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At least 199 records · Page 11Linked to original sources
PubMed1993

An amperometric flow-injection analysis biosensor for glucose based on graphite paste modified with tetracyanoquinodimethane.

A biosensor system using flow injection analysis (FIA) has been developed for the analysis of glucose in human serum. The system consists of the enzyme glucose oxidase incorporated into graphite paste modified with the electroactive material tetracyanoquinodimethane (TCNQ). TCNQ acts as an efficient mediator for oxidation of the reduced enzyme at 200 mV vs Ag/AgCl. The flow injection assay described has detection limits of 2 mM glucose using a 100-microliters sample injection through a 250-microliters sample loop. Data are presented to show the effect of sample injection volume and flow rate on the response of the FIA sensor. The biosensor exhibited excellent reproducibility for 800 injections. The loss of response after 800 injections was due to leaching of TCNQ from the graphite paste. Each assay takes 3 min giving a sample throughput of 20 per hour at a flow rate of 30 ml/h. The sensor was applied to the determination of glucose in human serum. The glucose measurements are in good agreement with those of a commercially available spectrophotometric method. Data showing the effect of interfering substances, ascorbic acid and acetaminophen, on the response of the sensor are also reported.

Acetaminophen↗
PubMed2006

Electrocatalytic reduction of hydrogen peroxide on modified graphite electrodes: application to the development of glucose biosensors.

The electrocatalytic activities of a series of compact graphites modified with microquantities of platinum metals (Pd or Pt+Pd) towards the electrochemical reduction of hydrogen peroxide were characterised. Operational parameters such as the optimal working potential, the influence of temperature and the resulting electrode characteristics were examined. The benefits of using graphite modified with Pt+Pd (mixture ratio 30%:70%) as the basic transducer in a glucose biosensor with improved sensitivity were demonstrated. It was proven that, under the working conditions chosen, the selected electrode (whether bare or covered with an enzyme layer) did not respond to any glutathione, uric acid or ascorbic acid (which all normally occur in biological fluids) present.

Biosensing Techniques↗
PubMed2000

Construction and analytical application of a biosensor based on stearic acid-graphite powder modified with sweet potato tissue in organic solvents.

A biosensor based on stearic acid-graphite powder modified with sweet potato (Ipomoea batatas (L.) Lam.) tissue as peroxidase source was constructed and applied in organic solvents. Several parameters were studied to evaluate the performance of this biosensor such as stearic acid-graphite powder and tissue composition, type and concentration of supporting electrolyte, organic solvents, water/organic solvent ratio (% v/v) and hydrogen peroxide concentration. After selection of the best conditions, the biosensor was applied for the determination of hydroquinone in cosmetic creams in methanol. At the peroxidase electrode hydroquinone is oxidized in the presence of hydrogen peroxide and the radical formed was reduced back electrochemically at -180 mV vs Ag/AgCl (3.0 mol L(-1) KCl). The reduction current obtained was proportional to the concentration of hydroquinone from 6.2 x 10(-5) to 1.5 x 10(-3) mol L(-1) (r = 0.9990) with a detection limit of 8.5 x 10(-6) mol L(-1). The recovery of hydroquinone from two samples ranged from 98.8 to 104.1% and an RSD lower than 1.0% for a solution containing 7.3 x 10(-4) mol L(-1) hydroquinone and 1.0 x 10(-3) mol L(-1) hydrogen peroxide in 0.10 mol L(-1) tetrabutylammonium bromide methanol-phosphate buffer solution (95:5% v/v) (n = 10) was obtained.

Antioxidants↗
PubMed1982

A micromethod for the analysis of zinc in plasma or serum by atomic absorption spectrophotometry using graphite furnace.

