Cylindrical Carbon Fiber Microelectrodes Research Articles

Surface-Roughened Graphene Oxide Microfibers Enhance Electrochemical Reversibility.

Here, we provide an optimized method for fabricating surface-roughened graphene oxide disk microelectrodes (GFMEs) with enhanced defect density to generate a more suitable electrode surface for dopamine detection with fast-scan cyclic voltammetry (FSCV). FSCV detection, which is often influenced by adsorption-based surface interactions, is commonly impacted by the chemical and geometric structure of the electrode's surface, and graphene oxide is a tunable carbon-based nanomaterial capable of enhancing these two key characteristics. Synthesized GFMEs possess exquisite electronic and mechanical properties. We have optimized an applied inert argon (Ar) plasma treatment to increase defect density, with minimal changes in chemical functionality, for enhanced surface crevices to momentarily trap dopamine during detection. Optimal Ar plasma treatment (100 sccm, 60 s, 100 W) generates crevice depths of 33.4 ± 2.3 nm with high edge plane character enhancing dopamine interfacial interactions. Increases in GFME surface roughness improve electron transfer rates and limit diffusional rates out of the crevices to create nearly reversible dopamine electrochemical redox interactions. The utility of surface-roughened disk GFMEs provides comparable detection sensitivities to traditional cylindrical carbon fiber microelectrodes while improving temporal resolution ten-fold with amplified oxidation current due to dopamine cyclization. Overall, surface-roughened GFMEs enable improved adsorption interactions, momentary trapping, and current amplification, expanding the utility of GO microelectrodes for FSCV detection.

Ferricyanide-backfilled cylindrical carbon fiber microelectrodes for in vivo analysis with high stability and low polarized potential.

The development of stable and reproducible methods for in vivo electrochemical monitoring of neurochemicals is of great physiological importance. In this study, we demonstrate ferricyanide-filled cylindrical carbon fiber microelectrodes (CFEs) of high stability and low polarized potential for in vivo electrochemical analysis. We first studied the voltammetric behavior of cylindrical CFEs by using a model system consisting of two separated cells each containing potassium ferricyanide (K3Fe(CN)6) or potassium ferrocyanide (K4Fe(CN)6). We observed that E1/2 values of the system were dependent on the ratio of the lengths of the cylindrical CFEs and of the concentrations of the redox species on both poles. Based on this property, we prepared the ferricyanide-backfilled cylindrical CFEs, and found that this kind of electrode exhibits a more stable current response and a lower polarized potential than the CFEs backfilled with KCl or Ru(NH3)6Cl3. Animal experiments with the ferricyanide-backfilled cylindrical CFEs demonstrate that this kind of electrode could be used for in vivo monitoring of neurochemical release with a high stability under some physiological conditions.

Epoxy insulated carbon fiber and carbon nanotube fiber microelectrodes

Carbon-fiber microelectrodes (CFMEs) are typically constructed from glass capillaries pulled to a fine taper or from a polyimide-coated capillary that is 90μm in outer diameter. Here, a new fabrication method is developed to insulate carbon-fiber microelectrodes with a thin epoxy coating. A polytetrafluoroethylene (Teflon) mold was laser etched with channels 30–40μm deep and wide and each channel filled with Armstrong C7 epoxy. A carbon fiber was laid into each channel so that the fiber extended past the mold, and the epoxy cured in an oven. One end of the fiber was trimmed to about 100μm to form a cylindrical carbon-fiber microelectrode, while the other end was attached to a pin and connected to a potentiostat. Epoxy-insulated electrodes were tested with fast-scan cyclic voltammetry. For dopamine, the sensitivity is similar to glass and polyimide-coated capillary electrodes with a linear range of 0.1–10μM and a LOD of 24nM. SU-8 epoxy was tested as an alternative insulator because it cures at a lower temperature using light, but it was more brittle. Carbon nanotube fibers were also successfully insulated with epoxy. Epoxy-insulated CFMEs were used to detect stimulated dopamine release in vivo. Epoxy-insulated electrodes are smaller in diameter than polyimide-coated capillary electrodes and amenable to mass production. They are advantageous for use in higher order mammals, where glass is not permitted, and with alternative electrode materials, such as carbon nanotube fibers, that cannot be fabricated in a capillary puller.

