Carbon Fiber Microdisk Electrode Research Articles

Oxygen Reduction Reactions in Low-Viscosity Ionic Liquids Based on Phosphonium Cations

There is high expectation for the lithium-oxygen (Li-O2) battery due to its high energy density because the cathodic active material is O2 in the air. The oxygen reduction reaction (ORR) at the cathode is of great importance, and it is strongly influenced by types of electrolytes. In particular, mass transport of oxygen molecules and its parameter, i. e. the solubility and the diffusion coefficient, are important for high performance of Li-O2 battery. Ionic liquids (ILs) have drawn attentions as an electrolyte for Li-O2 battery, because of their unique properties like incombustibility, low vapor pressure, good ionic conductivity and wide electrochemical potential window [1]. Major ILs used for such electrochemical devices include imidazolium, pyridinium, quaternary ammonium, pyrrolidinium and piperidinium derivatives. On the other hand, phosphonium ILs have the remarkable features such as their chemical and thermal stabilities, and lower viscosity, compared to the ammonium counterparts [2]. Therefore, we focused on the phosphonium ILs. Recently, much lower viscosity was reported that based on small phosphonium cation such as triethylalkylphosphonium cation or tri-n-butylalkylphosphonium cation [2,3]. All electrochemical measurements such as cyclic voltammetry (CV) and potential-step chronoamperometry (CA) were performed using a three electrode type electrochemical cell at 25oC. A glassy carbon (GC) disk electrode (d = 1 mm) or a carbon fiber (CF) microdisk electrode (d = 33 μm) were used as the working electrode. Counter electrode and reference electrode were Pt wire and Ag+/Ag in 0.1 M Ag-OTf / EMI-TFSA , respectively. CAs were performed on the CF microdisk electrode to estimate the solubility and the diffusion coefficient of oxygen in the ILs, individually, by the micro-electrode technique [4]. Two typical groups which consisted of triethylalkyl or tri-n-butylalkyl phosphonium cation were observed in the solubility correlated with molar volume. Moreover, two linear relationship between the diffusion coefficient and the viscosity of ILs were observed. In addition, the diffusion coefficients were correlated to occupied molar volumes which were calculated by the oxygen solubility in each IL. These result suggested that diffusion coefficient was not only affected the viscosity but also diffusion path originated from the cation structure. [1] A. Khan, C. Zhao, ACS sustainable Chem. Eng. 4, 506 (2016) [2] K. Tsunashima, M. Sugiya, Electrochem. Commun. 9, 2353 (2007) [3] K. Tsunashima, M. Sugiya, Electrochemistry, (2007), in press. [4] J. Maruyama, M. Inaba, T. Morita, Z. Ogumi, J. Electroanal. Chem. 504, 208 (2001)

Determination of Tryptophan, Glutathione, and Uric Acid in Human Whole Blood Extract by Capillary Electrophoresis with a One-Step Electrochemically Reduced Graphene Oxide Modified Microelectrode

In this work, a modified carbon fiber microdisk electrode (CFME) was created via the one-step electrochemical reduction of graphene oxide (GO) in a film on the surface of the electrode. This one-step electrochemically reduced GO-modified electrode (ERGO/CFME) displayed improved voltammetric responses to tryptophan (Trp), uric acid (UA), and glutathione (GSH) compared with those afforded by an unmodified CFME. The ERGO/CFME was used as the detector in capillary electrophoresis–electrochemical detection (CE-ECD) to separate and determine these species, and linear concentration ranges of 0.6–600 µmol L−1 for Trp, 0.8–600 µmol L−1 for GSH, and 1–600 µmol L−1 for UA were obtained using this electrode. The detection limits of this system were found to be 0.1 µmol L−1 for Trp and 0.6 µmol L−1 for GSH and UA. The ERGO/CFME showed good reproducibility, stability, and durability, and was successfully used to determine Trp, UA, and GSH in human whole-blood extracts. These results demonstrate that ERGO is a promising electrode material for use in electrochemical sensing and CE-ECD.

