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Biomedical subjects

Nathan S Lawrence

Publications and source records attributed to Nathan S Lawrence.

At least 19 recordsLinked to original sources
PubMed2006

Redox-sensitive copolymer: a single-component pH sensor.

A novel redox-active copolymer has been developed, capable of sensing pH. The polymer incorporates the redox-active but pH-insensitive vinylferrocene moiety as the internal reference signal, alongside the redox-active and pH-sensitive vinylanthracene moiety. The anthracene moiety undergoes a Nernstian potential shift with pH. The various monomer and homopolymer components have been examined in turn and the electrochemical response of the polymer characterized. Furthermore, the pH response was found to be stable over a wide temperature range.

Journal Article↗
PubMed2005

Boron-doped diamond microdisc arrays: electrochemical characterisation and their use as a substrate for the production of microelectrode arrays of diverse metals (Ag, Au, Cu)via electrodeposition.

A novel boron-doped diamond (BDD) microelectrode array is characterised with electrochemical and atomic force microscopic techniques. The array consists of 40 micron-diameter sized BDD discs which are separated by 250 microns from their nearest neighbour in a hexagonal arrangement. The conducting discs can be electroplated to produce arrays of copper, silver or gold for analytical purposes in addition to operating as an array of BDD-microelectrodes. Proof-of-concept is shown for four separate examples; a gold plated array for arsenic detection, a copper plated array for nitrate analysis, a silver plated array for hydrogen peroxide monitoring and last, cathodic stripping voltammetry for lead at the bare BDD-array.

Arsenic↗
PubMed2005

All-diamond microelectrode array device.

We report the development of all-diamond microelectrochemical devices, namely, a microelectrode array (MEA), in which a periodic array structure with well-defined diameters, distance, and hexagonal unit cell pattern is micromachined using a combination of state-of-the-art microwave-induced plasma growth and laser ablation shaping techniques to prepare and coat a patterned boron-doped diamond (BDD) substrate with an intrinsic diamond insulating layer. The active BDD element can be tuned to between 10 and 50 microm in diameter with a 10 times diameter center-to-center distance between two adjacent conducting elements, which are exactly coplanar to the dielectric surroundings. This type of device should enable applications in harsh conditions such as high temperature, high pressure, and resistive media under dynamic flow regimes.

Journal Article↗
PubMed2005

4-Nitrobenzylamine partially intercalated into graphite powder and multiwalled carbon nanotubes: characterization using X-ray photoelectron spectroscopy and in situ atomic force microscopy.

We report the characterization of partial intercalation of 4-nitrobenzylamine (4-NBA) into edge-plane or edge-plane-like defect sites on the surface of both graphite powder and "bamboo-like" multiwalled carbon nanotubes (MWCNTs) using X-ray photoelectron spectroscopy (XPS). By comparing the XPS spectra of 4-NBA derivatized graphite powder and MWCNTs with that of graphite powder treated with benzylamine in a similar fashion, we conclude that benzylamine itself does not undergo partial intercalation. Using in situ atomic force microscopy, we are able to observe the partial intercalation of 4-NBA into an edge-plane-like "step" defect on the surface of a highly ordered pyrolytic graphite crystal in real time. Together these observations provide further evidence for the partial intercalation of 4-NBA and lead us to propose a new hypothesis to explain this phenomenon.

Journal Article↗
PubMed2004

Microchip capillary electrophoresis with electrochemical detection of thiol-containing degradation products of V-type nerve agents.

A microchip protocol for the capillary electrophoresis separation and electrochemical detection of thiol-containing degradation products of V-type nerve agents is described. The microchip assay relies on the derivatization reaction of 2-(dimethylamino)ethanethiol (DMAET), 2-(diethylamino)ethanethiol (DEAET), and 2-mercaptoethanol (ME) with o-phthaldialdehyde in the presence of the amino acid valine along with amperometric monitoring of the isoindole derivatives. Both off-chip and on-chip derivatization reactions have led to highly sensitive and rapid detection of the thiol degradation products. Various parameters influencing the derivatization, separation, and detection processes were examined and optimized. These include the amino acid co-reagent, reagent-mixing ratio, reaction time, injection time, separation voltage, and detection potential. The chip microsystem offers a rapid (<4 min) simultaneous detection of micromolar concentrations of DMAET, DEAET, and ME. Linear calibration plots were observed for the V-type nerve agent thiol degradation products, along with good stability and reproducibility (RSD < 8.0%). Detection limits of 5 and 8 microM were obtained for the off-chip reaction of DMAET and DEAET, respectively, following a 2-s injection. The suitability for assays of environmental matrixes was demonstrated for the determination of DMAET and DEAET in untreated tap and river water samples. The favorable analytical performance makes the new microfluidic device attractive for addressing the needs of various security scenarios.