Silver-Nafion coated cylindrical carbon fiber microelectrode for amperometric monitoring of hydrogen peroxide heterogeneous catalytic decomposition

Cylindrical carbon fiber microelectrode coated with a silver layer electrodeposited from silver nitrate solution containing Pluronic F127, a surface active agent, and also a Nafion layer prepared using a dip-dry method forms a sensitive, selective and stable amperometric hydrogen peroxide sensor, operating in the cathodic region of potential. The layer formed at the electrode is composed of porous, partially crystalline silver deposits with a significant fraction of attached silver nanoparticles. Using amperometry in stirred solution at 0 mV (vs. Ag/AgCl), the electrode is capable to monitor hydrogen peroxide over its wide concentration range (1 × 10 −5 up to 0.1 M). The response of the electrode is sufficiently time-stable to enable the monitoring of hydrogen peroxide heterogeneous decomposition over manganese dioxide catalyst including the conditions where higher decomposition rates are observed, impossible to be monitored using batch titration.

Thin functionalized films on cylindrical microelectrodes for electrochemical determination of Hg(II)

Cylindrical carbon fiber microelectrodes (CFMEs) were used for the determination of Hg(II) in aquatic solutions using anodic stripping voltammetry (ASV). Optimizing the parameters, such as the potential and time of deposition, resulted in linear response of the fiber over a large range of Hg(II) concentrations (5 × 10 −8–1 × 10 −5 M). Furthermore, three methods of modification of glassy carbon and CFMEs with thiol or other Hg(II)-complexing groups, have also been examined. Electrodes were modified by electrochemically reducing 4,4′-disulfanediyldibenzenediazonium, followed by reduction of the disulfide bond, by electrodepositing sol–gel based on 3-mercaptopropyltrimethoxysilane and by spin-coating or adsorbing cellulose acetate thin films in which 4,13-didecyl-1,7,10,16-tetraoxa-4,13-diazacyclooctadecane (Kryptofix® 22DD) was incorporated. We found that all modified glassy carbon electrodes extracted Hg(II) under open-circuit potential, however, only the cylindrical microelectrode modified via the electrochemical reduction of the diazonium was capable of extracting mercuric ions. The interference of Pb(II), Cd(II), and Cu(II) with the determination of Hg(II) with 4,4′-disulfanediyldibenzenediazonium modified CFME was also studied.

Specific Oxygen-Containing Functional Groups on the Carbon Surface Underlie an Enhanced Sensitivity to Dopamine at Electrochemically Pretreated Carbon Fiber Microelectrodes

The in vivo use of carbon-fiber microelectrodes for neurochemical investigation has proven to be selective and sensitive when coupled with background-subtracted fast-scan cyclic voltammetry (FSCV). Various electrochemical pretreatments have been established to enhance the sensitivity of these sensors; however, the fundamental chemical mechanisms underlying these enhancement strategies remain poorly understood. We have investigated an electrochemical pretreatment in which an extended triangular waveform from -0.5 to 1.8 V is applied to the electrode prior to the voltammetric detection of dopamine using a more standard waveform ranging from -0.4 to 1.3 V. This pretreatment enhances the electron-transfer kinetics and significantly improves sensitivity. To gain insight into the chemical mechanism, the electrodes were studied using common analytical techniques. Contact atomic force microscopy (AFM) was used to demonstrate that the surface roughness was not altered on the nanoscale by electrochemical pretreatment. Raman spectroscopy was utilized to investigate oxide functionalities on the carbon surface and confirmed that carbonyl and hydroxyl functional groups were increased by electrochemical conditioning. Spectra collected after the selective chemical modification of these groups implicate the hydroxyl functionality, rather than the carbonyl, as the major contributor to the enhanced electrochemical signal. Finally, we have demonstrated that this electrochemical pretreatment can be used to create carbon microdisc electrodes with sensitivities comparable to those associated with larger, conventionally treated cylindrical carbon fiber microelectrodes.

