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PubMed1975

Interaction of nucleic acids with electrically charged surfaces. VI. A comparative study on the electrochemical behaviour of native and denatured DNAs at graphite electrodes.

Adsorption and electrochemical oxidation of deoxyribonucleic acid (DNA) at a pyrolytic graphite electrode (PGE) and a paraffin wax-impregnated spectroscopic graphite electrode (WISGE) were studied using differential pulse voltammetry. DNA is adsorbed at the surface of the graphite electrodes in a broad range of potentials including the potentials of electrochemical oxidation of DNA. Both native and denatured DNAs yield two single, well-defined and separated peaks, G and A, on the differential pulse voltammograms at the PGE and WISGE. The more negative peak, G, corresponds to electrochemical oxidation of guanine residues, whereas the more positive peak, A, corresponds to electrochemical oxidation of adenine residues. Peaks G and A of native DNA occur at the same potentials as peaks G and A of denatured DNA. However, electrochemical oxidation of adenine and guanine residues at graphite electrodes is markedly suppressed in native DNA. The heights of the peaks G and A represent a sensitive indicator of the helix-coil transition of DNA. An analysis of the product of interaction of a sample of native DNA with a large pyrolytic graphite electrode in the presence of formaldehyde at approximately neutral pH did not prove changes in the secondary structure of native DNA due to its interaction with the graphite electrode. It is suggested that the decreased differential pulse-voltammetric activity of native DNA is connected with its decreased flexibility.

Journal Article↗
PubMed2000

Graphite microparticles as coatings for quartz crystal microbalance-based gas sensors.

The use of graphite particles (1-2 microm) as coatings on quartz crystal microbalances (QCMs) for detection and monitoring of toluene and other volatile organic compounds (VOCs) is described. Unlike the more commonly used polymeric coatings with low glass transition temperatures (Tg), particulate graphitic coatings are not as susceptible to loss of acoustic energy when coating thickness or operational temperature increases. This situation enables the use of relatively thick coatings, which increases the absolute amount of vapor sorbed in the coating and, consequently, lowers the level of detection and enhances operation over a wide temperature range. The use of small size particles also results in a coating with a more porous structure, which facilitates uptake and release of VOCs in comparison to coatings made from high Tg polymers, which have a lower porosity. These attributes, coupled with the inherent stability of graphitic materials, make particulate graphite coatings especially suitable for applications at high temperatures. The advantages of using particulate graphite as a coating on QCMs are demonstrated by comparison to the performance of a few low-Tg polymers [i.e., poly(isobutylene) and poly(diphenoxyphosphazene)] and high-Tg polymers (i.e., polystyrene).

Crystallization↗
PubMed2002

Graphite-mediated reduction of 2,4-dinitrotoluene with elemental iron.

The mechanism and pathway through which 2,4-dinitrotoluene (DNT) is reduced with elemental iron were investigated through batch experiments performed utilizing the same iron surface area, with high-purity iron powder and Master Builders scrap iron. In addition to different kinetics and adsorption patterns, the distribution of two intermediates, 4-amino-2-nitrotoluene (4A2NT) and 2-amino-4-nitrotoluene (2A4NT), contrasted sharply. This suggests that different mechanisms are involved in DNT reduction with pure iron and scrap iron. We hypothesized that exposed graphite in scrap iron transferred reductants from iron to adsorbed nitroaromatic molecules. This hypothesis was supported by an experiment conducted using two-compartment dialysis cells in which ONT and pure iron powder were separated by a graphite sheet. Results indicate that graphite-mediated, indirect reduction of DNT occurred primarily through reduction of the ortho nitro group to form 2A4NT, whereas DNT reduction at the iron (hydr/oxide) surface occurred via para nitro reduction to give 4A2NT. Based on pH and product analysis, atomic hydrogen probably accounted for most of the reducing equivalents that passed through the graphite, reacting with adsorbed DNT mainly through ortho nitro reduction. In contrast, electron was a minor fraction of the reducing equivalents, reducing DNT mainly through para nitro reduction. The implications of graphite as a reaction site and conductor of electron and atomic hydrogen are discussed with respect to treatment processes involving iron.