Electrochemical detection of nitrate in PM2.5 with a copper-modified carbon fiber micro-disk electrode

The accurate measurement of nitrate in PM2.5 is essential for a complete understanding of the effects of aerosols on human health, the impact of aerosols on the radiative balance of the earth and the role of aerosols in visibility problems. In this paper, we present a novel, quick, easy, cheap and eco-friendly electroanalytical procedure for the determination of nitrate in PM2.5 samples using a carbon-fiber micro-disk electrode (CFMDE) coupled with square-wave voltammetry (SWV). Under optimal experimental conditions the nitrate SWV response increases linearly with nitrate concentration over a range of 0.003–2.0mmolL−1, and the detection limit is 1.10μmolL−1 (S/N=3). Nitrate contents in daily PM2.5 of Yangzhou in China were detected successfully by employing this novel method, and the results were compared well with those obtained by using ion chromatography. Then, we detected nitrate in two-hour PM2.5 filter samples via the standard addition method, and the concentrations were applied in an analysis of the daily change of nitrate contained in PM2.5 of Yangzhou. The research in this work indicates that the electrochemical method opens a new opportunity for fast, portable, and sensitive analysis of components in PM2.5.

Determination of glutathione in single HepG2 cells by capillary electrophoresis with reduced graphene oxide modified microelectrode.

Determination of intracellular bioactive species will afford beneficial information related to cell metabolism, signal transduction, cell function, and disease treatment. In this study, the electrochemically reduced graphene oxide modified carbon fiber microdisk electrode (ER-GOME) was used as a detector of CZE-electrochemical detection and developed to detect glutathione (GSH). The electrocatalytic activity of the modified microelectrode was characterized by cyclic voltammetry. Under optimized experimental conditions, the concentration linear range of GSH was from 1 to 60 μM. When the S/N ratio was 3, the concentration detection limit was 1 μM. Compared with the unmodified carbon fiber microdisk electrode, the sensitivity was enhanced more than five times. With the use of this method, the average contents of GSH in single HepG2 cells were found to be 7.13 ± 1.11 fmol (n = 10). Compared with gold/mercury amalgam microelectrode, which was usually used in determining GSH, the electrochemically reduced graphene oxide modified carbon fiber microdisk electrode was friendly to environment for free mercury. Furthermore, there were several merits of the novel electrochemical detector coupled with CE, such as comparative repeatability, easy fabrication, and high sensitivity, hold great potential for the single-cell assay.

Determination of ascorbic acid in individual liver cancer cells by capillary electrophoresis with a platinum nanoparticles modified electrode

Nanoparticles-based electrochemical sensor has received significant interest. In this work, platinum nanoparticles modified carbon fiber micro-disk electrode (PtNPsME) was electrochemically prepared and exploited as an amperometric detector for capillary electrophoresis. The prepared sensor displayed rapid and sensitive response towards ascorbic acid (AA), which was attributed to the electrocatalytic effect of platinum nanoparticles. Under optimized detection conditions, AA responded linearly from the range of 1.0μM to 8.0μM and 8.0μM to 1mM with correlation coefficients of 0.9981 and 0.9993. The concentration detection limits was 0.5μM (S/N=3). Compared with carbon fiber micro-disk electrode, the sensitivity was enhanced nearly four times. Acceptable repeatability of the microanalysis system was verified by ten consecutive injections of 0.5mM, 50μM and 5μM AA without capillary and electrode treatments, the relative standard deviations of peak areas and migration time were 4.9% and 2.5%; 5.3% and 2.5%; 5.9% and 2.8%, accordingly. Meanwhile, well-shaped electrophoretic peaks were observed, mainly due to the fast electron transfer of electroactive species on the PtNPsME. The developed capillary electrophoresis–electrochemistry setup was first applied to the determination of ascorbic acid in single liver cancer cells (HepG2). The mean amount of AA in HepG2 cells extract and the average amount in single cells could amount to 9.4±1.3fmol (n=3) and 12.9±4.3fmol (n=20), respectively. Several merits of the novel electrochemical sensor coupled with capillary electrophorsis, such as comparative repeatability, easy fabrication and high sensitivity, hold great potential for the single-cell assay.