Using in Vivo Electrochemistry To Study the Physiological Effects of Cocaine and Other Stimulants on the Drosophila melanogaster Dopamine Transporter

Dopamine neurotransmission is thought to play a critical role in addiction reinforcing mechanisms of drugs of abuse. Electrochemical techniques have been employed extensively for monitoring in vivo dopamine changes in the brains of model organisms including rats, mice, and primates. Here, we investigated the effects of several stimulants on dopamine clearance using recently developed microanalytical tools for in vivo electrochemical measurements of dopamine in the central nervous system of Drosophila melanogaster. A cylindrical carbon-fiber microelectrode was placed in the protocerebral anterior medial region of the Drosophila brain (an area dense with dopamine neurons) while a micropipette injector was positioned to exogenously apply dopamine. Background-subtracted fast-scan cyclic voltammetry was carried out to quantify changes in dopamine concentration in the adult fly brain. Clearance of exogenously applied dopamine was significantly decreased in the protocerebral anterior medial area of the wild-type fly following treatment with cocaine, amphetamine, methamphetamine, or methylphenidate. In contrast, dopamine uptake remained unchanged when identical treatments were employed in fumin mutant flies that lack functional dopamine transporters. Our in vivo results support in vitro binding affinity studies that predict these four stimulants effectively block normal Drosophila dopamine transporter function. Furthermore, we found 10 muM to be a sufficient physiological cocaine concentration to significantly alter dopamine transporter uptake in the Drosophila central nervous system. Taken together, these data indicate dopamine uptake in the Drosophila brain is decreased by psychostimulants as observed in mammals. This validates the use of Drosophila as a model system for future studies into the cellular and molecular mechanisms underlying drug addiction in humans.

Voltammetric detection of 5-hydroxytryptamine release in the rat brain.

5-Hydroxytryptamine (5-HT) is an important molecule in the brain that is implicated in mood and emotional processes. In vivo, its dynamic release and uptake kinetics are poorly understood due to a lack of analytical techniques for its rapid measurement. Whereas fast-scan cyclic voltammetry with carbon fiber microelectrodes is used frequently to monitor subsecond dopamine release in freely moving and anesthetized rats, the electrooxidation of 5-HT forms products that quickly polymerize and irreversibly coat the carbon electrode surface. Previously described modifications of the electrochemical waveform allow stable and sensitive 5-HT measurements in mammalian tissue slice preparations and in the brain of fruit fly larvae. For in vivo applications in mammals, however, the problem of electrode deterioration persists. We identify the root of this problem to be fouling by extracellular metabolites such as 5-hydoxyindole acetic acid (5-HIAA), which is present in 200-1000 times the concentration of 5-HT and displays similar electrochemical properties, including filming of the electrode surface. To impede access of the 5-HIAA to the electrode surface, a thin layer of Nafion, a cation exchange polymer, has been electrodeposited onto cylindrical carbon-fiber microelectrodes. The presence of the Nafion film was confirmed with environmental scanning electron microscopy and was demonstrated by the diminution of the voltammetric signals for 5-HIAA as well as other common anionic species. The modified microelectrodes also display increased sensitivity to 5-HT, yielding a characteristic cyclic voltammogram that is easily distinguishable from other common electroactive brain species. The thickness of the Nafion coating and a diffusion coefficient (D) in the film for 5-HT were evaluated by measuring permeation through Nafion. In vivo, we used physiological, anatomical, and pharmacological evidence to validate the signal as 5-HT. Using Nafion-modified microelectrodes, we present the first endogenous recording of 5-HT in the mammalian brain.