Carcinogens↗
PubMed2006

Interaction energy of a water molecule with a single-layer graphitic surface modeled by hydrogen- and fluorine-terminated clusters.

In ab initio calculations a finite graphitic cluster model is often used to approximate the interaction energy of a water molecule with an infinite single-layer graphitic surface (graphene). In previous studies, the graphitic cluster model is a collection of fused benzene rings terminated by hydrogen atoms. In this study, the effect of using fluorine instead of hydrogen atoms for terminating the cluster model is examined to clarify the role of the boundary. The interaction energy of a water molecule with the graphitic cluster was computed using ab initio methods at the MP2 level of theory and with the 6-31G(d = 0.25) basis set. The interaction energy of a water molecule with graphene is estimated by extrapolation of two series of increasing size graphitic cluster models (C(6n2)H(6n) and C(6n2)F(6n), n = 1-3). Two fixed orientations of water molecule are considered: (a) both hydrogen atoms of water pointing toward the cluster (mode A) and (b) both hydrogen atoms of water pointing away from the cluster (mode B). The interaction energies for water mode A are found to be -2.39 and -2.49 kcal/mol for C(6n2)H(6n) and C(6n2)F(6n) cluster models, respectively. For water mode B, the interaction energies are -2.32 and -2.44 kcal/mol for C(6n2)H(6n) and C(6n2)F(6n) cluster models, respectively.

Computer Simulation↗
PubMed2004

Comparative study of Li, Na, and K adsorptions on graphite by using ab initio method.

A comprehensive study has been conducted to compare the adsorptions of alkali metals (including Li, Na, and K) on the basal plane of graphite by using molecular orbital theory calculations. All three metal atoms prefer to be adsorbed on the "middle hollow site" above a hexagonal aromatic ring. A novel phenomenon was observed, that is, Na, instead of Li or K, is the weakest among the three types of metal atoms in adsorption. The reason is that the SOMO (single occupied molecular orbital) of the Na atom is exactly at the middle point between the HOMO and the LUMO of the graphite layer in energy level. As a result, the SOMO of Na cannot form a stable interaction with either the HOMO or the LUMO of the graphite. On the other hand, the SOMO of Li and K can form a relatively stable interaction with either the HOMO or the LUMO of graphite. Why Li has a relatively stronger adsorption than K on graphite has also been interpreted on the basis of their molecular-orbital energy levels.

Adsorption↗
PubMed1992

Curling and closure of graphitic networks under electron-beam irradiation.

The discovery of buckminsterfullerene (C60) and its production in macroscopic quantities has stimulated a great deal of research. More recently, attention has turned towards other curved graphitic networks, such as the giant fullerenes (Cn, n > 100) and carbon nanotubes. A general mechanism has been proposed in which the graphitic sheets bend in an attempt to eliminate the highly energetic dangling bonds present at the edge of the growing structure. Here, I report the response of carbon soot particles and tubular graphitic structures to intense electron-beam irradiation in a high-resolution electron microscope; such conditions resemble a high-temperature regime, permitting a degree of structural fluidity. With increased irradiation, there is a gradual reorganization of the initial material into quasi-spherical particles composed of concentric graphitic shells. This lends weight to the nucleation scheme proposed for fullerenes, and moreover, suggests that planar graphite may not be the most stable allotrope of carbon in systems of limited size.

Carbon↗
PubMed2005

Activation analysis of concrete and graphite in the experimental reactor RUS.

The decommissioning and dismantling of nuclear installations after their service life involves the necessary disassembling, handling and disposing of a large amount of radioactive equipment and structures. In particular, the concrete that has been used as a biological reactor shield and graphite that has been used as a moderator-reflector represent the majority of waste, requiring geological disposal. To reduce this undesirable volume to the minimum and to successfully plan the dismantling and disposal of radioactive materials to storage facilities, the activations of the structures should be accurately evaluated. In the framework of the decommissioning and the dismantling of the experimental reactor of the University of Strasbourg, detailed activation estimates have been conducted to characterise the graphite and the structural materials present in the reactor environment. For this purpose, the chemical compositions of fresh graphite samples and different types of concrete have been determined by activation analysis in the research reactors OSIRIS and ORPHEE of CEA Saclay (France). Then, the activations of graphite, concrete and other materials have been calculated in the whole reactor, as a function of the three main nuclear data libraries, i.e. ENDF, JEF and JENDL. In parallel, the activations of representative graphite and concrete samples have been measured experimentally. The comparison of theoretical predictions with experimental values validates the approach and the methodology used in the present study and tests the consistency and the reliability of the nuclear data used for activation analysis. We believe that a similar approach could also be used for the decommissioning of industrial nuclear reactors.