Biodegradation, Environmental↗
PubMed2004

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).

Amino Acids↗
PubMed2004

Biocatalytic carbon paste sensors based on a mediator pasting liquid.

The preparation and advantages of a new generation of carbon paste enzyme electrodes where the redox mediator acts also as the pasting liquid are described. The mediator pasting liquid concept is illustrated for amperometric biosensing of glucose in connection with either the tert-pentylferrocene or n-butylferrocene mediator/binder along with the glucose oxidase enzyme. The attractive performance and advantages of the new device is indicated from comparison to a conventional carbon paste biosensor using a mineral oil binder and the dimethylferrocene electron acceptor. The simplified preparation of the biosensor is coupled with a greatly improved sensitivity and an extended linear range. The mediator pasting liquid imparts high thermal stability onto the embedded enzyme and leads to good resistance to oxygen effects. Owing to the huge mediator reservoir, stability problems associated with the leaching of the mediator are greatly reduced. The fundamental aspects of the electrode behavior have been examined first in the absence of the enzyme. Variables affecting the performance of the new carbon paste biosensor have been investigated and optimized. Such use of the electron acceptor as a binder as well as the mediator offers considerable promise for the biosensing of numerous analytes of clinical and environmental significance.

Biosensing Techniques↗
PubMed2004

Effect of Cu(II) on the electrochemically initiated reaction of thiols with N, N-diethyl- p-phenylenediamine: methodology for the indirect voltammetric determination of Cu(II).

The effect of Cu(II) on the determination of homocysteine via its electrochemically initiated reaction with N, N-diethyl-p-phenylenediamine is examined. The presence of copper inhibited the detection process for homocysteine owing to a complexation reaction occurring. This provided an indirect route for the sensitive and selective determination of Cu(II), which produced a linear response over the range from 2.5 to 500 microM and a limit of detection of 2.5 microM. The detection pathway was examined in the presence of metallic and inorganic ions, with negligible interference observed.

Copper↗
PubMed2004

Abrasively immobilised multiwalled carbon nanotube agglomerates: a novel electrode material approach for the analytical sensing of pH.

We demonstrate for the first time that agglomerates of multiwalled carbon nanotubes (MWCNTs) can be formed in which the binder in the agglomerate is itself a redox-active molecular solid. Two separate agglomerates were formed by dissolving 9,10-phenanthraquinone (PAQ) or 1,2-napthaquinone (NQ) in acetone together with MWCNTs and adding an excess of aqueous solution to cause precipitation of agglomerates, approximately 10 microns in dimension, which consist of bundles of nanotubes running into and throughout the amorphous molecular solid that binds the agglomerate together. The nature of this structure, when immobilised on a substrate electrode and in contact with aqueous electrolyte solutions, gives rise to many three-phase boundaries, electrolyte|agglomerate|conductor, which is advantageous to the solid-state analytical electrochemistry of such a material as it imparts a larger electroactive surface area than other modified carbon electrodes. The two agglomerates each gave a voltammetrically measurable response to changes in pH; when abrasively immobilised on a basal plane pyrolitic graphite electrode a plot of peak potential against pH produced a linear response for both MWCNT-PAQ and MWCNT-NQ agglomerates over the pH range pH 1-12 and over the temperature range 20-70 degrees C.

Chemistry, Physical↗
PubMed2003

A thin-layer amperometric sensor for hydrogen sulfide: the use of microelectrodes to achieve a membrane-independent response for Clark-type sensors.

An electrochemical cell design of the Clark type including a thin layer of electrolyte in contact with a microelectrode has been successfully applied for the determination of sulfide utilizing its electrochemically initiated reaction with aqueous diethyl-p-phenylenediamine. The analytical parameters obtained were independent of the membrane used to separate the inner chamber and the outside sulfide-containing solution. The independence arises since the thickness of the diffusion layer associated with the microelectrode is small and, in contrast to the conventional macroelectrode Clark electrode, does not impinge on the membrane. This provides an improvement in gas sensor design and development as it obviates the need for membrane calibration and should simplify the application of Clark cells for variable-temperature measurements.

Electrochemistry↗
PubMed2003

Voltammetric characterization of a N,N'-diphenyl-p-phenylenediamine-loaded screen-printed electrode: a disposable sensor for hydrogen sulfide.