A micromethod is described for the analysis of zinc in plasma or serum by electrothermal atomisation and graphite furnace utilising less than 20 microliters sample volume. The sample matrix introduced an error when aqueous standards were used. Therefore standards were made up in serum which had been treated with Chelex 100 to remove as much zinc as possible. The residual zinc (less than 0.125 mg/l) was determined by flame analysis using the method of additions and each standard assigned a value equal to the sum of the residual zinc and the amount added. Furnace analyses using standards made up in Chelex-treated serum compared favourably with flame analyses over the whole range, and recovery experiments gave 98 +/- 2% of the expected value with 95 +/- 6% recovery of the added zinc. The within-batch coefficient of variation was 2.5%, day-to-day variation 5.6%. The mean concentration of zinc in paired samples of plasma and serum from 50 adults analysed by flame with viscosity-matched aqueous standards were: plasma (mean +/- SD) 0.821 +/- 0.085 mg/l; serum 0.821 +/- 0.07 mg/l. The same samples were analysed in the graphite furnace using serum standards, and the results were: plasma 0.812 +/- 0.082 mg/l; serum 0.815 +/- 0.08 mg/l. In contrast to earlier reports we were unable to demonstrate a significant difference between plasma and serum zinc concentration in the paired samples.

Adult↗
PubMed1992

Domain walls on graphite mimic DNA.

We show that domain walls on graphite are very likely to mimic features of extended macromolecules like DNA strands, when imaged with an STM. We explain with a simple model how different translational periods along a grain boundary originate from different relative orientations of the graphite lattice at the domain wall. We show how simple geometrical analysis of the images can be used to distinguish true macromolecular features from artifacts.

Artifacts↗
PubMed1994

Application of an electrochemical detector with a graphite electrode to liquid chromatographic determination of penicillamine and captopril in biological samples.

A high-performance liquid chromatographic determination of penicillamine and captopril in rat serum, liver and kidney samples is described. An electrochemical detector with a graphite working electrode at a potential of +0.9 V vs the Ag/AgCl reference electrode is used for the detection system. Linear responses of the peak height to the amount of samples injected were obtained in a range of 0.1-500 ng on-column and 0.5-500 ng on-column for penicillamine and captopril with correlation coefficients of 0.997 and 0.995, respectively. Detection limits at a signal-to-noise ratio of 3 were 20 and 300 pg for penicillamine and captopril, respectively. The graphite electrode has a long lifetime of about 4 months with continuous use, even with the high voltage supplied. The analytical application of this method to the determination of penicillamine and captopril in biological samples was successful.

Administration, Oral↗
PubMed2006

Development of a high analytical performance-tyrosinase biosensor based on a composite graphite-Teflon electrode modified with gold nanoparticles.

The design of a new tyrosinase biosensor with improved stability and sensitivity is reported. The biosensor design is based on the construction of a graphite-Teflon composite electrode matrix in which the enzyme and colloidal gold nanoparticles are incorporated by simple physical inclusion. Experimental variables such as the colloidal gold loading into the composite matrix, the enzyme loading and the potential applied to the bioelectrode were optimized. The Tyr-Au(coll)-graphite-Teflon biosensor exhibited suitable amperometric responses at -0.10 V for the different phenolic compounds tested (catechol; phenol; 3,4-dimethylphenol; 4-chloro-3-methylphenol; 4-chlorophenol; 4-chloro-2-methylphenol; 3-methylphenol and 4-methylphenol). The limits of detection obtained were 3 nM for catechol, 3.3 microM for 4-chloro-2-methylphenol, and approximately 20 nM for the rest of phenolic compounds. The presence of colloidal gold into the composite matrix gives rise to enhanced kinetics of both the enzyme reaction and the electrochemical reduction of the corresponding o-quinones at the electrode surface, thus allowing the achievement of a high sensitivity. The biosensor exhibited an excellent renewability by simple polishing, with a lifetime of at least 39 days without apparent loss of the immobilized enzyme activity. The usefulness of the biosensor for the analysis of real samples was evaluated by performing the estimation of the content of phenolic compounds in water samples of different characteristics.

Biosensing Techniques↗
PubMed2006

New antibodies immobilization system into a graphite-polysulfone membrane for amperometric immunosensors.