Algorithms↗
PubMed2005

Modelling of activation processes for GR-280 graphite at Ignalina NPP.

Ignalina Nuclear Power Plant (INPP) operates two RBMK-1500 water-cooled graphite-moderated channel-type power reactors. The total mass of graphite in the cores of both units at INPP is about 3600 tons. Modelling of activation processes in the reactor's structural materials is necessary for decommissioning planning, because large amounts of activated structural materials (graphite, stainless steel, concrete, etc.) should be managed as radioactive waste. Knowledge of radiological characteristics and a radioactive inventory of irradiated materials are essential in planning of the decommissioning processes. The purpose of this work was to perform conservative neutron activation analysis for decommissioning purposes of INPP. ORIGEN computer code was used for the calculations. Activity levels were calculated for different nuclides present in the graphite and estimates were made how these activity levels depend on irradiation time, neutron flux value and other parameters. Obtained results were compared with the data available from other investigations for GR-280 graphite.

Computer Simulation↗
PubMedDate not supplied

Nonasbestos ferruginous bodies in sputum from a patient with graphite pneumoconiosis: a case report.

BACKGROUND: Inhalation of graphite dust can cause lung disease, mostly in the form of mixed-dust pneumoconiosis in individuals working in the metallurgic industry or graphite mines. The morphologic landmark of graphite pneumoconiosis is nonasbestos ferruginous bodies with a black graphite core. CASE: In the sputum of an 81-year-old male, characteristic nonasbestos ferruginous bodies were identified. The occupational history confirmed long exposure to natural graphite in metallurgy. CONCLUSION: By the identification of typical ferruginous bodies, the screening of sputum smears may confirm the clinical and radiologic suspicion of pneumoconiosis and may contribute to determining its causes.

Aged↗
PubMed2005

Use of a subepithelial connective tissue graft to treat an area pigmented with graphite.

BACKGROUND: Pigmentations of the oral cavity are commonly noted in clinical practice and may have physiologic, non-physiologic, or pathologic etiologies. The most common non-physiologic localized pigmentation is the amalgam tattoo; another, less common, non-physiologic pigmentation is the graphite tattoo. Graphite tattoos may easily be confused with amalgam tattoos but have only infrequently been reported in the literature. METHODS: This case report discusses a patient with a localized pigmented lesion involving the free gingiva, attached gingiva, and alveolar mucosa. The patient was referred for esthetic enhancement of the area in question. Dental history revealed trauma to the area involving falling on a pencil as a child. The lesion was excised and submitted for histologic examination; a subepithelial connective tissue graft was harvested and utilized to prevent an esthetic defect. RESULTS: Histologic evaluation confirmed graphite within the soft tissue as the etiology of the discoloration. At the 2-month postoperative appointment, a highly esthetic outcome was obtained utilizing a connective tissue graft. CONCLUSIONS: Although less common than the amalgam tattoo, the graphite tattoo may be encountered in the course of routine dental examinations and should be included in a differential diagnosis of any localized pigmented lesion. Although histologic evaluation is necessary to rule out pathology, if the dental history is consistent with the clinical impression of a nonpathologic lesion, such as the graphite tattoo in this case report, treatment with an autogenous connective tissue graft at the time of excision can produce a highly esthetic result and avoid additional surgical procedures in the future.

Adolescent↗
PubMed2001

Analysis of high purity graphite and silicon carbide by direct solid sampling electrothermal atomic absorption spectrometry.