In Vivo Electrochemical Measurements of Exogenously Applied Dopamine in Drosophila melanogaster

Carbon-fiber microelectrodes coupled with electrochemical detection have been used extensively for the analysis of biogenic amines. In order to determine the functional role of these amines, in vivo studies have primarily used rats and mice as model organisms. Here, we report on the development of these microanalytical techniques for in vivo electrochemical detection of dopamine in the adult Drosophila melanogaster central nervous system (CNS). A triple-barrel micropipet injector was used to exogenously apply three different concentrations of dopamine, and a cylindrical carbon-fiber microelectrode was placed in the protocerebral anterior medial brain area where dopamine neurons are densely populated. Background-subtracted fast-scan cyclic voltammetry was used to measure dopamine concentration in the fly CNS. Distinct differences are shown for the clearance of exogenously applied dopamine in the brains of wild type flies versus fumin (fmn) mutants lacking a functional dopamine transporter. The current response due to oxidation of dopamine increased significantly from baseline for wild type flies following cocaine incubation. Interestingly, the current remained unchanged for mutant flies under the same conditions. These data confirm the accepted theory that cocaine blocks dopamine transporter function and validates the use of in vivo electrochemical methods to monitor dopamine uptake in Drosophila. Furthermore, after incubation with tetrodotoxin (TTX), a sodium channel blocker, there was a significant increase in peak oxidation current in the wild type flies; however, the current did not significantly change in the fmn mutant. These data suggest that factors that affect neuronal activity via ion channels such as TTX also influence the function of the dopamine transporter in Drosophila.

Flame Etching Enhances the Sensitivity of Carbon-Fiber Microelectrodes

Small sensors are useful for in vivo measurements and probing small spaces. In this paper, we compare two methods of fabrication of small, cylindrical carbon-fiber microelectrodes: flame-etching and electrochemical etching. With both methods, microelectrodes can be fabricated with tip diameters of 1 to 3 microm. Electrodes were tested with fast-scan cyclic voltammetry. Flame etching resulted in electrodes that have larger S/N ratios and higher currents per unit area for 1 microM dopamine than normal carbon-fiber microelectrodes or electrochemically etched electrodes. Therefore, the increased sensitivity is not just a property of size. The flame-etched surfaces had nanometer-scale surface features that were not observed on the other electrodes and exhibited increased sensitivity for other electroactive compounds found in the brain, including ascorbic acid, DOPAC, and serotonin. Faster kinetics and a faster response to a step change in dopamine were also observed, when the applied waveform was -0.4 to 1.0 V and back at 400 V/s. The sensitivity of the flame-etched electrodes was enhanced by overoxidizing the surface. The flame-etched electrodes were used to detect dopamine release in anesthetized rats after a single stimulation pulse. The small flame-etched electrodes will facilitate measurements of low concentrations in discrete brain regions or small organisms.

Flow injection and HPLC determination of furosemide using pulsed amperometric detection at microelectrodes

The flow-injection and HPLC determination of the diuretic drug furosemide using pulsed amperometric detection (PAD) at cylindrical carbon fibre microelectrodes (CFMEs) is reported. Experimental conditions such as pH (6.5) and buffer concentration (0.05 mol l −1 HPO 4 2−/H 2PO 4 −) were optimized using square-wave voltammetry (SWV). Repetitive flow-injection amperometric measurements at +1.25 V for furosemide showed a continuous decrease in the peak current, probably as a consequence of the microelectrode surface fouling. However, a suitable amperometric detection of furosemide was achieved using a PAD program consisting of a two-step potential waveform with alternating anodic and cathodic polarization. The anodic (detection) potential was +1.25 V (time of application 0.1 s), and the cathodic (cleaning) potential was −0.20 V (t=0.2 s). A linear calibration graph was obtained for furosemide in the 5.0×10 −7–1.0×10 −4 mol l −1 concentration range, with a limit of detection of 1.7×10 −7 mol l −1. HPLC-PAD at carbon fibre microelectrodes was used for the determination of furosemide in the presence of several thiouracil drugs and oxytetracycline (OTC). The mobile phase selected was a 25:75 acetonitrile:5.0×10 −3 mol l −1 NaH 2PO 4 (pH 5.0) mixture. A linear calibration graph was obtained for furosemide in the 1–100 μM range, with a limit of detection of 0.55 μM. The usefulness of this method for the determination of furosemide in real samples was evaluated by performing the analysis of commercial milk samples spiked with furosemide at a concentration level of 4.5×10 −7 mol l −1 (150 ng ml −1), as well as with other thiouracil drugs and OTC. A mean recovery of 95±5% furosemide was obtained.

