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At least 667 records · Page 37Linked to original sources

PubMed2005

Enzyme modification of platinum microelectrodes for detection of cholesterol in vesicle lipid bilayer membranes.

Platinum microelectrodes are modified with a lipid bilayer membrane incorporating cholesterol oxidase. Details for electrode surface modification are presented along with characterization studies of electrode response to cholesterol solution and to cholesterol contained in the lipid bilayer membrane of vesicles. Ferrocyanide voltammetric experiments are used to track deposition of a submonolayer of a thiol-functionalized lipid on the platinum electrode surface, vesicle fusion for bilayer formation on the thiolipid-modified surface, and incorporation of cholesterol oxidase in the electrode-supported thiolipid/lipid bilayer membrane. The data are consistent with formation of a lipid bilayer structure on the electrode surface that contains defects. Experiments for detection of cholesterol solubilized in cyclodextrin solution show steady-state current responses that correlate with cholesterol concentration. Direct contact between the electrode and a vesicle lipid bilayer membrane shows a response that correlates with vesicle membrane cholesterol content.

Cholesterol↗

PubMed2006

Label-free DNA detection based on modified conducting polypyrrole films at microelectrodes.

A label-free electrochemical detection method for DNA hybridization based on electrostatic modulation of the ion-exchange kinetics of a polypyrrole film deposited at microelectrodes is reported. Synthetic single-stranded 27-mer oligonucleotides (probe) have been immobilized at 2,5-bis(2-thienyl)-N-(3-phosphorylpropyl)pyrrole film formed by electropolymerization on the previously formed polypyrrole layer. The 27- or 18-mer target oligonucleotides were monitored via the electrochemically driven anion exchange of the inner polypyrrole film. The performance of the miniaturized DNA biosensor system was studied in respect to selectivity, sensitivity, reproducibility, and regeneration of the sensor. Control experiments were performed with a noncomplementary target of 27-mer DNA and 12 base-pair mismatched 18-mer sequences, respectively, and did not show any unspecific binding. Under optimized experimental conditions, the label-free electrochemical biosensor enabled the detection limits of 0.16 and 3.5 fmol for the 18- and 27-mer DNA strand, respectively. Furthermore, we demonstrate reusability of the electrochemical DNA biosensor after successful recovery of up to 100% of the original signal by regenerating the DNA "label-free" electrode with 50 mM HCl at room temperature.

Base Sequence↗

PubMed1996

Overoxidized polypyrrole-coated carbon fiber microelectrodes for dopamine measurements with fast-scan cyclic voltammetry.

Thin films of overoxidized polypyrrole have been electro-chemically coated onto carbon fiber microelectrodes and used for dopamine measurements with background-substracted, fast-scan cyclic voltammetry at a scan rate of 300 V/s. The films were stable on the electrode surface only when the electrodes were scanned to high potentials (1400 mV vs SSCE) in pH 7.4 aqueous buffer. Dopamine sensitivity and ascorbate and dihydroxyphenylacetic acid (DOPAC) rejection at the overoxidized polypyrrole-coated electrode were compared to those at carbon fiber electrodes coated with Nafion, a perfluorinated ion-exchange material. At 300 V/s, the overoxidized polypyrrole-coated electrode was almost 3 times more sensitive to dopamine than an uncoated disk electrode. Furthermore, the films were as effective as Nafion in the attenuation of the response to ascorbate and DOPAC, common interferences of dopamine in vivo. Overoxidized polypyrrole-coated electrodes maintained a stable response to dopamine for several hours when implanted in the rat brain. The electrochemical deposition procedure was effective at both elliptical and cylindrical electrodes. This is in contrast to the dip-coating procedures employed with Nafion films that lead to nonuniform coatings at cylindrical electrodes.

Animals↗

PubMed1996

In vivo brain glucose measurements: differential normal pulse voltammetry with enzyme-modified carbon fiber microelectrodes.

