotubes made by the chemical vapour deposition method (cvd-MWCNTs) contain copper oxide nanoparticles within them which are responsible for the analytical signal observed, in this example the electrochemical detection of glucose. Comparisons with copper(II) oxide particles immobilised onto a basal-plane pyrolytic graphite electrode produce almost identical voltammetric responses as the cvd-MWCNTs. Similar experiments performed using an edge-plane pyrolytic graphite electrode and with an ultra-pure sample of arc-MWCNTs (made using the electric arc discharge method and free from any residual metal nanoparticle catalysts) demonstrates that these electrode substrates are inactive towards even high concentrations of glucose. Thus the edge-plane-like defects present at the termini of the graphite sheets or the ends of the MWCNTs are not, as is usually the case, the electroactive sites towards this particular analyte. It is the copper impurities which are the electroactive species responsible and MWCNTs should not be considered as suitable substrates for the electrochemical detection of glucose contrary to numerous previous claims except in so far as they provide an elegant, if adventitious, method for “wiring” copper oxide nanoparticles.

The influence of edge-plane defects and oxygen-containing surface groups on the voltammetry of acid-treated, annealed and “super-annealed” multiwalled carbon nanotubes

Abstract: The role of edge-plane-like defects at the open ends of multiwalled carbon nanotubes (MWCNTs) and at hole defects in the tube walls is explored using cyclic voltammetry with two charged redox probes, namely potassium ferrocyanide and hexaamineruthenium(III) chloride in unbuffered aqueous solutions, and one neutral redox probe, norepinephrine, in pH 5.7 buffer. Further, the presence of oxygen-containing functional groups (such as phenol, quinonyl and carboxyl groups), which decorate the edge-plane defect sites on the voltammetric response of the MWCNTs, is also explored. To this end, three different pre-treatments were performed on the pristine MWCNTs made using the arc-discharge method (arc-MWCNTs). These were (a) arc-MWCNTs were subjected to acid oxidation to form acid-MWCNTs—open-ended MWCNTs also possessing numerous hole defects revealing a large number of edge-plane-like sites heavily decorated with surface functional groups; (b) acid-MWCNTs, which were subsequently vacuum-annealed at 900 °C to remove the functional groups but leaving the many undecorated edge-plane-like sites exposed (ann-MWCNTs); (c) ann-MWCNTs, which were subjected to a further vacuum “super-annealing” stage at 1,750 °C (sup-MWCNTs), which caused the hole defects to close and also closed the tube ends, thereby, restoring the original, pristine, almost edge-plane defect-free MWCNTs structure. The results of the voltammetric characterisation of the acid-, ann- and sup-MWCNTs provide further evidence that edge-plane-like sites are the electroactive sites on MWCNTs. The presence of oxygen-containing surface groups is found to inhibit the rate of electron transfer at these sites under the conditions used herein. Finally, the two charged, “standard” redox probes used were found to undergo strong interactions with the oxygen-containing surface groups present. Thus, we advise caution when using these redox probes to attempt to voltammetrically characterise MWCNTs, and by extension, graphitic carbon surfaces.

Steam Purification for the Removal of Graphitic Shells Coating Catalytic Particles and the Shortening of Single‐Walled Carbon Nanotubes

Abstract: Purification and shortening of single-walled carbon nanotubes (SWNTs) is carried out by treatment with steam. During the steam purification the graphitic shells coating the catalytic metal particles are removed. Consequently, the exposed catalytic particles can be easily dissolved by treatment with hydrochloric acid. No damage to the carbon nanotube tubular structure is observed, even after prolonged treatment with steam. Samples are characterized by HRTEM, TGA, magnetic measurements, Raman spectroscopy, AFM, and XPS.

Fabrication of carbon-nanotube-reinforced glass–ceramic nanocomposites by ultrasonic in situ sol–gel processing

Abstract: The addition of carbon nanotubes (CNTs) to ceramic or glass matrices has the potential to provide composites with novel properties but composites with a uniform dispersion of undamaged CNTs have proved difficult to make. This paper reports a processing method that overcomes these problems by using a powderless sol–gel route to produce a low-melting point aluminoborosilicate glass matrix, in combination with a dispersion method for the CNTs that is compatible with the sol. Single-walled or multi-walled CNTs were first functionalized by treatment with nitric acid followed by an ammonia–ethanol solution and were then dispersed in a glass precursor sol via ultrasonic processing. After gelation and calcination, the powder was hot pressed to make dense, well dispersed CNT–borosilicate glass composites with CNT contents from 0.5 to 5 wt%. Raman spectroscopy showed that the CNTs had suffered little damage during processing. The formation of cristobalite initially lead to microcracking but this was successfully suppressed by further additions of alumina. The CNTs suppressed cracking around hardness indentations and substantial crack bridging by the CNTs was observed.