Solid sampling electrothermal atomic absorption spectrometry using the boat technique and a transversely heated graphite tube was applied to direct analysis of graphite and silicon carbide powders for 14 and 12 impurity elements, respectively. With graphite, for all analytes under investigation, a very effective in-situ analyte/matrix separation was achieved. That was the case also for analytes in silicon carbide requiring atomization temperatures below 2400 degrees C. At higher atomization temperatures, the decomposition products of silicon carbide give rise to significant background, which can still be corrected. Sample amounts of up to 4 mg graphite and 8 mg silicon carbide per analysis cycle were applied. For all analytes in both materials, limits of detection at the lower ng g(-1) and sub-ng g(-1) level were achieved, excluding arsenic for which they were 50 ng g(-1) and 23 ng g(-1) for graphite and silicon carbide, respectively. Quantification was performed using calibration curves measured with aqueous standard solutions. The accuracy was checked by comparison of the results with those obtained by instrumental neutron activation analysis and by other independent methods.

Journal Article↗
PubMed2004

Affinity transformation from hydrophilicity to hydrophobicity of water molecules on the basis of adsorption of water in graphitic nanopores.

The interaction of water with hydrophobic surfaces is quite important in a variety of chemical and biochemical phenomena. The coexistence of water and oil can be realized by introduction of surfactants. In the case of water vapor adsorption on graphitic nanopores, plenty of water can be adsorbed in graphitic nanopores without surfactants, although the graphitic surface is not hydrophilic. Why are water molecules adsorbed in hydrophobic nanopores remarkably? This work can give an explicit insight to water adsorption in hydrophobic graphite nanopores using experimental and theoretical approaches. Water molecules are associated with each other to form the cluster of 1 nm in size, leading to a significant stabilization of the cluster in the graphitic nanopores. This mechanism can be widely applied to interfacial phenomena relating to coexistence of water and nanostructural materials of hydrophobicity.

Journal Article↗
PubMed2006

Cluster-support interactions and morphology of MoS2 nanoclusters in a graphite-supported hydrotreating model catalyst.

Supported MoS(2) nanoparticles constitute the active component of the important hydrotreating catalysts used for industrial upgrading and purification of the oil feedstock for the production of fossil fuels with a low environmental load. We have synthesized and studied a model system of the hydrotreating catalyst consisting of MoS(2) nanoclusters supported on a graphite surface in order to resolve a number of very fundamental questions related to the atomic-scale structure and morphology of the active clusters and in particular the effect of a substrate used in some types of hydrotreating catalysts. Scanning tunneling microscopy (STM) is used to image the atomic-scale structure of graphite-supported MoS(2) nanoclusters in real space. It is found that the pristine graphite (0001) surface does not support a high dispersion of MoS(2), but by introducing a small density of defects in the surface, highly dispersed MoS(2) nanoclusters could be synthesized on the graphite. From high-resolution STM images it is found that MoS(2) nanoclusters synthesized at low temperature in a sulfiding atmosphere preferentially grow as single-layer clusters, whereas clusters synthesized at 1200 K grow as multilayer slabs oriented with the MoS(2)(0001) basal plane parallel to the graphite surface. The morphology of both single-layer and multilayer MoS(2) nanoclusters is found to be preferentially hexagonal, and atom-resolved images of the top facet of the clusters provide new atomic-scale information on the MoS(2)-HOPG bonding. The structure of the two types of catalytically interesting edges terminating the hexagonal MoS(2) nanoclusters is also resolved in atomic detail in STM images, and from these images it is possible to reveal the atomic structure of both edges and the location and coverage of sulfur and hydrogen adsorbates.

Journal Article↗
PubMed2005

Why h atom prefers the on-top site and alkali metals favor the middle hollow site on the Basal plane of graphite.

In this work, the different adsorption properties of H and alkali metal atoms on the basal plane of graphite are studied and compared using a density functional method on the same model chemistry level. The results show that H prefers the "on-top site" while alkali metals favor the "middle hollow site" of graphite basal plane due to the unique electronic structures of H, alkali metals, and graphite. H has a higher electronegativity than carbon, preferring to form a covalent bond with C atoms, whereas alkaline metals have lower electronegativity, tending to adsorb on the highest electrostatic potential sites. During adsorption, there are more charges transferred from alkali metal to graphite than from H to graphite.