The enzyme glucose oxidase was immobilized on the surface of carbon fiber microelectrodes (CFMEs) either by cross-linking in glutaraldehyde vapor or by enzyme entrapment in electropolymerized films of m-phenylenediamine or resorcinol. The cross-linked enzymatic layer was, in the given conditions, covered with an additional membrane of Nafion or cellulose acetate. The prepared glucose sensors were tested using differential normal pulse voltammetry (DNPV, in which the scan comprises successive double pulses ("prepulse and pulse"), the prepulses are of increasing amplitude, and the current measured is the differential of the current existing between each prepulse and pulse). With properly chosen DNPV parameters, the response to glucose presented a peak at a potential of about 1 V versus an Ag/AgC1-reference, owing to the oxidation of enzymatically produced hydrogen peroxide. The calibration curves obtained (peak height/glucose concentration) were linear from 0.3-0.5 up to 1.5-6.5 mM and showed a sensitivity ranging from 1.4 up to 34.5 mA M-1 cm-2, depending on the sensor type. The DNPV response to glucose exhibited an essential insensitivity toward easily oxidizable interfering substances such as ascorbic acid and acetaminophen present at physiological concentrations. Peptides, the interfering species typical of the cerebral medium, were effectively retained by the above additional membranes. Concentration values of glucose in plasma and cerebrospinal fluid, determined in vitro from the DNPV peak height, agreed well with those measured by standard procedures. In the anesthetized rat, extracellular brain concentration of glucose was also monitored during administration of either insulin or glucagon. Under such pharmacological conditions, the changes observed in the peak height were in perfect agreement with the known effects induced by both substances.

Acetaminophen↗

PubMed1998

In vivo voltammetric detection of rat brain lactate with carbon fiber microelectrodes coated with lactate oxidase.

To allow rat brain lactate measurement in vivo, a specific sensor based on a carbon fiber (phi = 30 microns) microelectrode coated with lactate oxidase was prepared. Combined with the differential normal pulse voltammetry measurement method, such a sensor, with a sensitivity of 9.15 +/- 0.91 mA.M-1.cm-2, provided a lactate linear response in concentrations ranging from 0.1 to 2.0 mM. The measurements performed appeared to be essentially insensitive to usual interference caused by the electroactive compounds present in the brain (ascorbic acid and peptides). In vivo detection performed in the cortex of the anesthetized rat led to the determination of a lactate concentration of 0.41 +/- 0.02 mM. Moreover, to validate the results obtained in vivo, an ex vivo determination of the lactate level was also performed in samples of brain tissue, plasma, and cerebrospinal fluid, using both voltammetry and a clinical analyzer with colorimetric-based detection. A good correlation was observed between the sets of data established by both methods.

Animals↗

PubMed1999

Effect of pH and surface functionalities on the cyclic voltammetric responses of carbon-fiber microelectrodes.

Carbon electrodes are useful for the detection of oxidizable species with cyclic voltammetry. In particular, carbon-fiber microelectrodes have been employed for the measurement of several neurotransmitters in brain tissue. However, during cyclic voltammetry with carbon-fiber electrodes the current varies with changes in concentration of some inorganic cations as a result of their interaction with surface functional groups. The electrode's response to the hydronium ion is a particular concern because its voltammetric response occurs over a broad range of potentials that overlap those of neurotransmitters of interest such as dopamine. This is especially a problem in vivo because simultaneous changes of dopamine and pH frequently occur in brain tissue. In this work, voltammetric current changes are shown to arise from pH dependent shifts in the peak potentials of background voltammetric waves that arise from species confined to the carbon-fiber electrode surface. Polishing the electrode with alumina suspended in cyclohexane in an environment containing lowered oxygen, a method previously demonstrated to remove oxides from the carbon surface, leads to a substantial reduction in the sensitivity to pH changes. However, this is accompanied by a loss in signal amplitude for dopamine. The dopamine response can be restored using the cation exchanger Nafion without significantly increasing the pH response. To investigate which oxide functional groups play a direct role in the electrode's current responses to changes in pH, surface-confined carbonyl and alcohol functionalities were chemically modified. In both cases, the modification did not affect the carbon-fiber electrode's responsiveness to changes in pH. Nonetheless, the polishing technique proved to be effective in reducing pH interferences in in vivo applications.

Animals↗

PubMed2000

Simultaneous determination and speciation of zinc, cadmium, lead, and copper in natural water with minimum handling and artifacts, by voltammetry on a gel-integrated microelectrode array.