Determination of dopamine in rat less differentiated pheochromocytoma cells by capillary electrophoresis with a palladium nanoparticles microdisk electrode

A simple method was described for the direct detection of dopamine (DA) in single rat pheochromocytoma cells (PC12 cells) by capillary electrophoresis with a palladium nanoparticles modified carbon fiber microdisk electrode (PdNPsCFME). The Pd nanoparticles could be electrodeposited on the surface of the carbon fiber microdisk electrode (CFME) with a simple pretreatment procedure. The PdNPsCFME provided an enhanced effective electrode surface and high catalytic activity toward dopamine compared to traditional CFME. Under optimized detection conditions, dopamine responded linearly from the range of 0.2 μM to 2 μM and 2 μM to 0.1 mM with correlation coefficients of 0.9979 and 0.9990. The concentration detection limit was 0.1 μM (S/N = 3). The relative standard deviations of the migration time and peak current were 1.4% and 3.5%, respectively. This method had been successfully applied to determine DA in single PC12 cells and the amount of DA in ten PC12 cells ranged from 1.2 to 6.3 fmol. The above results demonstrated that CE coupled with PdNPsCFME is convenient, sensitive, and will become an attractive alternative method for single-cell analysis.

Nanomolar detection of p-nitrophenol via in situ generation of p-aminophenol at nanostructured microelectrodes

A new method and a new buffer medium, expected to be practical for miniaturized electrochemical immunoassays, were developed for rapid detection of nanomolar levels of p-nitrophenol (p-NP). The method exploits rapid reduction of p-NP to p-aminophenol (p-AP) by fast scan cyclic voltammetry at 500 V s(-1) at nanostructured carbon fiber microdisk electrodes (~7 μm dia.) fabricated from polyacrylonitrile (PAN) fibers. Large surfaces of the nanostructured microdisks facilitate the rapid reduction of p-NP to p-aminophenol (p-AP), as confirmed by the overlap with the analytical signals of the standards, which is then rapidly detected by fast scan cyclic voltammetry. A new 2-amino-2-(hydroxymethyl)propane-1,3-diol (Tris)-HAc buffer medium was developed in order to allow adaptation of this detection strategy to alkaline phosphatase (ALP)-based immunoassays. Tris-HAc is a stable medium, while the traditional Tris-HCl buffer medium produces large residual faradaic currents attributed to chloride oxidation. Addition of sodium acetate to a Tris-HAc buffer medium allows sensitivity enhancement by a factor of 2 to 0.85 nA μM(-1), similar to the best sensitivity reported at the nanostructured PAN carbon fiber microdisk sensors.

ELECTROCHEMICAL INVESTIGATION OF CYSTEAMINE AT CARBON FIBER MICRODISK ELECTRODE

The electrooxidation of cysteamine compound was carried out using convolutive cyclic voltammetry, linear sweep voltammetry and chronoamperomtry techniques at a carbon fiber microdisk electrode in 0. 1 M perchloric acid. The electrooxidation potential ( E0'' ) of cysteamine occurs at + 0.921 V. The mechanistic pathway of electrooxidation process at carbon fiber microelectrode is loss of 1 electron per molecule. The electrode process is controlled mainly by diffusion. The chemical and electrochemical parameters of the investigated system were determined experimentally and verified theoretically via digital simulation method. The unique properties of microdisk electrodes offer a number of particular advantages for their use in analytical applications. In the steady-state mass transport to the microdisk is dominated by quasi-hemispherical diffusion. As a consequence, the mass transport to and from the microdisk electrode surface is efficient, reproducible and insensitive to random convection. In analytical applications based on accumulation and stripping techniques this obviates the need for stirring during the preconcentration step, improves the precision of the measurement, and reduces analysis times. In addition, the small electrode surface area of the microdisk means that the double layer capacitance of the electrode is small, thus significantly reducing the charging current. As a consequence, the ratio of the steady-state Faradaic current to the non-Faradaic current is increased and the detection limit is enhanced.1 The small size of the electrodes also means that the currents passed at the microelectrode are small resulting in negligible ohmic drop (iR drop) even in the absence of a supporting electrolyte.2,3 This has dual advantages in analytical applications that sample preparation can be simplified and that the possibility of impurities from the added electrolyte is eliminated. Microelectrodes are thus particularly well-suited for trace analysis. Pt, Au, Hg and carbon microelectrodes have been used in stripping analyses for the determination of trace metals by a number of authors.4-8 In contrast, studies using microelectrodes for the determination of organic species are far less numerous and are restricted to the use of carbon fiber microelectrodes. 9-19