Voltammetric determination of chloramphenicol in milk at electrochemically activated carbon fibre microelectrodes

Electrochemical activation of cylindrical carbon fibre microelectrodes (CFMEs) by repetitive square-wave (SW) voltammetric scans between 0.0 and +2.6 V has shown to increase dramatically the cathodic current of chloramphenicol (CAP). This is attributed to the increase of the carbon fibre surface area due to its fracture and the appearance of deep fissures along the main fibre axis. Cyclic voltammograms of CAP suggested a contribution to the total current from most likely thin-layer electrolysis. The current density:concentration ratio calculated for the reduction of CAP at an activated fibre was 6.5×10 7 μA cm −2 mol −1 l. A method for the determination of CAP at low concentrations levels, using SW voltammetry at electrochemically activated cylindrical microelectrodes, was developed. Different chemical and electrochemical variables involved in the activation step were optimised. Calibration graphs were linear ( r=0.9990) in the 1.0×10 −7 to 1.0×10 −5 mol l −1 concentration range, with a slope of (2.69+0.03) × 10 5 μA mol −1 l. A limit of detection of 4.7×10 −8 mol l −1 (15 ng ml −1) was found. The SWV method was applied for the determination of CAP in milk samples spiked with the antibiotic at two concentration levels: 64 and 320 ng ml −1. Recoveries >97% were obtained in all cases by following a very simple experimental procedure.

Voltammetric Determination of Methylthiouracil in Animal Feed Using Carbon Fiber Microelectrodes

Cylindrical carbon fiber microelectrodes (CFMEs) (8 μm in diameter, 8 mm in length) are used to develop a voltammetric method for the determination of the thyreostatic drug methylthiouracil (MTU) in animal feed samples. Methanol, containing 0.05 mol L−1 tetrabutylammonium perchlorate as supporting electrolyte, was selected as working medium because of the high MTU solubility in this solvent, which is also used for the extraction of MTU from real samples. Reproducible electroanalytical responses were obtained with no need of applying cleaning treatments to the CFME. The net-current square-wave (SW) oxidation signal of MTU was selected to be exploited analytically. Under the optimized SW conditions, a linear calibration plot for MTU was obtained in the 1.0×10−6–1.0×10−4 mol L−1 range, with a detection limit of 3.8×10−7 mol L−1. The SW voltammetric behavior of other thyreostatic drugs was checked. A simple extraction procedure with methanol, followed by a clean up step, was used for the determination of MTU in a commercial dry fodder sample spiked with this drug at the 500 μg g−1 level. The detection limit in this type of sample was found to be 8.6×10−6 mol L−1.

Carbon fibre microelectrodes modified with rhodium for the electrocatalytic determination of hydrazine

The electroanalytical capabilities of cylindrical carbon fibre microelectrodes (CFMEs) modified with electrodeposited rhodium for their use as amperometric microsensors in the detection of hydrazine under flow-injection conditions are discussed. Metallized CFMEs were prepared potentiostatically at −0.80 V in an acetate buffer solution of pH 4.5 containing 100 mg l −1 Rh(III), and characterised by cyclic voltammetry. Rh-CFMEs showed electrocatalytic ability towards the oxidation of hydrazine and some other organic compounds. A good stability of the metallised microelectrodes was found when they were used for the amperometric detection of hydrazine under flowing conditions, with no need to regenerate the electrode surface. Moreover, the signal-to-noise ratio was remarkably better at the Rh-CFME than at a Rh-modified glassy carbon electrode of conventional size. A limit of detection of 6.2×10 −7 mol l −1 hydrazine (approximately 20 μg l −1) was obtained. The effect of the presence of potential interferents was studied and the flow-injection method was applied to the determination of hydrazine in boiler water samples spiked with 0.64 mg l −1 analyte.