The paper reports a new approach based on a gel-integrated Hg-plated-Ir-based microelectrode array (GIME), for measuring Cu, Pb, Cd, and Zn speciation in natural waters. This paper focuses on the quantitative discrimination between mobile and colloidal metal species (size limit of a few nanometers), for which most classical separation techniques present severe drawbacks. Previous papers have shown qualitatively that GIME combined with square wave anodic stripping voltammetry (SWASV) has the basic characteristics required to discriminate between these two fractions directly on the unperturbed sample. In addition, because of the large sensitivity provided by GIME, complexation parameters (equilibrium constants and site concentrations) can be determined in little-perturbed samples, particularly without metal addition or with small addition compared with natural concentrations. The advantages of this procedure are exemplified and the possible artifacts occurring when titrating the sample with metals, in particular intermetallic compound formation and other problems, are discussed in detail. The present paper shows that the characteristics of GIME make it a unique tool to get quantitative information on metal speciation at nanomolar or even subnanomolar levels, with only minor sample handling.

Cadmium↗

PubMed2005

Three-dimensional mapping of oxygen distribution in wastewater biofilms using an automation system and microelectrodes.

The three-dimensional oxygen distribution in wastewater biofilms was evaluated using combined oxygen microelectrodes and an automation system. The biofilms were sampled from rotating biological contactors treating domestic wastewater. The samples studied were mature biofilms with a thickness from 630 to 1600 microm. It was demonstrated that the dissolved oxygen concentration could be depleted at the biofilm surface. The heterogeneity of the dissolved oxygen distribution was high in sections further away from the biofilm surface in the water layer. The study showed that the concentration and level of heterogeneity of dissolved oxygen inside the biofilms decreased with depth, forming stratification. The oxygen concentration in biofilms changed generally from a high degree of heterogeneity near the biofilm surface to a low degree of heterogeneity in deep sections of biofilms, indicating a cell--cluster-like structure near the surface and more compact base layer close to the substratum. The three-dimensional oxygen distribution maps revealed pockets of dissolved oxygen in deep sections of biofilms. The dissolved oxygen concentrations of these pockets in the biofilm samples ranged from 0.4 to 1.0 mg/L at 760 microm depth. The three-dimensional oxygen distribution maps produced relevant knowledge of functional and structural characteristics of biofilms used for the treatment of wastewater.

Automation↗

PubMed2006

Community structure, abundance, and in situ activity of nitrifying bacteria in river sediments as determined by the combined use of molecular techniques and microelectrodes.

The community structure, spatial distributions, and in situ activity of ammonia-oxidizing bacteria (AOB) representing the Betaproteobacteria and nitrite-oxidizing bacteria (NOB) representing the genus Nitrospira in three different river sediments with different pollution sources and levels along the Niida River, Hachinohe, Japan, were investigated by the combined use of 16S rRNA gene-cloning analysis, real-time quantitative polymerase chain reaction (RTQ-PCR) assays, and microelectrodes. The goal of this research was to evaluate the contribution of nitrifying activity in the sediment to the overall nitrogen elimination rate in this river. The 16S rRNA gene-cloning analysis revealed that the community structures of AOB and Nitrospira-like NOB are present in three sediments. On the basis of the results of 16S rRNA gene-cloning analysis, the RTQ-PCR assay using a TaqMan probe was developed and optimized for the quantification of the Nitrospira-like NOB. In the sediments, AOB specific 16S rRNA genes were detected in the range of 10(6) to 10(7) copies/cm3 and evenly distributed over the sampled sediment depth (0-5 mm), whereas the Nitrospira-like NOB 16S rRNA gene copy numbers per cm3 were 1-2 orders of magnitude higher than the AOB copy numbers. Under light conditions, intensive oxygenic photosynthesis occurred in the surface and increased the maximal O2 concentration and O2 penetration depth in all sediments. This concomitantly stimulated nitrifying bacteria present in diurnally anoxic deeper zones and expanded nitrification zones, which consequently increased the total NH4+ consumption rate in the sediment (i.e., total NH4+ flux into the sediment). The results suggested that the in situ nitrifying activity was restricted mainly to the surface 2 mm of the sediment and linked with photosynthetic activity, which obviously plays an important role in nitrogen elimination in this river.

Ammonia↗

PubMed2004

Steady-state detection of cholesterol contained in the plasma membrane of a single cell using lipid bilayer-modified microelectrodes incorporating cholesterol oxidase.