Effects of covalent functionalization on the biocompatibility characteristics of multi-walled carbon nanotubes.

Abstract: We report the effect of chemical modification of multi-walled carbon nanotubes (MWNTs) on their activation of the human serum complement system, as well as the adsorption of human plasma proteins on MWNTs. Four different types of chemically-modified MWNTs were tested for complement activation via the classical and alternative pathways using haemolytic assays. Human plasma protein binding was also tested using an affinity chromatography technique based on carbon nanotube-Sepharose matrix. Covalent functionalization of MWNTs greatly altered the level of activation of the complement system via the classical pathway. For example, MWNTs functionalised with epsilon-caprolactam or L-alanine showed respectively >90% and >75% reduction in classical pathway activation compared with unmodified MWNTs. These results demonstrate for the first time that these types of chemical modification are able to alter considerably the levels of specific complement proteins bound by pristine MWNTs (used as a control experiment). The reduced levels of complement activation via the classical pathway, that are likely to increase biocompatibility, were directly correlated with the amount of C1q protein bound to chemically modified carbon nanotubes. An inverse correlation was also observed between the amount of complement factor H bound to chemically modified MWNTs and the level of complement consumption via the alternative pathway. Binding of human plasma and serum proteins to pristine and modified MWNTs was highly selective. The chemical modifications studied generally increased nanotube dispersibility in aqueous media, but diminished protein adsorption.

The role of photoinduced defects in TiO2 and its effects on hydrogen evolution from aqueous methanol solution.

Abstract: The hydrogen evolution from aqueous methanol solutions was found to follow two stages of zero order kinetics during photoreactions using TiO2 as the photocatalyst. Maximal hydrogen evolution was found at the 10% (v/v) methanol solution. X-ray photoelectron spectroscopy (XPS) shows that Ti(1566) defects are formed on the surface of TiO2 and X-ray powder diffraction (XRD) indicates that Ti(1566) defects are also formed in the bulk after photoreaction. Formation of defects is also shown by broadening of Bragg peaks and blue shifts and peak broadening in Raman spectroscopy. The defect disorder results in the increase of hydrogen evolution. UV−vis diffuse reflection spectra confirm that new absorptions in the visible light region are related to the defect content. At high methanol concentration, XPS implies that the active sites of the surface are blocked by hydroxyl groups, which results in the decrease of hydrogen evolution. TEM images showed that the photoreaction occurred on the surface of the photocatalys...

The Electrocatalytic Properties of Arc‐MWCNTs and Associated ‘Carbon Onions’

Abstract: For the first time we report on the electrochemical characteristics of nanometer sized polyhedral graphite onions dispersed amongst arc-MWCNTs. These are formed during the electric arc discharge method of producing ultrapure MWCNTs (arc-MWCNTs). The carbon onions are randomly dispersed amongst the arc-MWCNTs which are produced with very little amorphous carbon deposits or other unwanted impurities and are formed as closed-ended tubes. By comparison with commercially available open-ended hollow-tube multiwalled carbon nanotubes made using the chemical vapor deposition method (cvd-MWCNTs), a glassy carbon electrode (GCE), an edge-plane pyrolytic graphite electrode (eppg) and basal plane pyrolytic graphite (bppg) electrode, we can speculate that it is the edge-plane-like defect sites that are the electroactive sites responsible for the apparent ‘electrocatalysis’ seen with a wide range of analytes including: ferrocyanide, ruthenium hexaamine(III), nicotinamide adenosine dinucleotide (NADH), epinephrine, norepinephrine, cysteine, and glutathione. The arc-MWCNTs themselves are produced as closed-ended tubes with very few, if any, edge-plane-like defect sites evident in their HRTEM characterization. Therefore we speculate that it is the carbon onions dispersed amongst the arc-MWCNTs which have incomplete graphite shells or a rolled-up ‘Swiss-roll’ structures that posses the edge-plane-like defect sites and are responsible for the observed voltammetric responses. Carbon onions are no more or no less ‘electrocatalytic’ than open-ended MWCNTs which in turn are no more electrocatalytic than an eppg electrode. As the carbon onions are ubiquitous in MWCNTs formed using the arc-discharge method the authors advise that caution should be taken before assigning any electrocatalytic behavior to the MWCNTs themselves as any observed electrocatalysis likely arises from the carbon onion impurities.

