Investigation of modified basal plane pyrolytic graphite electrodes: definitive evidence for the electrocatalytic properties of the ends of carbon nanotubes

The basis of the electrocatalytic nature of multi-wall carbon nanotubes is suggested to reside in electron transfer from the ends of nanotubes, which structurally resemble the behaviour of edge plane (as opposed to basal plane) graphite, and is demonstrated via the comparison of the electrochemical oxidation of epinephrine and the electrochemical reduction of ferricyanide at nanotube-modified electrodes using different types of graphite electrodes and with C(60)-modified electrodes.     

New electrodes for old: from carbon nanotubes to edge plane pyrolytic graphite

Different types of carbon based electrodes have emerged over the last few years, significantly changing the scope and sensitivity of electro-analytical methods for the measurement of diverse targets from metal ions through gases to biological markers. This Highlight article shows how the use of carbon nanotube modified electrodes has led to a fundamental understanding of the location and nature of electron transfer processes on graphitic electrodes and to the realisation that edge plane pyrolytic graphite may represent, at present, an optimal electrode material of this type for electroanalysis.     

Edge plane pyrolytic graphite electrodes in electroanalysis: an overview.

The recent development, behavior and scope of edge plane pyrolytic graphite electrodes in electroanalysis are overviewed. Similarities to, and advantages, over multi-walled CNT modified electrodes are noted and the wide scope of applications, ranging through gas sensing, stripping voltammetry and biosensing, illustrated.     

Abrasive immobilization of carbon nanotubes on a basal plane pyrolytic graphite electrode: application to the detection of epinephrine

The performance of a basal plane pyrolytic graphite (bppg) electrode modified with carbon nanotubes is described. Abrasive immobilization of multiwall carbon nanotubes on a bppg electrode was achieved by gently rubbing the electrode surface on a filter paper supporting carbon nanotubes. The resulting electrode showed excellent mediation of epinephrine oxidation: a decrease in the overvoltage of the epinephrine electro-oxidation (200-500 mV) was observed as well as a dramatic increase in the peak current (4 times) compared to that seen at a bare bppg electrode. The oxidation peaks of epinephrine and ascorbic acid which overlap on bare bppg electrode were separated successfully (by ca. 220 mV) at the surface of the modified bppg electrode. The modified electrode showed good stability in comparison to most modified carbon nanotubes electrodes prepared by alternative methods.     

Exploring the electrocatalytic sites of carbon nanotubes for NADH detection: an edge plane pyrolytic graphite electrode study

The electrocatalytic properties of multi-walled carbon nanotube modified electrodes toward the oxidation of NADH are critically evaluated. Carbon nanotube modified electrodes are examined and compared with boron-doped diamond and glassy carbon electrodes, and most importantly, edge plane and basal pyrolytic graphite electrodes. It is found that CNT modified electrodes are no more reactive than edge plane pyrolytic graphite electrodes with the comparison with edge plane and basal plane pyrolytic graphite electrodes allowing the electroactive sites for the electrochemical oxidation of NADH to be unambiguously determined as due to edge plane sites. Using these highly reactive edge plane sites, edge plane pyrolytic graphite electrodes are examined with cyclic voltammetry and amperometry for the electroanalytical determination of NADH. It is demonstrated that a detection limit of 5 microM is possible with cyclic voltammetry or 0.3 microM using amperometry suggesting that edge plane pyrolytic graphite electrodes can conveniently replace carbon nanotube modified glassy carbon electrodes for biosensing applications with the relative advantages of reactivity, cost and simplicity of preparation. We advocate the routine use of edge plane and basal plane pyrolytic graphite electrodes in studies utilising carbon nanotubes particularly if 'electrocatalytic' properties are claimed for the latter.     

Electrocatalysis at graphite and carbon nanotube modified electrodes: edge-plane sites and tube ends are the reactive sites

Carbon, and particularly graphite in its various forms, is an attractive electrode material. Two areas of particular interest are modified carbon electrodes and carbon nanotube electrodes. In this article we focus on the relationship between surface structure and electrochemical and chemical reactivity of electrodes based on these materials. We overview recent work in this area which has led us to believe that much of the catalytic activity, electron transfer and chemical reactivity of graphitic carbon electrodes is at surface defect sites, and in particular edge-plane-like defect sites. We also question the claimed special "catalytic" properties of carbon nanotube modified electrodes.     

Exploration of gas sensing possibilities with edge plane pyrolytic graphite electrodes: nitrogen dioxide detection

The voltammetric response of nitrogen dioxide in aqueous sulfuric acid using an edge plane pyrolytic graphite electrode has been explored and contrasted with that from basal plane pyrolytic graphite, glassy carbon or boron-doped diamond electrodes. Edge plane graphite electrode is found to produce an excellent voltammetric signal in comparison with other carbon-based electrodes exhibiting a well-defined analytically useful voltammetric redox couple in 2.5 M sulfuric acid which is absent on the alternative electrodes.     