Platinum microelectrodes modified with a lipid bilayer membrane incorporating cholesterol oxidase are used for detection of cholesterol contained in the plasma membrane of a single cell. Amperometric responses are consistent with enzymatic catalysis being rate limiting and cholesterol diffusing laterally in the plasma membrane to the electrode contact site. Importantly, electrode response appears to correlate with the cholesterol content of the cell plasma membrane. The electrodes should be useful for characterizing cellular cholesterol tracking pathways involved in pathogenesis of disease.

Animals↗

PubMed2006

Selective functionalization of independently addressed microelectrodes by electrochemical activation and deactivation of a coupling catalyst.

We demonstrate selective functionalization of independently addressed microelectrodes by electrochemical activation and deactivation of a coupling catalyst. 1,2,3-Triazole formation between terminal acetylenes and organic azides is efficiently catalyzed by copper(I) complexes (a Sharpless "click" reaction), while the oxidized copper(II) complexes are inactive. By electrochemically activating or deactivating the catalyst by switching its redox state, we demonstrate control over triazole formation between surface-immobilized azides and ethynylferrocene. The reaction proceeds on the time scale of minutes using submicromolar concentration of reactants and catalyst, requires mild potentials for catalyst activation and deactivation, and works in aqueous and mixed aqueous-organic solvents. By appropriate biasing of each electrode, we selectively modify one of two chemically identical 10-mum-wide electrodes separated by 10 mum in an interdigitated array. The ability to switch on or off the reaction by electrical addressing together with the chemoselectivity of this reaction makes Cu(I)-catalyzed triazole formation an ideal method for the chemical modification of multielectrode arrays.

Catalysis↗

PubMed2006

Use of an electrochemically etched platinum microelectrode for ascorbic acid mapping in oranges.

A positionable platinum microelectrode fabricated by electrochemical etching was used to monitor the concentration of ascorbic acid in fruits and vegetables. Studies carried out with ascorbate oxidase confirmed the suitability of the amperometric sensor to measure selectively the ascorbic acid content. The results obtained with the proposed method for ascorbic acid determination in orange juices compared well with those found by iodimetry with coulometrically generated iodine. The standard deviation calculated by measuring limiting current values in voltammograms was found to be 3% (n = 150). The sensor allowed the evaluation of the spatial distribution of ascorbic acid concentration in oranges by in-situ measurements. Ascorbic acid concentration maps show that in a perpendicular cut the concentration is higher near the peel to the center of the fruit. In a parallel cut, the concentration increases with the distance to the stem. A correlation between the ripening stage and the ascorbic acid concentration was also observed from electrochemical measurements, the content being higher in mature fruits.

Ascorbic Acid↗

PubMed2005

In situ, time-resolved normal incidence reflectance spectroscopy of polycrystalline platinum microelectrodes in aqueous electrolytes.

In situ normal incidence reflectance spectra of polycrystalline Pt microelectrodes have been monitored as a function of the applied potential in aqueous 0.5 M H(2)SO(4) using a He-Ne laser source (633 nm) and a beam splitter/microscope objective arrangement. Data recorded during voltammetric cycles in Ar-purged solutions revealed a linear correlation between the normalized change in reflectance, DeltaR/R = (R(s) - R(ref))/R(ref) (where R(s) and R(ref) are the light intensities measured by the detector at the sampling, s, and reference potentials, ref, respectively), and the extent of oxidation of the Pt surface over a wide coverage range. Reflectance spectra were also collected in CO-saturated 0.5 M H(2)SO(4) during chronocoulometric measurements involving judiciously selected limits for both the potential step and duty cycle parameters. Analysis of these results made it possible to extract contributions to the current derived from oxide formation during oxidation of adsorbed and bulk CO, based strictly on the optical response.

Adsorption↗

PubMed2006

In situ, time-resolved reflectance spectroscopy in the microsecond domain: oxidation of adsorbed carbon monoxide on polycrystalline pt microelectrodes in aqueous solutions.

The dynamics of the electrooxidation of adsorbed CO, COads, on polycrystalline Pt microelectrodes has been examined in CO-saturated 0.5 M H2SO4 and 0.5 M HClO4 aqueous solutions, using in situ, time-resolved, normalized differential reflectance spectroscopy lambda = 633 nm). Attention was focused on the unique dependence of COads oxidation on the potential at which the adsorbed full CO monolayer is assembled (i.e., hydrogen adsorption/desorption vs the double-layer region) using both fast linear scan voltammetry and potential step techniques. As evidenced from the data collected, COads oxidation at a fixed potential proceeds at slower rates when the monolayer is formed in the double- layer region compared to when it is formed in the hydrogen adsorption/desorption region. Possible explanations for this effect are discussed.