Permeable porous 1–3 nm thick overoxidized polypyrrole films on nanostructured carbon fiber microdisk electrodes

Ionically conducting 1–3 nm thick porous films of overoxidized polypyrrole (OPPY) were electrodeposited on nanostructured 7 μm diameter carbon fiber microdisk electrodes. The microdisk electrodes were fabricated from two types of polyacrylonitrile (PAN) carbon fibers, PAN T650 and PAN HCB. The electrodes were nanostructured by electrochemical etching of the microdisk electrode surface. Ultrathin porous polypyrrole (PPY) films were electrodeposited by the electropolymerization of pyrrole (PY) to PPY by a short (10 ms) single potential pulse. During the electropolymerization, the polymer “precipitated” on the nanostructured surface producing ultrathin porous film. OPPY films were fabricated by constant potential overoxidation of PPY. In steady-state voltammetry of ferricyanide, the nanostructured electrodes behave as a random array of microscopic nodules and pores. At potential scan rates of 0.050 V s −1 diffusion fields at the 300–600 nm nodules on the 7 μm diameter microdisk electrode overlap. The surface area of the electroactive nanofeatures decreases after deposition of insulating OPPY. Kinetics of ferricyanide at bare and OPPY-coated nanostructured electrodes reflect the electrode surface area, as predicted by the model for charge transfer at a partially blocked surface. A model reflecting the 58–94% coverage of the nanostructured electrodes by OPPY was developed to address the high permeability of the porous OPPY-coated microdisk electrodes.

Simultaneous multiple enantioseparation with a one-pot imprinted microfluidic channel by microchip capillary electrochromatography

A multi-template imprinted microchannel was prepared by a one-pot in situ imprinting process. The imprinted microchannel led to a novel chip-based strategy for simultaneous multiple enantioseparation. The one-pot imprinting process formed a multi-template imprinted porous thin layer (about 2 μm) on the inner wall of the capillary, which was characterized by scanning electron microscopy, infrared spectroscopy, and solid-state UV-vis spectroscopy. By fixing the imprinted capillary to a support substrate composed of poly(dimethylsiloxane) on a glass slide, a multi-analyte microchip was thus conveniently constructed. Using L-tyrosine (L-Tyr) and L-tryptophan (L-Trp) as the template molecules, two pairs of enantiomers were simultaneously baseline separated in a 6 cm separation channel within 120 s under the optimized preparation and electrochromatographic conditions. The separation showed excellent efficiency. The linear ranges for amperometric detection of four analytes using a carbon fiber microdisk electrode at +1.2 V (vs. Ag/AgCl) were from 20 to 500 μM for racemic Tyr and Trp. This multi-template imprinting strategy could be expanded for simultaneous separation and detection of additional pairs of enantiomers within a short analytical time. It could open up a promising avenue for high-throughput screening of chiral compounds.