Differential pulse stripping voltammetry of mercury(II) ions at a cylindrical carbon-fiber microelectrode in the presence of au(III)ions

The determination of Hg(II) ions in aqueous solutions containing Au(III) ions was studied using differential pulse stripping voltammetry at a cylindrical carbon-fiber (d= 30 μm) microelectrode fabricated from a pitch at 2800°C. At the [Au(III)]/[Hg(II)] ratio higher than 25, the anodic voltammogram of mercury accumulated in the potential range from 0.2 to –0.2 V for 40–200 s exhibited a current peak at 0.62–0.72 V. The peak height and area were directly proportional to the concentration of Hg(II) in the range (1–1000) × 10–10M. The results of determining Hg(II) in waters of different origin are reported.

Analytical voltammetry in low-permitivity organic solvents using disk and cylindrical microelectrodes. Determination of thiram in ethyl acetate

The electrochemical behavior of the N, N-dialkyldithiocarbamate pesticide thiram at a Au microdisk electrode (10 μm φ) and at cylindrical carbon fiber microelectrodes (CFMEs) (8 μm φ, 8 mm length) in low-permitivity organic solvents such as toluene and ethyl acetate is reported. The obtained cyclic voltammograms were very different in shape depending on the microelectrode and the solvent used, as well as on thiram concentration. Analytically useless responses were obtained with the gold microelectrode. However, a good fitting of the voltammetric signals at cylindrical CFMEs with the theoretical model proposed by Aoki was found. The influence of variables such as the concentration of thiram and of the background electrolyte, the length of the CFMEs and the potential scan rate was tested by CV. The oxidation responses obtained by square-wave voltammetry at carbon fiber microelectrodes in ethyl acetate were found to be analytically suitable to develop a method for the determination of thiram at low concentration levels. A linear calibration graph in the 1.0×10 −6–6.0×10 −4 mol l −1 concentration range was obtained, with a slope of (1.2±0.1)×10 4 μA l mol −1. The limit of detection was 4.3×10 −7 mol l −1 thiram, and the relative standard deviation at a 5.0×10 −5 mol l −1 concentration level ( n=10) was of 1.6%. A good reproducibility of the electroanalytical measurements was found with no need of cleaning or pretreatment procedures of the CFMEs surface. The SWV method was applied with good results to the determination of thiram in spiked grapes, by measuring directly the net current in the analyte extract in ethyl acetate. Commercial fungicides and pharmaceutical preparations containing thiram were also analysed.

Use of binary metal deposits on a cylindrical carbon-fiber microelectrode in the voltammetric determination of metal ions

The effect of binary metal deposits on a cylindrical carbon-fiber microelectrode on the determination of metals by direct and stripping voltammetry was studied. The electrolytic deposition of a binary system of copper and thallium, cadmium, lead, or mercury on the electrode in an alkaline solution resulted in the disappearance of the electroreduction peak of dissolved oxygen in the potential range from -0.8 to -1.4 V and in a decrease in the background current. Under the conditions of limited diffusion, the peak currents of Ni(II), Co(II), and Zn(II) in differential pulse voltammograms were 3–7 times higher than those calculated for a reversible electrode process under the conditions of semi-infinite diffusion. Because of this, the determination limit for metal ions in direct voltammetry was lowered to 1 X 10-6 M. With a binary copper-thallium system, the peak current of zinc(II) reduction can be be detected in the presence of 5000-fold molar amounts of copper(II). The deposition of binary copper-lead and copper-thallium systems under the conditions of limited diffusion reduced the effect of negative interaction between the components of these systems and made possible the determination of lead(II) and thallium(I) by stripping voltammetry using additional peaks.

