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At least 217 records · Page 12Linked to original sources

PubMed2001

Chiral supercritical fluid chromatography on porous graphitic carbon using commercial dimethyl beta-cyclodextrins as mobile phase additive.

Using dimethylated-beta-cyclodextrin mixtures (MeCD) as chiral selectors in CO2-polar modifier mobile phase and porous graphitic carbon as solid-phase, chiral supercritical (or subcritical) fluid chromatography was performed. The adsorbed quantity of MeCD onto the porous graphitic carbon (Hypercarb) was measured for various chiral selector concentrations using the breakthrough method with evaporative light scattering detector. The effects of MeCD concentration in the mobile phase, the nature of the polar modifier, the outlet pressure, the column temperature and the nature of the commercial MeCD mixture on the retention and the enantioselectivities were studied. For a given solute, the enantioselectivity is greatly dependent on the commercial MeCD mixture used. The retention mechanism was also studied. From the data, we find that the dominant mechanism for the chiral discrimination is the diastereoisomeric complexation in the mobile phase.

Chromatography, Supercritical Fluid↗

PubMed1990

Separation of anionic and cationic compounds of biomedical interest by high-performance liquid chromatography on porous graphitic carbon.

The separation of small, ionizable compounds of biomedical interest on porous graphitic carbon is described. The retention of anionic compounds is dominated by electronic interaction between the solute and the delocalized electron clouds on the graphitized carbon, while cationic compounds are mainly retained by reversed-phase interaction with the hydrophobic carbon surface. Anionic and cationic compounds can be separated simultaneously with a mobile phase containing an electronic modifier (e.g., trifluoroacetic acid) and an organic modifier (e.g., acetonitrile) for elution. Examples of applications include the measurement of oxalic acid in urine, the determination of creatine and creatinine in urine and in serum, the separation of basic drugs (remoxipride and FLA 981) and the simultaneous analysis of pertechnetate anion and the cationic technetium-amine complexes.

Anions↗

PubMedDate not supplied

Ambient STM and in situ AFM study of nitrite reductase proteins adsorbed on gold and graphite: influence of the substrate on protein interactions.

Trimeric Achromobacter cycloclastes Cu-containing nitrite reductase (CuNIR) proteins adsorbed on gold and graphite have been studied by ambient STM and in situ AFM. STM resolves them individually and in layers, distinguishing the sub-molecular individual units of the trimer. The Cu atoms are not visible to STM. STM shows that individual CuNIR denatures as it adsorbs on Au, although a deformed trimeric shape can be identified in some cases. CuNIR forms disordered layers on gold. On graphite, ordered self-assembled layers of CuNIR have been resolved by in situ AFM and ambient STM forming parallel rows whose separation distance corresponds to the size of one of the units of the trimer, 5nm. Ambient STM can achieve better resolution than in situ AFM in the images of the layers. We observe differences between domains showing the parallel row structure and unstructured parts of the CuNIR layer by in situ phase imaging AFM.

Adsorption↗

PubMed2003

NMR study of n-dodecane adsorbed on graphite.

In this brief contribution we demonstrate that 1H and 2H NMR spectroscopy can be an effective method of investigating adsorption from liquids at the solid-liquid interface. The method is illustrated here with the adsorption of a simple alkane adsorbed on graphite, in particular the system n-dodecane and graphite at coverages of 1 and 5 monolayers. Static single-pulse proton nuclear magnetic resonance and static quadrupolar echo deuterium nuclear magnetic resonance spectra were recorded for both coverages. The experimental NMR results presented here show features clearly consistent with earlier calorimetric and neutron scattering work and demonstrate the formation of solid adsorbed layers that coexist with the bulk adsorbate with both isotopes. This ability to probe both deuterated and protonated materials simultaneously illustrates that this experimental approach can be readily extended to investigate the adsorption behaviour of multicomponent mixtures.

Adsorption↗

PubMed2003

Graphite-Teflon composite bienzyme amperometric biosensors for monitoring of alcohols.

Composite graphite-Teflon electrodes, in which the enzymes alcohol oxidase (AOD) and horseradish peroxidase (HRP), as well as the mediator ferrocene, are incorporated into the electrode matrix, are reported for the reliable monitoring of alcohols in food and beverages. The bienzyme electrodes are constructed by simple physical inclusion of the enzymes and the mediator in the bulk of graphite-70% Teflon rigid cylindrical pellets. The composite biosensors are robust and reusable because of the renewability of the electrode surface by polishing. Reproducible amperometric responses at 0.00 V were obtained with different electrodes constructed from the same pellet and from different pellets. No significant loss of the enzymes activities was found after at least 3 months of storage at 0 degrees C. The detection limits obtained by amperometry in stirred solutions can be advantageously compared with those achieved with commercial sensors for alcohols. The bienzyme electrodes are suitable to be used under flow-injection conditions, as well as for amperometric detection in HPLC. The bioelectrodes were employed for the determination of ethanol in beers, wines and liquors, using both batch- and flow-injection modes, and for the determination of methanol in wines and liquors by HPLC with amperometric detection. Only a dilution of the beverages was needed as sample treatment in all cases.