Highly Sensitive Detection of Exocytotic Dopamine Release Using a Gold-Nanoparticle-Network Microelectrode

Here we report a new type of microelectrode sensor for single-cell exocytotic dopamine release. The new microsensor is built by forming a gold-nanoparticle (AuNP) network on a carbon fiber microelectrode. First a gold surface is obtained on a carbon fiber microdisk electrode by partially etching away the carbon followed by electrochemical deposition of gold into the pore. The gold surface is chemically functionalized with a sol-gel silicate network derived from (3-mercaptopropyl)trimethoxysilane (MPTS). A AuNP network is formed by immobilizing Au nanoparticles onto the thiol groups in the sol-gel silicate network. The AuNP-network microelectrode has been characterized by scanning electron microscopy (SEM) and steady-state voltammetry. The AuNP-network microelectrode has been used for amperometric detection of exocytotic dopamine secretion from individual pheochromocytoma (PC12) cells. The results show significant differences in the kinetic peak parameters including shorter rise time, decay time, and half-width as compared to a bare carbon fiber electrode equivalent. These results indicate AuNP-network microelectrodes possess an excellent sensing activity for single-cell exocytotic catecholamine release, specifically dopamine. Moreover, key advantageous properties inherent to bare carbon fiber microelectrodes (i.e., rigidity, flexibility, and small size) are maintained in addition to an observed prolonged shelf life stability and resistance to cellular debris fouling and dopamine polymerization.

Molecularly imprinted magnetic nanoparticles as tunable stationary phase located in microfluidic channel for enantioseparation

A microfluidic device integrated with molecularly imprinted magnetic nanoparticles as stationary phase was designed for rapid enantioseparation by capillary electrochromatography. The nanoparticles were synthesized by the co-polymerization of methacrylic acid and ethylene glycol dimethacrylate on 3-(methacryloyloxy)propyltrimethoxysilane-functionalized magnetic nanoparticles (25-nm diameter) in the presence of template molecule, and characterized with infrared spectroscopy, thermal gravimetric analysis, and transmission electron microscope. The imprinted nanoparticles (200-nm diameter) could be localized as stationary phase in the microchannel of microfluidic device with the tunable packing length by the help of an external magnetic field. Using S-ofloxacin as the template molecule, the preparation of imprinted nanoparticles, the composition and pH of mobile phase, and the separation voltage were optimized to obtain baseline separation of ofloxacin enantiomers within 195 s. The analytical performance could be conveniently improved by varying the packing length of nanoparticles zone, showing an advantage over the conventional packed capillary electrochromatography. The linear ranges for amperometric detection of the enantiomers using carbon fiber microdisk electrode at +1.0 V (vs. Ag/AgCl) were from 1.0 to 500 μM and 5.0 to 500 μM with the detection limits of 0.4 and 2.0 μM, respectively. The magnetically tunable microfluidic device could be expanded to localize more than one kind of template-imprinted magnetic nanoparticles for realizing simultaneous analysis of different kinds of chiral compounds.

Development of CE–dual opposite carbon‐fiber micro‐disk electrode detection for peak purity assessment of polyphenols in red wine

A new dual opposite carbon-fiber micro-disk electrode detector was fabricated and tested for hyphenation with CE in the polyphenol determination. Under optimized conditions, CE-dual opposite carbon-fiber micro-disk electrode was found able to baseline separate and determine five important polyphenols (trans-resveratrol, (+)-catechin, (-)-epicatechin, quercetin and gallic acid) in red wine within 16 min with low detection limit (0.031-0.21 mg/L) and satisfactory repeatability (2.0-3.3% RSD, n=5). The opposite dual electrode enables simultaneous determination of CE eluents for current ratio measured at +0.8 and +1.0 V versus Ag/AgCl for the peak purity assessment. The capability to identify the presence of co-migrating impurities in given polyphenol peaks was demonstrated in a mixed standard solution with overlapping (+)-catechin and (-)-epicatechin peaks and in commercial red wine with unknown impurities and confirming the reliability for polyphenol quantitation in red wine with matching migration time and current ratio.