The voltammetric response of a 10% (by weight) N,N'-diphenyl-p-phenylenediamine (DPPD) and 90% (by weight) carbon and binder screen-printed electrode has been examined in aqueous media over a range of pH using cyclic voltammetry both in the presence and in the absence of sulfide. In the absence, the screen-printed electrode undergoes an initial oxidative process on the surface of the solid organic particles to form an insoluble layer of the corresponding cation radical salt, DPPD(*)(+)X(-), where X(-) is an anion present in the solution. The charge transfer is thought to occur at the three-phase boundary between solid DPPD, carbon, and the aqueous solution. At higher potentials, a second oxidative wave is observed that is attributed to the oxidation of the bulk DPPD with intercalation of the anion species present to form a solid phase of DPPD(*)(+)X(-). The two voltammetric processes were found to stabilize after repetitive scanning, after which time, sulfide was added to the solution. The voltammetric response was found to respond to sulfide by showing a decrease in both the oxidative and reductive waves, which can be attributed to the sulfide effectively blocking the three-phase boundary. The response was found to be independent of the electrode used and at pH 4 produced a linear range from 20 to 165 microM, and a limit of detection of 7.5 microM for sulfide detection was achieved.

Journal Article↗
PubMed2003

Amperometric determination of sulfide at a pre-oxidised nickel electrode in acidic media.

The electrochemical response of a pre-oxidised nickel electrode to increasing additions of sulfide has been examined and shown to produce a stripping-like voltammetric wave. A mechanism is described based on the formation of nickel sulfide at the electrode surface from a non-electroactive nickel oxide layer. The analytical utility of the approach has been examined and a linear range from 1 to 140 microM and a limit of detection of 0.8 microM is achievable, depending on the accumulation time.

Air Pollutants↗
PubMed2003

Physical adsorption of N,N'-diphenyl-p-phenylenediamine onto carbon particles: application to the detection of sulfide.

The derivatisation of carbon powder by physical adsorption of N,N'-diphenyl-p-phenylenediamine (DPPD) onto the surface of graphite particles (1-2 microm diameter) results in usefully functionalised carbon. The derivatised carbon powders have been studied using cyclic voltammetry by (i) abrasive immobilisation of the powder onto a basal plane pyrolytic graphite electrode (bppg) and (ii) by the manufacture of carbon-epoxy electrodes containing the derivatised carbon. The electrochemical response of the DPPD modified carbon has been examined in the presence of sulfide and possible reactions identified for the behaviour of each electrode substrate. The analytical application of the carbon-epoxy electrode for the sensing of sulfide is reported.

Journal Article↗
PubMed2002

Advances in the voltammetric analysis of small biologically relevant compounds.

The problems associated with attempting to apply voltammetric techniques to the analysis of biologically relevant organics within complex media are identified and, through reviewing the very recent literature (1999-mid-2001), possible solutions are described. The boundaries of the search were limited to research targeted at the resolution of specific problems, associated with quantitative determinations. Various strategies have emerged to counter problems of poor sensitivity and selectivity and these have been summarized and critically appraised. Where possible, the characteristics of each approach have been distilled into a table format to ease comparison. Emphasis has been placed on the collation of information that will improve the intrinsic electrode response and as such should be of value to those interested in pursuing electroanalytical methodologies regardless of context.

Biological Factors↗
PubMed2002

Ultrasonic extraction of iron from non-aqueous liquids.

A novel procedure for the extraction of iron from predominately organic solvents has been described. An ultrasonic probe was used to create a microemulsion with a small quantity of nitric acid such that labile iron could be released into the aqueous vesicles and subsequently quantified after phase separation. The analytical and operational viability of using a simple colorimetric assay based on the coordination of aminothiol ligands (principally homocysteine) was evaluated in terms of signal sensitivity, selectivity and stability. The use of homocysteine provided a linear range for iron(III) from 9 microM to 50 microM with a corresponding limit of detection of 2 microM (based on 3s(b)). The effectiveness of the approach was assessed through the recovery of 0.3 ppm iron from a sample of commercial kerosene and the results compared with those obtained through attempting to quantify the iron under passive (ultrasonically silent) conditions.

Journal Article↗
PubMed2002

An electrochemical adaptation of Ellman's test.

The electrochemically initiated reaction of thiols with N,N-diethyl-p-phenylenediamine has been coupled with an existing colorimetric sensing reaction developed by Ellman as a means of providing an electrochemical adaptation of the latter whereby the total thiol species present in a sample can be determined. The detection methodology has been proven to be robust with a linear range for cysteine from 2-120 microM, a limit of detection of 1.17 microM and has shown selectivity against a wide range of potential interferences. The efficiency of the methodology has been examined in the determination and recovery of thiol species in three growth tissue media, which contain a number of common biological interferences.

Journal Article↗