A single-wall carbon nanotubes modified edge plane pyrolytic graphite sensor for determination of methylprednisolone in biological fluids

An electrochemical protocol based on reduction is developed to determine methylprednisolone using single-wall carbon nanotubes (SWNTs) modified edge plane pyrolytic graphite electrode (EPPGE). To obtain a good sensitivity, instrumental variables were studied using Square Wave Voltammetry (SWV). The voltammetric results indicate that SWNTs modified EPPGE remarkably enhances the reduction of methylprednisolone which leads to considerable improvement of peak current with shift of peak potential to less negative values. The voltammetric current showed a linear response for methylprednisolone concentration in the range 5-500 nM with a sensitivity of 98 nA nM⁻¹. The limit of detection was estimated to be 4.5 × 10⁻⁹ M. The developed method is used for the determination of methylprednisolone in pharmaceutical dosages and human blood plasma samples of patients undergoing treatment with methylprednisolone. The major metabolites present in blood plasma did not interfere with the present investigation as they did not exhibit reduction peak in the experimental range used. A comparison of results with high performance liquid chromatography (HPLC) indicates a good agreement.     

Edge plane pyrolytic graphite electrode covalently modified with 2-anthraquinonyl groups: theory and experiment

An edge plane pyrolitic graphite (EPPG) electrode was modified by electrochemical reduction of anthraquinone-2-diazonium tetrafluoroborate (AQ2-N(2)(+)BF(4)(-)), giving an EPPG-AQ2-modified electrode of a surface coverage below a monolayer. Cyclic voltammograms simulated using Marcus-Hush theory for 2e(-) process assuming a uniform surface gave unrealistically low values of reorganisation energies, λ, for both electron transfer steps. Subsequently, two models of surface inhomogeneity based on Marcus-Hush theory were investigated: a distribution of formal potentials, E', and a distribution of electron tunneling distances, r(0). The simulation of cyclic voltammograms involving the distribution of formal potentials showed a better fit than the simulation with the distribution of tunneling distances. Importantly the reorganization energies used for the simulation of E' distribution were similar to the literature values for adsorbed species.     

Electroanalysis of kaempferol using pyrolytic graphite and a hemoglobin/polysorbate-20 modified electrodes

The electrochemical behavior of kaempferol, one of the most common flavonoids, was studied using a pyrolytic graphite electrode and a hemoglobin/polysorbate-20 modified electrode. The modified electrode was found to be applicable for kaempferol determination. The linear calibration was in the range from 4 × 10⁻⁷ to 4 × 10⁻⁵ M, with a limit of detection (LOD) of 1 × 10⁻⁷ M. The relative standard deviation (RSD) was 3.3% for ten successive determinations. The proposed method is sensitive, has a high reproducibility, and a wide detection range. The text was submitted by the authors in English.     

Electrochemical detection of amino acids at carbon nanotube and nickel-carbon nanotube modified electrodes

The oxidation and enhanced detection of traditionally 'non-electroactive' amino acids at a single-wall carbon nanotube (SWNT) surface and at a nickel hydroxide film electrochemically deposited and generated upon the SWNT layer is reported. Different CNT are compared, with Nafion-dispersed SWNT offering the most favorable layer for constant-potential amperometric detection. Factors affecting the oxidation process, including the pH or applied potential, are assessed. The response of the SWNT-Nafion coated electrode compares favorably with that of copper and nickel disk electrodes under flow injection analysis (FIA) conditions. The electrodeposition of nickel onto the SWNT-Nafion layer (Ni-CNT) led to a dramatic enhancement of the analytical response (vs. that observed at the SWNT or nickel electrodes alone). The oxidative process at the Ni(OH)(2) layer has been studied and the increase in sensitivity rationalized. In the presence of amino acid the Ni-CNT layer undergoes an electrocatalytic process in which the amino acid reduces the newly formed NiO(OH) species. Furthermore, the attractive response of both the CNT and Ni-CNT layers has allowed these electrodes to be used for constant-potential FIA of various amino acids and indicates great promise for monitoring chromatographic effluents. Once again an improved signal was observed at the Ni-CNT electrode compared to nickel deposited upon a bare glassy carbon electrode (Ni-GC).     