Determination of styrene and styrene additives using cylindrical microelectrodes in acetone

Cylindrical carbon fibre microelectrodes (CFMEs) (8 mm in diameter, 8 mm in length) were used to develop voltammetric methods for the determination of the antioxidant 4-tert-butylcatechol (TBC) in styrene, and of styrene in polystyrene samples. Acetone, containing 0.01 mol l−1 sodium perchlorate as supporting electrolyte, was selected as a suitable working medium. A good adjustment of the voltammetric responses to that predicted by the theory was found. Reproducible electroanalytical responses were obtained with no need of applying cleaning treatments to the CFME. Using square-wave voltammetry (SWV), linear calibration graphs were obtained for TBC and styrene in the 1.0 × 10−6–1.0 × 10−4 mol l−1 range, with detection limits of 4.3 × 10−7 mol l−1 and 4.0 × 10−7 mol l−1, respectively. The sensitivity of the method for styrene was around four orders of magnitude better than that reported for another microelectrode-based electroanalytical method. Voltammograms from mixtures of TBC and styrene showed two well-defined oxidation peaks with a difference between the peak potentials of 600 mV. Commercial styrene samples containing 0.005% of TBC were analyzed with good results. In addition, the method was applied to the determination of styrene in spiked polystyrene yoghurt packagings.

Microcylinder Polymer Modified Electrodes as Amperometric Detectors for Liquid Chromatographic Analysis of Catecholamines

The use of cylindrical carbon fiber microelectrodes (CFMEs) of various mm in length modified with poly(3-methylthiophene) (P3MT) films as amperometric microsensors for the LC-EC detection of catecholamines is reported. Cyclic voltammetry at the modified microelectrodes of dopamine, epinephrine, catechol, serotonin and ascorbic acid showed electrocatalytic ability of the modified surface. This allowed the application of low potentials (a0.35 V) for the amperometric detection of the above-mentioned compounds. Flow injection measurements carried out using a 2:98 v=v methanol:0.025 mol L ˇ1 H2PO4 ˇ =HPO4 2ˇ solution of pH 7.0, showed no evidence of fouling of the modified microelectrode surface. No cleaning or regeneration treatment was needed when working with the same polymer modified microelectrode during the whole working day. Moreover, RSD values for ip lower than 5 % were obtained when amperometric measurements were carried out with three different P3MT-coated CFMEs. These modified microelectrodes have also shown to be suitable for amperometric detection in HPLC. A mobile phase composed of 2:98 % methanol:phosphate buffer solution of pH 7.0 allowed a good separation of mixtures of ascorbic acid, epinephrine, dopamine, serotonin and catechol with detection limits ranging between 14 and 48 pmol. Furthermore, as an application, the determination of those compounds in spiked serum samples, was accomplished with good results.

Continuous monitoring of amino acids and related compounds with poly(3-methylthiophene)-coated cylindrical carbon fiber microelectrodes

The use of cylindrical carbon fiber microelectrodes (CFMEs) modified with poly(3-methylthiophene) (P3MT) films as amperometric microsensors for continuous monitoring of some amino acids such as tyrosine (Tyr), tryptophan (Trp), L-dopa, and related compounds such as tyramine and ascorbic acid, as well as small peptides such as Trp–Ala and Tyr–Gly, is discussed. Important practical advantages with respect to conventional glassy carbon (GC) electrodes, and also with respect to P3MT-coated GC electrodes of conventional size are demonstrated. The electrocatalytic ability of the modified surface allows the possibility of applying moderate potentials for the amperometric detection of the above mentioned compounds. Flow-injection experiments carried out using a phosphate buffer solution of pH 7.0 as the carrier, and an applied potential of +0.8 V showed that no cleaning or regeneration pretreatment was needed when working with the same polymer modified electrode during the whole working day. The effect of the presence of acetonitrile or methanol in the flowing solution was evaluated. A flow-injection method with amperometric detection was developed for the determination of L-dopa in pharmaceutical preparations. Finally, the modified microelectrodes have also shown suitable for amperometric detection in liquid chromatography (LC). A mobile phase composed of 5 : 95 (v/v) methanol : phosphate buffer solution of pH 7.0 allowed a good separation of mixtures of ascorbic acid, L-dopa, Tyr, tyramine and Trp, with detection limits of around 160 pmol. Furthermore, the possibility of detection of Tyr- and Trp-containing oligopeptides was also demonstrated.