Microdisk Array Research Articles

Effect of the Microelectrode Geometry on the Diffusion Behavior and the Electroanalytical Performance of Hg-Electroplated Iridium Microelectrode Arrays Intended for the Detection of Heavy Metal Traces

Mercury-electroplated iridium microband arrays intended for heavy metal determination purposes were successfully fabricated by means of microelectronic processing techniques. Iridium microbands of 5 μm width each and lengths of 50 μm, 100 μm and 5 mm were investigated and their diffusion behavior and analytical performance compared with those of 5 μm microdisk arrays. The limiting current responses of disk arrays and 5 mm length microband arrays were found to be in good agreement with the radial and hemicylindrical flux expressions, respectively. In contrast, microband arrays having an aspect ratio (length/width) below 50 exhibit diffusion profiles that are characterized by a mixture of both radial and hemicylindrical diffusion components. Such a behavior is thought to be a consequence of edge effects at the extremity of the bands. To perform square-wave anodic stripping voltammetry (SWASV) analysis, the iridium microelectrode arrays were successfully Hg-electroplated and a complete surface coverage was obtained even on microbands having an aspect ratio as high as 1000. The electroanalytical performance of the various Hg-electroplated iridium microelectrode array geometries was evaluated by measuring Cd and Pb ion concentrations in synthetic solutions by means of SWASV over a concentration range as wide as 0.1 to 50 μg L−1. Their detection efficiency (as expressed by the redissolution current normalized to the microelectrodes surface) is shown to be significantly lower than that of microdisk arrays having the same critical dimension (5 μm). Moreover, not only the detection efficiency but also the detection limits were found to decrease as the length of the microbands increases.

A Theoretical Model for Immobilized Enzyme Inhibition Biosensors

The development and experimental verification of a theoretical model for immobilized enzyme inhibition biosensors is reported. The model predicts that the percentage of inhibited enzyme (I %), after exposure to an inhibitor, is linearly related to both the inhibitor concentration ([I]B) and the square root of incubation time (t1/2): I%=Kt1/2[I]B×100 The model is based on diffusion limited inhibitor transport and takes into account the heterogeneous nature of enzyme inhibition that results from immobilization of an enzyme at the surface of a transducer. Experimental verification of the model was achieved using an electrochemical (amperometric) acetylcholine sensor. The immobilized enzyme used in this study was acetylcholinesterase, while the organophosphorus pesticide, paraoxon, was employed as the inhibitor. A second enzyme, choline oxidase, was co-immobilized to facilitate electrochemical detection of the substrate. To satisfy the conditions required by the proposed model a specially designed biosensing device was fabricated employing a microdisk array as the transducer. Acetylcholinesterase (0.00025 units) and choline (1.0 units) were co-immobilized onto the array using a water-based polyurethane/polyethylene oxide colloidal dispersion matrix. A dialysis membrane was used as an active barrier to prevent fouling of the array and to restrict enzyme leaching. The device was used for both batch and flow injection analysis and experimental results verifying the proposed model are presented using both modes.

Electroporation Introduction of Diclofenac Sodium into Human Erythrocytes and Its Determination

A method of electroporation for introducing diclofenac sodium (a drug) into human erythrocytes was developed. The conditions (the pulse strength, pulse duration and pulse number) for the introduction of diclofenac sodium into erythrocytes by electroporation were investigated. The diclofenac sodium in single human erythrocytes introduced by the electroporation was analyzed qualitatively and quantitatively by capillary zone electrophoresis (CZE) with end-column ampermetric detection at a carbon microdisk array electrode.