Electrochemical Opening of Single-Walled Carbon Nanotubes Filled with Metal Halides and with Closed Ends

Abstract: The electrochemical opening of closed-ended, pristine single-walled carbon nanotubes (SWCNTs) upon the application of either a sufficiently oxidizing or reducing electrode potential is reported. Hitherto, it has been unclear whether the side walls of SWCNTs are electrochemically active, or whether, like their multiwalled counterparts (MWCNTs), the electroactive sites on SWCNTs also reside at the edge-plane-like defects at the open ends of the tubes. Evidence is presented herein that suggests the latter case is true, i.e., that SWCNTs require edge-plane sites to be electroactive. Comparisons of the voltammetric response of end-closed SWCNTs (EC-SWCNTs), end-open (EO-SWCNTs), and SWCNTs encapsulating a metal halide filling (MX@SWCNTs, where MX represents either NaI or CuI) in aqueous electrolytes indicate that SWCNTs undergo electrochemical opening if the applied electrode potential is greater than +1.2 V vs SCE or less than −1.5 V vs SCE. This was further confirmed using ex situ X-ray photoelectron spectro...

An electrochemical comparison of manganese dioxide microparticles versus α and β manganese dioxide nanorods: mechanistic and electrocatalytic behaviour

Abstract: The comparative electrochemical behaviour of both α- and β-nanorods of manganese dioxide (MnO2) and microparticles of predominantly β-phase manganese dioxide is investigated at pHs close to neutral. In order to understand the observed voltammetric behaviour of all three materials the mechanisms of electrodeposition of MnO2 onto a graphite electrode surface from a solution of Mn(II) at pH 3–7 is also reported. It is proposed that two competing mechanistic pathways operate, both invoking MnOOH as an intermediate species, which are an ECE or a DISP process, respectively. At low pH values (

Highly hydrophilic and stable polypeptide/single-wall carbon nanotube conjugates

Abstract: A polymeric dispersing agent for single-wall carbon nanotubes (SWCNTs) in water was synthesized by 4-(pyren-1-yl)butanoylation of the amine groups of poly-L-lysine. The pyrene content of 4-(pyren-1-yl)butanoylated polylysine (PBPL) was optimized so that a maximal dispersion effect of the SWCNTs was obtained. High molecular weight PBPL (>300000 g mol−1) exceeds the widely used surfactant sodium dodecylsulfate (SDS) as a dispersing agent yielding a 27% higher SWCNT concentration in dispersion. The high stability of the PBPL/SWCNT conjugates is demonstrated by removing the conjugates from dispersion through filtration, washing, and redispersion in only water. The resulting redispersions do not contain free PBPL and the importance of having SWCNT dispersions in the absence of free dispersing agent is discussed. For SDS dispersions, poor redispersion properties have been observed. The binding of PBPL onto the SWCNTs was studied by atomic force microscopy, optical absorbance spectroscopy, and fluorescence spectroscopy.

Interactions between tripodal porphyrin hosts and single walled carbon nanotubes: an experimental and theoretical (DFT) account

Abstract: We describe a strategy for dispersing single walled nanotubes (SWNTs) in organic solvents using supramolecular coating with tailor-made, structurally adjustable tripodal porphyrin receptors. These have the ability to recognise SWNTs from DMF–toluene mixtures. HR TEM, TM AFM and SEM measurements showed that the hosts wrap around individual arc-made SWNTs, giving monodispersed composites in DMF which are stable for weeks. The dispersed composites were studied by Raman and fluorescence spectroscopies, including laser scanning confocal microscopy (for aggregates incorporated in cells). These porphyrin hosts encapsulate SWNTs based on π–π type interactions in an analogous manner to those observed by single crystal X-ray diffraction in their C60 complexes. These interactions may be further reinforced by porphyrin–porphyrin stacking. Density functional theory (DFT) calculations predicted the structure of the SWNT : porphyrin host complex or composite formed and the binding energies of tripodal porphyrin hosts in composites with either C60 or SWNT as guests.