Efficient electron transport across nickel powder modified basal plane pyrolytic graphite electrode: Sensitive detection of sulfhydryl degradation products of the V-type nerve agents

This communication describes the electron transport behaviour of basal plane pyrolytic graphite electrodes (BPPGEs) modified with nickel powder (BPPGE-Ni), single-walled carbon nanotube (BPPGE-SWCNT) and BPPGE-Ni decorated with SWCNT via drop-dry method (BPPGE-Ni-SWCNT). The BPPGE-Ni gave enhanced Faradaic response for the redox probe (Ferricyanide/Ferrocyanide species) and also displayed enhanced electrocatalytic behaviour towards the detection of degradation products of V-type nerve agents, dimethylaminoethanethiol (DMAET) and diethylaminoethanethiol (DEAET) with high sensitivity (∼23 × 10⁻³ AM⁻¹) and low detection limits (4.0–9.0 μM range). When compared to the notable electrodes and detection protocols reported in the literature, BPPGE-Ni exhibits more promising features required for a simple, highly sensitive, fast and less expensive electrode for the detection of these V-type nerve agents in aqueous solution. The efficient response of the BPPGE-Ni is attributed to the high microscopic surface area of the nickel powder. The poor response of the BPPGE-Ni-SWCNT suggests that nickel impurity in the SWCNT did not show any detectable impact on its electron transfer kinetics.     

Simultaneous determination of adenosine and inosine using single-wall carbon nanotubes modified pyrolytic graphite electrode

Voltammetric determination of adenosine and inosine has been carried out at single-wall carbon nanotubes (SWNTs) modified pyrolytic graphite electrode (PGE) at pH 7.2 using Osteryoung square wave voltammetry (OSWV). The modified electrode exhibits remarkable electrocatalytic properties towards adenosine and inosine oxidation with a peak potential of approximately 1229 mV and 1348 mV, respectively. Linear calibration curves are obtained over the concentration range 0.5 microM to 1.0 mM in adenosine and 10 microM to 1.0 mM in inosine with sensitivity of 1.0 microA microM(-1) and 1.9 microA microM(-1) for adenosine and inosine respectively. The limit of detection for adenosine and inosine was found to be 0.51x10(-7) M and 2.04x10(-7) M, respectively. The proposed method was also used to estimate these compounds in human blood plasma and urine samples and the method was validated using HPLC.     

Manufacture and evaluation of carbon nanotube modified screen-printed electrodes as electrochemical tools

Carboxylated multiwalled carbon nanotubes (MWCNT-COOH) dissolved in a mixture of DMF:water were used to modify the surfaces of commercially available screen-printed electrodes (SPEs). The morphology of the MWCNT-COOH and the modified SPEs was characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), respectively. SEM analysis showed a porous structure formed by a film of disordered nanotubes on the surface of the working electrode.The modification procedure with MWCNT-COOH was optimised and it was applied to unify the electrochemical behaviour of different gold and carbon SPEs by using p-aminophenol as the benchmark redox system. The analytical advantages of the MWCNT-COOH-modified SPEs as voltammetric and amperometric detectors as well as their catalytic properties were discussed through the analysis, for instance, of dopamine and hydrogen peroxide. Experimental results show that the electrochemical active area of the nanotube-modified electrode increased around 50%. The repeatability of the modification methodology is around 6% (R.S.D.) and the stability of MWCNT-COOH-modified SPEs is ensured for, at least, 2 months.     

Voltammetry and amperometric detection of tetracyclines at multi-wall carbon nanotube modified electrodes

The voltammetric behaviour and amperometric detection of tetracycline (TC) antibiotics at multi-wall carbon nanotube modified glassy carbon electrodes (MWCNT-GCE) are reported. Cyclic voltammograms of TCs showed enhanced oxidation responses at the MWCNT-GCE with respect to the bare GCE, attributable to the increased active electrode surface area. Hydrodynamic voltammograms obtained by flow-injection with amperometric detection at the MWCNT-GCE led us to select a potential value E _det = +1.20 V. The repeatability of the amperometric responses was much better than that achieved with bare GCE (RSD ranged from 7 to 12%), with RSD values for i _p of around 3%, thus demonstrating the antifouling capability of MWCNT modified electrodes. An HPLC method with amperometric electrochemical detection (ED) at the MWCNT-GCE was developed for tetracycline, oxytetracycline (OTC), chlortetracycline and doxycycline (DC). A mobile phase consisting of 18:82 acetonitrile/0.05 mol L⁻¹ phosphate buffer of pH 2.5 was selected. The limits of detection ranged from 0.09 μmol L⁻¹ for OTC to 0.44 μmol L⁻¹ for DC. The possibility to carry out multiresidue analysis is demonstrated. The HPLC-ED/MWCNT-GCE method was applied to the analysis of fish farm pool water and underground well water samples spiked with the four TCs at 2.0 × 10⁻⁷ mol L⁻¹. Solid-phase extraction was accomplished for the preconcentration of the analytes and clean-up of the samples. Recoveries ranged from 87 ± 6 to 99 ± 3%. Under preconcentration conditions, limits of detection in the water samples were between 0.50 and 3.10 ng mL⁻¹.     