Alcoholic Beverages↗

PubMed1999

Graphite-Teflon composite bienzyme electrodes for the determination of L-lactate: application to food samples.

A bienzyme amperometric graphite-Teflon composite biosensor, in which lactate oxidase (LOD) and peroxidase, together with the mediator ferrocene, are incorporated into the electrode matrix, was developed for the determination of L-lactate in food samples such as wine and yogurt by using both batch- and flow-injection modes. This bienzyme electrode was fabricated by simple physical inclusion of the enzymes and the mediator in the bulk of the graphite-Teflon matrix. A Teflon content of 70%, an applied potential of 0.00 V, and a pH of 7.4 were employed as working conditions. The composite bioelectrode exhibited long-term operation because of the renewability of its surface by polishing. Reproducible amperometric responses were achieved with different electrodes fabricated from different composite matrices, and no significant loss of the enzyme activity occurred after 6 months of storage at 4 degrees C. Detection limits for L-lactate of 1.4 and 0.9 microM were obtained by batch amperometry in stirred solutions and flow-injection with amperometric detection, respectively. An interferences study with different substances which may be present in wine and yogurt together with L-lactic acid demonstrated very good selectivity for the determination of this analyte. The bienzyme composite electrode was applied to the determination of L-lactic acid in red wine and shaken yogurt, and the methods were validated by comparing these results with those obtained by applying a recommended reference method.

Biosensing Techniques↗

PubMed2003

Electrochemical behaviour of human adrenodoxin on a pyrolytic graphite electrode.

Adrenodoxin (Adx) functions as a redox protein in the delivery of electrons to all mitochondrial cytochromes P450. In order to further characterize the human form of this protein, direct electrochemistry of human adrenodoxin (Hadx) has been observed for the first time on a pyrolytic graphite electrode (PGE) modified with poly-L-lysine. A single well-defined redox wave was observed with a midpoint potential of -448+/-3 mV vs. Ag/AgCl (sat. KCl) at scan rates of 10 mV/s and over the pH range 4.0-8.0. At slow scan rates, the reduction process was close to being electrochemically reversible whereas, at faster scan rates, only quasi-reversibility was observed. A correlation was observed between the peak separation (DeltaE) for the cyclic voltammograms and pH over a wide range of scan rates. The variation of DeltaE with pH was at a minimum (optimum reversibility) at pH 7.0 for all scan rates tested. This correlation may suggest that the direct electrochemistry method could possibly provide a means for determining protein or enzyme activity. The electron transfer rate constant, k(s), was determined to be 0.28 s(-1) at pH 7.0 and a small pH dependence was observed. The results obtained in this study demonstrate the facile nature of direct electron transfer for human adrenodoxin, and provide an estimate of the midpoint reduction potential at a pyrolytic graphite electrode via electrostatic immobilisation.

Adrenodoxin↗

PubMed1993

Surfactant-modified graphite surfaces in biological analysis: ionic strength and ion charge effects.

Previous work with rough pyrolytic graphite and glassy carbon electrodes demonstrated that surfactant assembly at these two structurally and chemically different surfaces was similar, with surfactants adsorbing head on. Improvement in response of biological molecules was attributed to the formation of a dynamic, renewable surface. Furthermore, surfactants provided a favorable hydrophobic-hydrophilic environment for the response of selected catechols. Surfactants could also be used to control selectivity if needed. In this work ionic strength and cation charge effects on response of catechols in surfactant solutions are investigated. The results illustrate the relative importance of electrostatic and hydrophobic interactions at graphite to the response of probes with combined hydrophobic and hydrophilic properties. Furthermore, the results show that although catechol-surface interactions are significantly affected by ionic strength and electrolyte charge, surfactant-surface and surfactant-probe interactions are not significantly affected, indicating that surfactants serve as an effective ionic buffer. The results presented here indicate that surfactant assembly at the surface is best at low ionic strength. These results also verify the importance of a favorable hydrophobic and hydrophilic environment to catechol response which can be provided by the surfactants when needed.

3,4-Dihydroxyphenylacetic Acid↗

PubMed2002

A colloidal graphite-coated emitter for sheathless capillary electrophoresis/nanoelectrospray ionization mass spectrometry.