Adsorption↗

PubMed1997

Measurements of oxygenation and perfusion in skeletal muscle using multiple microelectrodes.

This paper describes an apparatus to measure tissue oxygenation and perfusion (as measured by the wash-in rate of gaseous hydrogen) simultaneously at multiple points in muscle using needle microelectrodes. The development of suitable electrodes and apparatus is described, as well as the development of the method and its validation. In particular, the potential for tissue damage secondary to electrode insertion, the need for in vivo voltammetric determination of the operating potential and the extent of any electrode-tissue and of electrode-electrode interactions are explored, and are shown to be insufficient in magnitude to affect the technique. Its subsequent use to characterise oxygenation and perfusion in rabbit skeletal muscle at rest is also described. In resting tibialis anterior muscle of the rabbit the mean pO2 was 18 +/- 13.3 mm Hg and the mean perfusion was 4.4 +/- 1.3 ml s-1 100 g-1. There was a heterogeneity in simultaneously-measured values of pO2 and perfusion at different points within muscle, and also a temporal variation at the same site. The spans between the highest and lowest simultaneously-measured values of pO2 in muscle ranged from 14 to 80 mm Hg, and for perfusion, from 1 to 12 mls-1 100 g-1. No significant correlation was evident from histological examination between either pO2 or perfusion and surrounding fibre type or capillary density.

Animals↗

PubMed1980

Free calcium in Xenopus embryos measured with ion-selective microelectrodes.

A regulatory role for intracellular free calcium has been suggested in cell division and intercellular communication via gap junctions. Previous measurements of cytoplasmic free Ca2+ during cell division and uncoupling have been made with the Ca2+-sensitive photoprotein aequorin. We now report experiments using Ca2+-sensitive microelectrodes to monitor free Ca2+ in the cells and intercellular fluid of early embryos of the amphibian Xenopus laevis. In addition to measuring basal levels, we have looked for changes in free Ca2+ during cell division and during electrical uncoupling of the normally coupled embryonic cells, induced by acidification of the intracellular medium.

Animals↗

PubMed1980

Free calcium in heart muscle at rest and during contraction measured with Ca2+ -sensitive microelectrodes.

Direct measurements of free Ca2+ in heart cells are needed for an understanding of the regulation of contractility. We developed and used Ca2+ -sensitive microelectrodes with fine tips, stable properties and ample sensitivity to free Ca2+ in the sub-micromolar range. In quiescent ventricular muscle, measurements which passed tests for electrode sealing and cell viability gave a mean free Ca2+ concentration of 0.26 microM. During contractures, we recorded Ca2+ transients rising as high as 10 microM. In studying the effects of catecholamines on free Ca2+ and force, we found evidence that adrenaline can reduce myofibrillar Ca2+ sensitivity in intact heart muscle.

Animals↗

PubMed1986

In vivo measurement of regional brain and tumor pH using [14C]dimethyloxazolidinedione and quantitative autoradiography. II: Characterization of the extracellular fluid compartment using pH-sensitive microelectrodes and [14C]sucrose.

We measured the extracellular (interstitial) pH (pHe) of RG-2 rat gliomas using H+-sensitive microelectrodes and estimated the volume of tumor extracellular space based on the tissue-plasma ratio of [14C]sucrose. The average RG-2 pHe was 7.63 +/- 0.15 (mean +/- SD, n = 6), whereas the average pHe of contralateral brain tissue was 7.34 +/- 0.10 (n = 3) and arterial pH was 7.36 +/- 0.02. RG-2 extracellular space water volume was estimated to be 0.3 ml water/g tissue. In separate experiments in normal, nontumored rats, intracellular pH (pHi) was calculated for nine gray and white matter regions based on measurements of tissue and plasma [14C]dimethyloxazolidinedione concentration. pHi values ranged from 6.80 to 6.94, and no consistent gray-white differences were observed. Our data suggest that tumor pHi is not more acidic than that of normal brain tissue and that the observed alkalinity of primary brain tumors is due to the presence of a large alkaline extracellular space.

Animals↗