Enantioseparation and Amperometric Detection of Chiral Compounds by in Situ Molecular Imprinting on the Microchannel Wall

The molecular imprinting technique was first introduced into the microchannel of a microfluidic device to form in situ the imprinted polymer for fast enantioseparation of chiral compounds. The molecularly imprinted polymer (MIP) was in situ chemically polymerized on the microchannel wall using acrylamide as the functional monomer and ethylene glycol dimethacrylate as the cross-linker, and characterized by scanning electron microscopy, atomic force microscopy, and infrared spectroscopy. Under the optimized conditions, such as optimal preparation of MIP, composition and pH of mobile phase, and separation voltage, the model enantiomers, tert-butoxycarbonyl-D-tryptophan (Boc-D-Trp) and Boc-L-Trp, could be baseline separated within 75 s. The linear ranges for amperometric detection of the enantiomers using carbon fiber microdisk electrode at +1.2 V (vs Ag/AgCl) were from 75 to 4000 microM and 400 to 4000 microM with the detection limits of 20 and 140 microM, respectively. The MIP-microchip electrophoresis provided a powerful protocol for separation and detection of Boc-Trp enantiomers within a short analytical time. The molecular imprinting on microchannel wall opens a promising avenue for fast enantioscreening of chiral compounds.

Real-Time Monitoring of Oxidative Burst from Single Plant Protoplasts Using Microelectrochemical Sensors Modified by Platinum Nanoparticles

Oxidative bursts from plants play significant roles in plant disease defense and signal transduction; however, it has not hitherto been investigated on individual living plant cells. In this article, we fabricated a novel sensitive electrochemical sensor based on electrochemical deposition of Pt nanoparticles on the surface of carbon fiber microdisk electrodes via a nanopores containing polymer matrix, Nafion. The numerous hydrophilic nanochannels in the Nafion clusters coated on the electrode surface served as the molecular template for the deposition and dispersion of Pt, which resulted in the uniform construction of small Pt nanoparticles. The novel sensor displayed a high sensitivity for detection of H(2)O(2) with a detection limit of 5.0 x 10(-9) M. With the use of this microelectrochemical sensor, the oxidative burst from individual living plant protoplasts have been real-time monitored for the first time. The results showed that oxidative burst from single protoplasts triggered by a pathogen analogue were characterized by quanta release with a large number of "transient oxidative microburst" events, and protoplasts from the transgenic plants biologically displayed better disease-resistance and showed a distinguished elevation and longer-lasting oxidative burst.

Real-time monitoring of NO release from single cells using carbon fiber microdisk electrodes modified with single-walled carbon nanotubes

In this paper, a novel NO electrochemical microsensor, which is fabricated by modifying the surface of a carbon fiber microdisk electrode (CFMDE, diameter: 5–7 μm) with single-walled carbon nanotubes (SWNTs) and Nafion membrane, is reported for the first time. The modification of SWNTs dramatically improves the sensitivity of CFMDEs, and the detection limit for NO is 4.3 nM that is nearly 10 times lower than that from the bare one and lower than most NO electrochemical sensors reported before. The Nafion membrane offers a good barrier to some interferents such as nitrite and ascorbic acid without losing response speed to NO. The sensor has been successfully applied to the measurement of NO release from single isolated human umbilical vein endothelial cells (HUVECs). Real-time amperometric data show that the addition of l-arginine ( l-arg) or acetylcholine (ACh) can cause a quick increase in NO production with a maximum concentration of 232 ± 44 nM ( n = 5) and 159 ± 29 nM ( n = 5), respectively.

Analysis of nephroloxic and carcinogenic aristolochic acids in Aristolochia plants by capillary electrophoresis with electrochemical detection at a carbon fiber microdisk electrode