A colloidal graphite-coated emitter is introduced for sheathless capillary electrophoresis/nanoelectrospray ionization time-of-flight mass spectrometry (CE/ESI-TOFMS). The conductive coating can be produced by brushing the capillary tip to construct a fine layer of 2-propanol-based colloidal graphite. The fabrication involves a single step and requires less than 2 min. Full cure properties develop in approximately 2 h at room temperature and then the tip is ready for use. The coated capillary tip is applied as a sheathless electrospray emitter. The emitter has proven to bear stable electrospray and excellent performance for 50 microm i.d. x 360 microm o.d. and 20 microm i.d. x 360 microm o.d. capillaries within the flow rate of 80-500 nL/min; continuous electrospray can last for over 200 h in positive mode. Baseline separation and structure elucidation of two clinically interesting basic drugs, risperidone and 9-hydroxyrisperidone, are achieved by coupling pressure-assisted CE to ESI-TOFMS using the described sheathless electrospray emitter with a bare fused-silica capillary at pH 6.7. It is found that the signal intensity of m/z in sheathless CE/ESI-TOFMS at pH 6.7 is approximately 50 times higher than that at pH 9.0 for the two analytes, although the electroosmotic flow (EOF) at pH 9.0 provides sufficient flow rate (approximately 150 nL/min) to maintain electrospray.

Colloids↗

PubMed2003

A high-throughput method for the conversion of CO2 obtained from biochemical samples to graphite in septa-sealed vials for quantification of 14C via accelerator mass spectrometry.

The growth of accelerator mass spectrometry as a tool for quantitative isotope ratio analysis in the biosciences necessitates high-throughput sample preparation. A method has been developed to convert CO(2) obtained from carbonaceous samples to solid graphite for highly sensitive and precise (14)C quantification. Septa-sealed vials are used along with commercially available disposable materials, eliminating sample cross contamination, minimizing complex handling, and keeping per sample costs low. Samples containing between 0.25 and 10 mg of total carbon can be reduced to graphite in approximately 4 h in routine operation. Approximately 150 samples per 8-h day can be prepared by a single technician.

Biochemistry↗

PubMed2003

Graphite-coated nanoelectrospray emitter for mass spectrometry.

A new, more rapid method for coating nanoelectrospray emitters with graphite is to use a vacuum deposition chamber and a graphite carbon electrode. This method allows for mass production of nanoelectrospray emitters in a short period of time. The emitters are laser-pulled borosilicate glass micropipets and have tapers of around 4 microm i.d. The conductive coating applied to the emitter is only 20-30 nm thick, allowing for optical transparency with the borosilicate emitters. The conductive coating is stable for a number of hours at the high voltages used for nanoelectrospray ionization and is durable in both positive and negative ion modes-even during electrical discharge. This stability will make it possible to couple these emitters with online separations such as capillary liquid chromatography or capillary electrophoresis.

Coated Materials, Biocompatible↗

PubMed1998

Fast-scan cyclic voltammetry of protein films on pyrolytic graphite edge electrodes: characteristics of electron exchange.

The rapid electron-exchange characteristics of metalloproteins adsorbed at a pyrolytic graphite "edge" electrode have been studied by analog dc cyclic voltammetry at scan rates up to 3000 V s-1. The voltammetry of four proteins, azurin (a "blue" copper protein) and three 7Fe ferredoxins, reveals oxidation and reduction peaks that display only modest increases in width and peak separation as the scan rate is raised. This is indicative of a substantially homogeneous population of noninteracting centers which undergo rapid electron exchange with the electrode. Both the Butler--Volmer and Marcus models have been tested. The electrochemical kinetics, as reflected by k0 (the rate at zero overpotential), are too fast to allow the determination of reorganization energies by this method. Nonetheless, the rapid and energetically coherent nature of the electron transfer enables the cyclic oxidation and reduction of protein redox centers to be examined on a time scale sufficiently short to recognize coupled processes occurring in the millisecond time domain, which are characteristic of the protein under investigation. Two of the ferredoxins display increasingly asymmetric voltammetry as the scan rate is increased, which is attributed to the coupling of electron transfer to conformational (or orientational) changes. For azurin, the use of higher electrolyte concentrations enables studies to be made at scan rates up to 3000 V s-1, from which a standard electron-transfer rate constant in the region of 5000 s-1 is obtained. At these high scan rates, azurin still shows very symmetrical voltammograms but with peak shapes displaying a more gradual decrease in current, at increasing overpotential, than is predicted using realistic values of the reorganization energy. The ability to measure even faster rate constants and access coupled reactions occurring in shorter time domains is likely to be limited by complex processes occurring on the graphite surface.