Journal Article↗
PubMed2001

Effect of interfering substances on current response of recombinant peroxidase and glucose oxidase-recombinant peroxidase modified graphite electrodes.

Graphite electrodes have been modified with different forms of horseradish peroxidase (HRP). These included native HRP, wild-type recombinant HRP, and two single-point recombinant HRP mutants, N70V and N70D. The mediator-less response of these electrodes to H2O2 was studied indicating that electrodes modified with recombinant HRP forms are more stable than those modified with native HRP. Various interfering compounds were investigated for their effect on the current response to H2O2. It was found that interferences such as acetaminophen and dopamine affected the response by mediating the electron transfer (ET) between graphite and peroxidases. The mediating behaviour manifested itself as an increased current of the electrode to H2O2. The interfering effect was less pronounced for the electrodes modified with recombinant HRPs possessing better electronic coupling with the graphite surface. The interfering behaviour of acetaminophen on the response for glucose with the bienzyme electrode containing co-immobilised glucose oxidase and HRP was mainly ascribed to mediation of ET between graphite and HRP. It was experimentally proven that a high efficiency of direct ET between graphite and recombinant HRP substantially reduces the interfering effect of acetaminophen.

Journal Article↗
PubMed2003

A comparison of the electrochemical stabilities of metal, polymer and graphite coated nanospray emitters.

Chronoamperometry (CA) and cyclic voltammetry (CV) were used to compare the electrochemical behavior of metal, polymer and graphite coated nanospray emitters. It is shown that electrochemical reactions occurring at the emitter surface limit the lifetime of the noble metal coated nanospray emitters while the graphite coated nanospray emitters show good electrochemical stabilities. Although the surface of the graphite coated emitters may be passivated at positive potentials, the conductive coating is not lost as for the noble metal coated nanospray emitters. The graphite coated nanospray emitters still produced a stable nanospray signal despite the presence of a passivated surface. The polymer (i.e. polyaniline) coated nanospray emitters showed very low electrochemical activity and could not be thoroughly tested by CA. The relative short lifetimes seen in the electrochemical tests are qualitatively comparable with those obtained in nanospray experiments, in which only the outmost tip of the emitter is electrochemically active. However, the electrochemical stress during CA far exceeds the stress during ESI, which implies that CA can be used to perform quick and simple estimates of emitter stabilities. To our knowledge, this is the first time the electrochemical behavior of metal, polymer and graphite coated nanospray emitters has been compared.

Journal Article↗
PubMed2000

Laser-induced graphitization on a diamond (111) surface.

We report an atomistic simulation study of laser-induced graphitization on the diamond (111) surface. Our simulation results show that the diamond to graphite transition occurs along different pathways depending on the length of the laser pulse being used. Under nanosecond or longer laser pulses, graphitization propagates vertically into bulk layers, leading to the formation of diamond-graphite interfaces after the laser treatment. By contrast, with femtosecond (0.2-0.5 ps) laser pulses, graphitization of the surface occurs layer by layer, resulting in a clean diamond surface after the ablation. This atomistic picture provides an explanation of recent experimental observations.

Journal Article↗
PubMed1997

The initial reactions of graphite and gold with blood.

The initial reactions of graphite and gold with blood were investigated by short-time exposure to capillary blood and detection of surface-adsorbed plasma proteins and cells with an immunofluorescence technique. Antibodies specific to fibrinogen, complement factors C1q and C3c, prothrombin/thrombin, von Willebrand factor, and platelet- and leukocyte-membrane antigens were used. The fluorescence intensity was quantitated by computer-aided image analysis. Fibrinogen was the most abundant plasma protein immobilized on either surface, and dense populations of platelets adhered to the protein layer. Complement factors and prothrombin/thrombin were found on the graphite surface, localized in fibrin clots or related to platelets. Platelets were activated (expression of selectin CD62) on both surfaces but more extensively so on the gold surface. Activation of polymorphonuclear granulocytes (PMNGs), measured as expression of integrin CD11b, was seen on both surfaces but with different kinetics. On the graphite surface, the CD11b expression was only transient whereas on gold it increased with time. Our data indicate that graphite is more thrombogenic than gold but less inflammatory.

Adsorption↗