Aristolochic acids (AAs) are the main bioactive ingredients in the most of Aristolochia plants, which are used to make dietary supplements, slimming pills and Traditional Chinese Medicines (TCMs). Excessive ingestion of AAs can lead to serious nephropathy. Therefore, quantitative analysis and quality control for the plants containing AAs is of great importance. In this paper, capillary electrophoresis (CE) with electrochemical detection (ED) at a 33 μm carbon fiber microdisk electrode (CFE) has been applied to detect AA-I and AA-II in Aristolochia plants. Under the optimum conditions: detection potential at 1.20 V, 2.0 × 10 −2 mol L −1 phosphate buffer solution (PBS) (pH 10.0), injection time 25 s at a height of 17 cm and separation voltage at 12.5 kV, the AA-I and AA-II were baseline separated within 5 min. Low detection limits for AA-I and AA-II were 4.0 × 10 −8 mol L −1 and 1.0 × 10 −7 mol L −1, respectively. Wide linear ranges were from 4.0 × 10 −8 mol L −1 to 1.9 × 10 −5 mol L −1 and 1.0 × 10 −7 mol L −1 to 5.0 × 10 −5 mol L −1 for AA-I and AA-II, respectively. The proposed method has been successfully applied to analyze AAs contents in plant extracts. The results indicated that the contents of AAs in each part of Aristolochia debilis Sieb. Et Zucc. plant were different. Meanwhile, the CE–ED method was utilized for fingerprint analysis of medicine herbs. Six herbs ( Radix aristolochiae, Fructus aristolochiae, Herba aristolochiae, Caulis aristolochiae manshuriensis, Caulis clematidis armandii, Caulis akebiae) were well distinguished by comparing their electropherograms obtained by CE–ED method.

Electrochemical Detection of Methimazole by Capillary Electrophoresis at a Carbon Fiber Microdisk Electrode

We developed a sensitive, simple and low cost method to determine methimazole based on capillary electrophoresis with electrochemical detection (CE-EC) at a carbon fiber microdisk electrode (CFE). We investigated the effects of detection potential, the concentration and pH value of the phosphate buffer, and injection time as well as separation voltage on the detection of methimazole. Under the optimized conditions: the detection potential at 1.30 V, 10 mmol/ L phosphate buffer (pH 7.0), injection time 30 s at a height of 20 centimeter and separation voltage at 15 kV, the linear range was obtained from 1.0 � 10 � 7 to 2.0 � 10 � 4 mol/L, covering 3 orders of magnitude with a correlation coefficient of 0.9995. The LOD (S/N ¼ 3) obtained was 5.0 � 10 � 8 mol/L. The RSD of migration time and peak current for 2.0 � 10 � 4 mol/L methimazole was 1.04% and 1.54% (n ¼ 10), respectively. The method was also used to analyze methimazole tablets and human urine sample.

Microchip Capillary Electrophoresis with an End-Channel Amperometric Detector and Its Preliminary Application

An end-channel amperometric detector with a guide tube for working electrode was designed and integrated on a home-made glass microchip. The guide tube was directly patterned and fabricated at the end of the detection reservoir, which made the fixation and alignment of working electrode relatively easy. The fabrication was carried our in a two-step etching process. A 30 mum carbon fiber microdisk electrode and Pt cathode were also integrated onto the amperometric detector. The baseline separation of dopamine (DA), catechol (CA) and epinephrine (EP) was achieved within 80 s. Relative standard deviations of not more than 5.2% were obtained for both peak currents and migration times of DA and CA (n=5). Using standard adding method, DA in urine and plasma samples was detected. The recoveries were in the range of 83%-103%.

Voltammetry and determination of metronidazole at a carbon fiber microdisk electrode

The electrochemistry of metronidazole, 1-(hydroxyethyl)-2-methyl-5-nitroimidazole, was investigated at a carbon fiber microdisk electrode in pH 9 Britton Robinson buffer. Under these conditions, the reduction of metronidazole is controlled by both mass transport to the microdisk and adsorption with an equilibrium constant of 4 × 10 3 mol −1 dm 3 and a saturation coverage of 0.88 × 10 −8 mol cm −2. The adsorption and accumulation of metronidazole on the surface of the carbon fiber allows its determination at low concentrations by square wave adsorptive stripping voltammetry. A detection limit for metronidazole of 5 × 10 −7 mol dm −3 and a R.S.D. of 3.7% at 1 × 10 −6 mol dm −3 ( n = 4) were obtained with a two electrode system with no stirring during the accumulation step. Based on this method, a simple procedure for the determination of metronidazole in urine is described which requires no pre-treatment of the sample before analysis.