Design of a new hypoxanthine biosensor: xanthine oxidase modified carbon film and multi-walled carbon nanotube/carbon film electrodes

A new and simple-to-prepare hypoxanthine biosensor has been developed using xanthine oxidase (XOD) immobilised on carbon electrode surfaces. XOD was immobilised by glutaraldehyde cross-linking on carbon film (CF) electrodes and on carbon nanotube (CNT) modified CF (CNT/CF). A comparison of the performance of the two configurations was carried out by the current response using amperometry at fixed potential; the best characteristics being exhibited by XOD/CNT/CF modified electrodes. The effects of electrolyte pH and applied potential were evaluated, and a proposal is made for the enzyme mechanism of action involving competition between regeneration of flavin adenine dinucleotide and reduction of hydrogen peroxide. Under optimised conditions, the determination of hypoxanthine was carried out at ?0.2 V vs. a saturated calomel electrode (SCE) with a detection limit of 0.75 μM on electrodes with CNT and at ?0.3 V vs. SCE with a detection limit of 0.77 μM on electrodes without CNT. The applicability of the biosensor was verified by performing an interference study, reproducibility and stability were investigated, and hypoxanthine was successfully determined in sardine and shrimp samples.     

Studies of adsorption and total heterogeneity properties of pure and modified carbon nanotube surfaces

Modified carbon multiwall nanotubes were prepared via the oxidation process by means of 65 % nitric acid and/or ferric nitrate dissolved with 65 % nitric acid. Using special thermogravimetry (Q-TG), sorptometry, and AFM methods physicochemical properties of pure and modified nanotube surfaces were investigated. A numerical and analytical procedure for the evaluation of total heterogeneous properties on the basis of liquid thermodesorption from the sample surfaces under the quasi-equilibrium conditions is presented. The calculations of the fractal dimensions of carbon nanotubes using the thermogravimetry Q-TG, sorptometry, and AFM data are presented.     

Electrochemical sensors and biosensors based on redox polymer/carbon nanotube modified electrodes: A review

The aim of this review is to present the contributions to the development of electrochemical sensors and biosensors based on polyphenazine or polytriphenylmethane redox polymers together with carbon nanotubes (CNT) during recent years. Phenazine polymers have been widely used in analytical applications due to their inherent charge transport properties and electrocatalytic effects. At the same time, since the first report on a CNT-based sensor, their application in the electroanalytical chemistry field has demonstrated that the unique structure and properties of CNT are ideal for the design of electrochemical (bio)sensors. We describe here that the specific combination of phenazine/triphenylmethane polymers with CNT leads to an improved performance of the resulting sensing devices, because of their complementary electrical, electrochemical and mechanical properties, and also due to synergistic effects. The preparation of polymer/CNT modified electrodes will be presented together with their electrochemical and surface characterization, with emphasis on the contribution of each component on the overall properties of the modified electrodes. Their importance in analytical chemistry is demonstrated by the numerous applications based on polymer/CNT-driven electrocatalytic effects, and their analytical performance as (bio) sensors is discussed.     

Vertically aligned carbon nanotube based electrodes: Fabrication, characterisation and prospects

We present a methodology to fabricate carbon nanotube based electrodes using plasma enhanced chemical vapour deposition. The metal catalyst nanoparticles used to promote nanotube growth are removed using a water plasma treatment in combination with an acid attack. The final integrated microelectrode-based devices present excellent electrocatalytic properties that make them suitable for electrochemical applications. The presented methodology enables the construction of highly regular and dense vertically aligned carbon nanotube (VACNT) forests that can be confined within the patterned bounds of a desired surface. These VACNT electrodes display very low capacitive currents and are amenable to further chemical modifications.     

Anthraquinone monosulfonate adsorbed on graphite shows two very different rates of electron transfer: surface heterogeneity due to basal and edge plane sites

Graphitic electrodes find widespread use throughout electrochemistry; understanding their fundamental electrochemical properties is imperative. It is widely thought that graphite edge plane sites exhibit faster rates of electron transfer as compared to basal plane sites. Hitherto the different rates of electron transfer at the edge and basal sites have been inferred indirectly using diffusional systems. To avoid possible complications we alternatively study a surface-bound system to simplify the interpretation. The voltammetric response of graphitic-surface-bound anthraquinone monosulfonate (AQMS) with varying pH, reveals two distinct voltammetric responses, ascribed as being due to the basal and edge plane sites; where the pK(a) s associated with the reduced anthraquinone are found to differ for the two sites. Through modelling of the system based upon a "scheme of squares" mechanism it is possible to conclude that both the thermodynamics and kinetics of the species differ in the two environments in which the rate of electron transfer at the basal plane site is shown to be 2-3 orders of magnitude slower than that of the edge plane site. This work provides the first example of a voltammetric response which is purely due to electron transfer at a basal plane site. Further, we believe this is the first time a full "scheme of squares" model has been used for the quantitative analysis of a diffusionless 2H(+)2e(-) system.