Determination of Histamine by Capillary Zone Electrophoresis with End-Column Amperometric Detection at a Carbon Fiber Microdisk Array Electrode

Determination of Histamine by Capillary Zone Electrophoresis with End-Column Amperometric Detection at a Carbon Fiber Microdisk Array Electrode

Electroporation Introduction of Diclofenac Sodium into Human Erythrocytes and Its Determination

A method of electroporation for introducing diclofenac sodium (a drug) into human erythrocytes was developed. The conditions (the pulse strength, pulse duration and pulse number) for the introduction of diclofenac sodium into erythrocytes by electroporation were investigated. The diclofenac sodium in single human erythrocytes introduced by the electroporation was analyzed qualitatively and quantitatively by capillary zone electrophoresis (CZE) with end-column ampermetric detection at a carbon microdisk array electrode.

Determination of Histamine by Capillary Zone Electrophoresis with End-Column Amperometric Detection at a Carbon Fiber Microdisk Array Electrode

Capillary zone electrophoresis (CZE) was employed for the analysis of histamine using an end-column amperometric detector with a carbon fiber microdisk array electrode. The optimum conditions of separation and detection are: 40 mmol/L NaH2PO4–Na2HPO4 (pH 7.0) for the buffer solution, 12.5 kV for the separation voltage, 5 kV and 10 s for the injection voltage and the injection time, and 1.30 V (vs. SCE) for the detection potential. The limit of detection is 3.3×10−6 mol/L or 13 fmol (signal to noise, S/N=3). The relative standard derivation was 0.45% for the migration time and 3.2% for the electrophoretic peak current. The method can be applied to determining histamine in rat peritoneal mast cells. The recovery of the method is between 92 and 99%.

Simultaneous determination of Cd, Pb, and Cu metal trace concentrations in water certified samples and soil extracts by means of Hg-electroplated-Ir microelectrode array based sensors

Mercury-electroplated-iridium microelectrode array based sensors were successfully used for the simultaneous determination of Cd, Pb and Cu trace concentrations by means of square wave anodic stripping voltammetry (SWASV). These recently developed sensors consist of an array of (42×42) 6-μm-diameter iridium microdiscs onto which Hg microdrops are electroplated. The Ir microdisc arrays were achieved by using microfabrication processing techniques and a Hg charge of 15 mC was identified to be the optimum amount to be electroplated onto the 6-μm-diameter microdisc array. The microelectrode array based sensors were used to analyze both water certified samples covering a wide range of metal concentrations (from 1 to ∼1000 ppb) and soil extracts collected at different soil depths (from the surface to about 1 m depth in the Rouyn-Noranda region, Que., Canada). Different physico-chemical forms of the three metals were extracted from the soil samples using barium chloride, sodium pyrophosphate and oxalic acid/ammonium oxalate solvents. For the water samples, the measured Cd, Pb and Cu concentrations are found to be in a good agreement with the certified values (with an averaged recovery for the three elements of (103±15)%). On the other hand, all the Cd forms extracted from the soil are found to be ASV-electroactive since a satisfactory agreement was obtained between the concentrations measured by SWASV and those determined by means of graphite furnace atomic absorption spectrometry (GFAAS). While Pb and Cu concentration profiles in soil were found to be particularly dependent on the extractant used, Cd concentration profile exhibited the same trend for the three extractants. The differences in metal concentrations between extractants are discussed and the predominant physico-chemical metal forms are established as a function of the horizon in the soil profile. Finally, the developed SWASV sensors are shown to be successful to measure reliably metal concentrations as low as 50 ppt in soil extracts with a preconcentration time of only 10 min.

Applications of a copper microparticle-modified carbon fiber microdisk array electrode for the simultaneous determination of aminoglycoside antibiotics by capillary electrophoresis

A copper microparticle-modified carbon fiber microdisk array electrode was fabricated and employed in capillary electrophoresis for the simultaneous determination of the five aminoglycoside antibiotics (AGs) including netilmicin, tobramycin, lincomycin, kanamycin and amikacin. The array electrode exhibited high catalytic activity for AGs, good reproducibility and stability. Under the optimum separation conditions (separation voltage of 6.2 kV, electrophoretic medium of 125 mM NaOH), the five AGs above were baseline separated within 20 min. At a working electrode potential of 0.7 V (versus saturated calomel electrode), the calibration curves were linear over two orders of magnitude of concentration, and the detection limits ( S/ N=3) were below 2 μM except for lincomycin (6.7 μM). The developed method was successfully employed for the simultaneous determination of the five AGs studied in pharmaceutical injections. The feasibility of this method for the simultaneous determination of lincomycin, kanamycin and amikacin in urine sample was also demonstrated.