Archaeal Proteins↗

PubMed2006

Functionalized single graphene sheets derived from splitting graphite oxide.

A process is described to produce single sheets of functionalized graphene through thermal exfoliation of graphite oxide. The process yields a wrinkled sheet structure resulting from reaction sites involved in oxidation and reduction processes. The topological features of single sheets, as measured by atomic force microscopy, closely match predictions of first-principles atomistic modeling. Although graphite oxide is an insulator, functionalized graphene produced by this method is electrically conducting.

Electric Conductivity↗

PubMed2006

In situ synthesis of platelet graphite nanofibers from thermal decomposition of poly(ethylene glycol).

Platelet graphite nanofibers (PGNFs) were synthesized by in situ thermal decomposition from a mixture containing poly(ethylene glycol) (PEG) serving as the carbon source and nickel chloride (NiCl(2)) serving as the catalytic precursor. The mixture was conducted by thermal treatment under a nitrogen atmosphere at 750 degrees C and results found PGNFs with high purity and a uniform diameter distribution formed without hydrocarbon gases in the process. Observations using field-emission scanning electron microscopy and high-resolution transmission electron microscopy revealed PGNFs with a high degree of graphitization, well-ordered graphene layers, and uniform diameters of 10-20 nm. Thermogravimetry-differential scanning calorimetry-mass spectrometry was employed to study the thermal decomposition phenomena of the mixture (PEG/NiCl(2)) before the thermal process. The analysis clarified the in situ synthesis growth mechanism of PGNFs from the mixture.

Graphite↗

PubMed2006

Bioluminescence of monolayers of firefly luciferase immobilized on graphite.

We report on the immobilization of the firefly protein luciferase on the hydrophobic surface of graphite. Observation by liquid-phase atomic force microscopy of islands with a height consistent with the size of a single molecule confirmed that the protein was contained within a monomolecular layer. The enzyme activity was assayed by single-photon counting of the bioluminescence, which is the catalytic product of luciferase. Attachment to the surface modified the efficiency of the enzyme, but the introduction of the substrates luciferin and ATP resulted in the reactivation of the enzyme. The functionalized graphite surface was employed as a cathode in a bioelectrochemical cell. This demonstrated that the electric field caused a substantial loss of enzyme catalytic activity.

Adenosine Triphosphate↗

PubMed1997

Miniaturized graphite sensors doped with metal-bathophenanthroline complexes for the selective potentiometric determination of uric acid in biological fluids.

Miniaturized poly(vinyl chloride) matrix membrane sensors in an all-solid-state graphite support, responsive to urate anion, were developed. The membranes incorporate lipophilic ion-pair complexes of urate anion with ruthenium(III), iron(II), nickel(II) and copper(I) bathophenanthroline (4,7-diphenyl-1,10-phenanthroline) counter cations. The sensors demonstrate a near-Nernstian response to urate over the concentration range 1 x 10(-2)-1 x 10(-5) mol l-1 and have micromolar detection limits and good selectivity properties. The response is virtually unaffected by pH changes in the range 7-10 and the response times are 5-10 s in aqueous solutions and in human serum and urine samples. A flow injection detector incorporating an iron(II) bathophenanthroline-urate graphite sensor was used for continuous monitoring of uric acid. The minimum detectable concentration was approximately 8 micrograms ml-1 and the sample throughput was approximately 120 h-1. Direct potentiometric determination of uric acid in the static and hydrodynamic modes of operation over the range 15 micrograms ml-1-1.5 mg ml-1 showed average recoveries of 98.7 and 97.8% with RSDs of 0.6 and 0.7%, respectively. Application of the method to the determination of uric acid in human serum and urine gave results that compared favourably with those obtained by the standard spectrophotometric method.

Flow Injection Analysis↗

PubMed1994

Amperometric biosensor for phenols based on a tyrosinase-graphite-epoxy biocomposite.

A new biocomposite, based on the incorporation of the enzyme tyrosinase into a graphite-epoxy resin matrix, was used for the effective biosensing of phenolic compounds. The enzyme retains its bioactivity on confinement in the epoxy resin environment. This renewable (polishable) and rigid bioprobe offers convenient quantification for various phenolic substrates. The fast response (steady-state time = 25 s) accrues from the close proximity of the enzyme and graphite sites. The influence of various experimental variables was explored for optimum biosensing performance. Flow-injection monitoring of phenolic compounds at a rate of 50 samples h-1 yielded a detection limit of 1 x 10(-6) mol l-1 and a relative standard deviation of 1.4% (n = 40).

Biosensing Techniques↗

PubMed2004

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.

Catalysis↗