Polysulfone membrane is used for the first time for the preparation of electrochemical immunosensors. A disposable immunosensor based on a porous conductor polymer graphite-polysulfone-electrode has been developed using a phase inversion technique for the determination of anti-rabbit IgG (anti-RIgG) as a model analyte. To construct the sensor, a conductor membrane was deposited on the surface of working graphite-epoxy composite (GEC) electrode. The membrane was characterized by SEM. This sensor was based on the competitive assay between free and labeled anti-RIgG for the available binding sites of immobilized rabbit IgG (RIgG). Incubation parameters were optimized in this work. The immunological reaction was detected using an enzymatic-labeling procedure (HRP enzyme) combined with the amperometric detection using H(2)O(2) as substrate and hydroquinone as mediator. This sensor shows stability during a week and a good reproducibility. The current was monitored amperometrically at -0.1 V versus SCE and this method showed a linear range of the anti-RIgG from 1 to 6 microg/ml. The detection limit was determined to be 0.77 microg/ml.

Adsorption↗
PubMed2005

Influences of pH and current on electrolytic dechlorination of trichloroethylene at a granular-graphite packed electrode.

Electrolytic dechlorination using a granular-graphite packed cathode is an alternative method for the remediation of chlorinated organic compounds. Its effectiveness under various conditions needs experimental investigation. Dechlorination of trichloroethylene (TCE) was conducted under various conditions in an electrolytic reactor with a platinum-gauze anode and a granular-graphite packed cathode. The higher the applied current, the more TCE was eliminated and more hydrogen and oxygen gasses were generated. Current efficiency decreased with a decrease in TCE concentration during each dechlorination experiment. But, the current efficiency concentration coefficient (CECC), which was defined as current efficiency divided by concentration, was a better indicator of current efficiency. The CECC was not significantly affected by current, but it varied with pH value. The pH effects were results of the involvement of electrolytes in the proton reduction and the electron transfer at the cathode. A lower pH value favored TCE dechlorination in potassium chloride, which is an electrolyte that was not involved in cathode reactions with protons and electrons. In ammonium acetate and potassium nitrate, which involve proton reduction and/or electron transfer, the pH value affected TCE dechlorination through proton limitation and electron competition.

Electrodes↗
PubMed2004

Analysis of the mechanism of retention on graphitic carbon by a computational chemical method.

Retention mechanism on a graphitic carbon was analyzed by computational chemical calculation. The model graphitic carbon phase was a large polycyclic aromatic hydrocarbon (PAH) and analytes were carbohydrates and hydrocarbons separated by liquid and gas chromatography. Molecular mechanics calculation was fast and suggested their retention order and main retention force. Molecular orbital package calculation (MOPAC) demonstrated their complex form.

Graphite↗
PubMed2005

Separation and detection of the isomeric equine conjugated estrogens, equilin sulfate and delta8,9-dehydroestrone sulfate, by liquid chromatography--electrospray-mass spectrometry using carbon-coated zirconia and porous graphitic carbon stationary phases.

Equilin-3-sulfate and delta8,9-dehydroestrone-3-sulfate are two isomers found in equine conjugated estrogens that differ in structure only by the position of a double bond in the steroid B-ring. These geometric isomers were not resolved on a C18 column during the analysis of conjugated estrogen drug products by LC-MS using acetonitrile-ammonium acetate buffer as the mobile phase. While no separations of these two isomers were observed on C18 or other alkyl-bonded silica based phases using a variety of mobile phase conditions, partial separations were achieved on phenyl bonded silica phases with a resolution of 1.5 on a diphenyl phase, and baseline separations were readily achieved on two carbonaceous phases with resolutions routinely exceeding three on graphitic carbon-coated zirconia (Zr-CARB) and resolutions as high as 19 on porous graphitic carbon (Hypercarb). An examination of a selected few conjugated estrogens in the complex drug substance by LC-MS on Hypercarb is presented.

Chromatography, Liquid↗
PubMed2006

Liquid chromatography on porous graphitic carbon with atmospheric pressure photoionization mass spectrometry and tandem mass spectrometry for the analysis of glycosphingolipids.