Imaging Size-Selective Permeation through Micropatterned Thin Films Using Scanning Electrochemical Microscopy

This paper describes a new approach for quantification of rates of molecular transport through patterned, or otherwise heterogeneous, porous films supported on conductive substrates. Scanning Electrochemical Microscopy (SECM) has been used to image molecular sieving of redox active probes by thin, electropolymerized films of Fe(5-amino-1,10-phenanthroline)3(2+) on micropatterned and microdisk array electrodes. Films as thin as 12 nm completely block redox mediators with average molecular diameters greater than 12 A, whereas smaller diameter probes (radii 5-8 A) were observed to permeate selectively. SECM tip currents measured for three different redox permeants/mediators are observed to decrease with increasing polymer thickness, consistent with a transport model that includes partitioning into and diffusion within the polymer films. Permeabilities, PDf, within the poly[Fe(5-NH2-phen)3(2+)] films have been quantitatively determined from the SECM tip currents and are in excellent agreement with data previously obtained from rotatingdisk electrochemistry. This new methodology provides a versatile approach for quantitative investigation of membrane transport and permeation selectivity with good lateral spatial resolution.

Measurement of chloramphenicol by capillary zone electrophoresis following end-column amperometric detection at a carbon fiber micro-disk array electrode

Capillary zone electrophoresis was employed for the measurement of chloramphenicol using end-column amperometric detection with a carbon fiber micro-disk array electrode, at a constant potential of −1.00 V vs. saturated calomel electrode. The effect of oxygen in the buffer has been investigated. It is found that when the area of the carbon fiber electrode is smaller than 1.1 mm 2, the interference of oxygen can be overcome. In this procedure deoxygenation is not necessary. The effect of pH, the concentration of the buffer and the high separation voltage across the capillary on the migration time, electrophoretic peak current and separation efficiency has been studied. The optimum conditions of separation and detection are 8.4×10 −4 mol/l HOAc–3.2×10 −3 mol/l NaOAc for the buffer solution, 20 kV for the separation voltage, 5 kV and 5 s for the injection voltage and the injection time, respectively. The calibration plot was found to be linear in the range 5×10 −6 to 1×10 −3 mol/l and the limit of detection is 9.1×10 −7 mol/l or 1.4 fmol ( S/N=2). The relative standard deviation is 1.1% for the migration time and 2.3% for the electrophoretic peak current. The method was applied to the determination of chloramphenicol in human serum.

A new method of quantification of pipemidic-acid by capillary zone electrophoresis with end-column amperometric detection.

Capillary zone electrophoresis was employed for the determination of pipemidic acid using an end-column amperometric detection with a carbon fiber microdisk array electrode, at a constant potential of -1.10 V vs. saturated calomel electrode. The optimum conditions of separation and detection were 1.2 x 10(-4) mol/LNaOAc - 8.8 x 10(-4) mol/ LHOAc for the buffer solution, 20 kV for the separation voltage, 5 kV and 10 s for the injection voltage and the injection time. The limit of detection was 1.05 x 10(-7) mol/L or 189 amol (S/N=3). The relative standard deviation was 0.31% for the migration time and 2.0% for the electrophoretic peak current. The method was applied to determining pipemidic acid in human serum.