The study of several structural variations (the length, the degree of unsaturation and hydroxylation of the alkyl chains, the number and nature of osidic residues) helped understand the behaviour of neutral glycosphingolipids (GSLs) on porous graphitic carbon stationary phase (PGC). Atmospheric pressure photoionization mass spectrometry (APPI) and tandem mass spectrometry were used to perform the detection and the identification of molecular species in positive mode where [M+H](+) and [M-H(2)O+H](+) ions provided structural information on the fatty acid and the sphingoid base. The retention of GSLs increased with the hydrocarboneous volume of their alkyl chains and with the number of osidic residues in agreement with hydrophobic properties and polar retention effect of graphite, respectively. The presence of polar groups, such as OH-group or double bond within alkyl chains, decreased their retention. The coupling of chromatography on PGC with APPI tandem mass spectrometry detection appeared a powerful technique to discriminate isobaric molecules. Isobaric solutes differing by the position of two double bonds or by the repartition of hydrocarboneous skeleton were discriminated according to their chromatographic comportment or their mass spectrum, respectively. Among isobaric molecules, only few structures differing by the nature of osidic residue were not discriminated (i.e. glucosylceramide and galactosylceramide with similar ceramide skeleton were co-eluted and no difference in mass spectra was observed).

Atmospheric Pressure↗
PubMed2006

Observation of single-stranded DNA on mica and highly oriented pyrolytic graphite by atomic force microscopy.

Atomic force microscopy was used to image single-stranded DNA (ssDNA) adsorbed on mica modified by Mg(2+), by 3-aminopropyltriethoxysilane or on modified highly oriented pyrolytic graphite (HOPG). ssDNA molecules on mica have compact structures with lumps, loops and super twisting, while on modified HOPG graphite ssDNA molecules adopt a conformation without secondary structures. We have shown that the immobilization of ssDNA under standard conditions on modified HOPG eliminates intramolecular base-pairing, thus this method could be important for studying certain processes involving ssDNA in more details.

Aluminum Silicates↗
PubMed2004

Renewable Protein A modified graphite-epoxy composite for electrochemical immunosensing.

A novel rigid and renewable transducing material for electrochemical immunosensing, based on Protein A bulk-modified graphite-epoxy biocomposite (ProtA-GEB) is reported. Protein A is able to bind to the Fc region of antibodies and provide an affinity matrix for antibody immobilisation onto the transducer. The rigid conducting biocomposite acts not only as a transducer, but also as a reservoir for protein A. After use, the electrode surface can be renewed by a simple polishing procedure, highlighting a clear advantage of this new approach with respect to classical immunoassays. The performance of ProtA-GEB transducers was compared with surface-modified transducers based on a simple dry adsorption procedure, where both Protein A and directly the antibody were adsorbed onto the surface of graphite-epoxy composite (ProtA/GEC and IgG/GEC, respectively). The application of the new biocomposite material in electrochemical immunosensing was studied using a model competitive immunoassay. The immunological reaction was detected using an enzymatic-labeling procedure together with the amperometric detection through a suitable substrate (H(2)O(2)) for the enzyme (HRP). The enzymatic labelling was performed using a two-step procedure based on the biotin/streptavidin interaction as well as a one-step procedure using an antibody labelled with the enzyme. Electrochemical and microscopic characterisation of ProtA-GEB transducer, optimisation of the immunosensor design as well as the stability of this material are also reported.

Biosensing Techniques↗
PubMed2005

Electrochemical detection of enzyme labeled DNA based on disposable pencil graphite electrode.

Electrochemical biosensor for the detection of DNA hybridization using the reduction signal of alpha-naphthol is described. A pencil graphite electrode was used as a working electrode. Capture probes were covalently attached on to the pencil graphite electrode surface (PGE) at the 5' end amino group by using N-(dimethylamino)propyl-N'-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysulfosuccinimide (NHS) as a coupling agent on to PGE. After capture probe immobilization on to PGE surface; probe was hybridized with complementary biotinylated oligonucleotide. Alkaline phosphatase labeled with extravidin (Ex-AP) binds to biotinylated hybrid via biotin-avidin interaction. alpha-Naphthyl phosphate (alpha-NAP) was added and the reaction between alkaline phosphatase (AP) and alpha-NAP was occurred consequently as a substrate of AP, alpha-NAP reduction signal was obtained from this reaction, at -0.100 V by using differential pulse voltammetry (DPV). Other experimental parameters were studied such as; optimizations of hybridization time, and the concentrations of capture probe, biotinylated oligonucleotide and enzyme.

Alkaline Phosphatase↗
PubMed2002

Porous graphitic carbon shows promise for the rapid screening partial DPD deficiency in lymphocyte dihydropyrimidine dehydrogenase in Chinese, Indian and Malay in Singapore by using semi-automated HPLC-radioassay.

OBJECTIVE: Dihydropyrimidine dehydrogenase (DPD) catalyzes the degradation of thymine, uracil, and the chemotherapeutic drug 5-Fluorouracil. In general reverse-phase high pressure liquid chromatography is the standard method for separating 5-[2-(14)C]Fluorouracil and 5-[2-(14)C]Fluoro-5,6-dihydrouracil. However, the use of 100% aqueous solution (as HPLC mobile phase) may collapse the C-18 bonded phase and result in a retention time shift. The aim of this study is to develop a rapid, reproducible, sensitive method for screening partial DPD deficiency in healthy volunteers. DESIGN AND METHODS: The activity of DPD was measured using 5-[2- (14)C]Fluorouracil (5-[2-(14)C]FUra) followed by separation of substrate and product 5-[2-(14)C]FUraH(2) with a 15 x 4.6 mm I.D., 5 microm particle size (d(p)) porous graphitic carbon (PGC) column (Hypercarb(R)) and HPLC with online detection of the radioactivity. This was standardized using the protein concentration of the cytosol (NanoOrange(R) Protein Quantitation). RESULTS: Complete baseline separation of 5-[2-(14)C]Fluorouracil (5-[2-(14)C]FUra) and 5-[2-(14)C]Fluoro-5,6-dihydrouracil (5-[2-(14)C]FUraH(2)) was achieved using a porous graphitic carbon (PGC) column. The detection limit for 5-[2-(14)C]FUraH(2) was 0.4 pmol. CONCLUSIONS: By using linear gradient separation (0.1% Trifluoroacetic acid [TFA] in water to 100% Methanol) protocols in concert with PGC columns (Hypercarb(R)), we have demonstrated that a PGC column has a distinct advantage over C-18 reverse phase columns in terms of column stability (pH 1-14). This method provides an improvement on the specific assay for DPD enzyme activity. It is rapid, reproducible and sensitive and can be used for routine screening for healthy and cancer patients for partial and profound DPD deficiency before treatment with 5- FUra.

Adult↗
PubMed2000

Assessment of polycrystalline graphites as sorbents for solid-phase microextraction of nonionic surfactants.

Two polycrystalline graphites (pencil lead and glassy carbon) were used as sorbents for solid-phase microextraction of a nonionic alkylphenol ethoxylate surfactant (Triton X-100). Analyses were performed by reversed-phase HPLC-fluorescence detection. The presence of the benzene ring in the congeners of Triton X-100 also allowed their direct detection at lambda(ex) = 230 nm and lambda(em) = 310 nm. Variables such as time of adsorption, time of desorption and concentration of surfactant in water were evaluated. The method limit of detection was found to be 0.5 microg/l for Triton X-100, with a linear dynamic range of 0.5-150 microg/l. Results were compared to those obtained using polymeric fibers such as PDMS/DVB and Carbowax/TPR. The chemical resistance and low cost of the polycrystalline graphites are advantageous over commercially available SPME fibers.

Graphite↗
PubMed2001

Liquid chromatography of polyglycerol fatty esters and fatty ethers on porous graphitic carbon and octadecyl silica by using evaporative light scattering detection and mass spectrometry.

A liquid chromatographic method has been developed for the analysis of polyglycerol fatty esters and fatty ethers which are non-ionic surfactants. Two methods were compared using either octadecyl silica or porous graphitic carbon. The octadecyl silica system with a hydroorganic mobile phase enables to compare the hydrophobic behavior of the compounds. The porous graphitic carbon enables the separation of the diastereoisomers. Detection of these non-UV-absorbing compounds was achieved by evaporative light scattering detection. Identification of isomers was performed by mass spectrometry.

Chromatography, Liquid↗