A simple capillary zone electrophoretic assay for reserpin using end-column amperometric detection at a carbon fiber micro-disk array electrode

Capillary zone electrophoresis was employed for the determination of reserpin using an end-column amperometric detection with a carbon fiber micro-disk array electrode at a constant potential of +1.30 V vs. saturated calomel electrode. The optimum conditions of separation and detection are 1.0 × 10 −4 mol/l Tris–8.4 × 10 −5 mol/l HCl for the buffer solution, 20 kV for the separation voltage, 5 kV and 10 s for the injection voltage and the injection time, respectively. The limit of detection is 8.0 × 10 −7 mol/l or 1.4 fmol ( S/N = 2). The relative standard deviation is 2.5% for the migration time and 1.5% for the electrophoretic peak current. The method was applied to the determination of reserpin in human serum.

Quantitation of caffeine by capillary zone electrophoresis with end-column amperometric detection at a carbon microdisk array electrode.

Capillary zone electrophoresis is employed for the determination of caffeine using end-column amperometric detection with a carbon fiber microdisk array electrode at a constant potential of 1.45 V versus a saturated calomel electrode. The optimum conditions of separation and detection are 0.1 52mM NaH2PO4-0.648mM Na2HPO4 for the buffer solution, 20 kV for the separation voltage, 5 kV for the injection voltage, and 10s for the injection time. The limit of detection is 2.9 x 10(-4)mM or 1.2 fmol (signal-to-noise ratio = 2). The relative standard deviation is 0.68% for the migration time and 2.3% for the electrophoretic peak current. The method is applied to determining caffeine in human serum and a cola drink.

Electrochemical behavior of redox species at carbon fibre microdisk array electrode modified with mixed-valent molybdenum(VI, V) oxide

In this study, electrode responses to a large number of electroactive species with different standard potentials at the molybdenum oxide-modified carbon fibre microdisk array (CFMA) electrode were investigated. The results demonstrated that the electrochemical behavior for those redox species with formal potentials more positive than ∼0.0 V at the molybdenum oxide-modified CFMA electrode were affected by the range and direction of the potential scan, which were different from that at a bare CFMA electrode. If the lower limit of the potential scan was more positive than the reduction potential of the molybdenum oxide film, neither the oxidation nor the reduction peaks of the redox species tested could be observed. This indicates that electron transfer between the molybdenum oxide film on the electrode and the electroactive species in solution is blocked due to the existence of a high resistance between the film and electrolyte in these potential ranges. If the lower limit of the potential scan was more negative than the reduction potential of the molybdenum oxide film (∼−0.6 V), the oxidation peaks of these species occurred at the potentials near their formal potentials. In addition, the electrochemical behavior of these redox species at the molybdenum oxide-modified CFMA electrode showed a diffusionless electron transfer process. On the other hand, the redox species with formal potentials more negative than ∼-0.2 V showed similar reversible voltammetric behaviors at both the molybdenum oxide-modified CFMA electrode and the bare electrode. This can be explained by the structure changes of the film before and after reduction of the film. In addition we also observed that the peak currents of some redox species at the modified electrode were much larger than those at a bare electrode under the same conditions, which has been explained by the interaction between these redox species and the reduction state of the molybdenum oxide film.

Development of Hg-electroplated-iridium based microelectrode arrays for heavy metal traces analysis

We report on the successful development of Hg-plated-iridium based microelectrode arrays intended to heavy metal trace analysis. The iridium microdisk arrays were achieved by using microelectronic processing techniques. These conducting-base arrays consist of 30×30 of 5 μm dia. iridium microdisks spaced 180 μm from each other. They were systematically characterized by means of cyclic voltammetry and found to exhibit a steady-state voltammetric behavior. Prior to metal trace analysis, mercury was electroplated on the iridium microdisk arrays. The latters were found to sustain the mercury film formation-and-dissolution operation at least 15 times. The electroanalytical performances of the fabricated Hg-plated-iridium microelectrode arrays were evaluated by means of multiple square wave anodic stripping voltammetry (MSWASV) in synthetic solutions of Pb and Cd ions. Over the (500 ng l −1–15 μg l −1) concentration range, the detected signal of both Cd and Pb was found to exhibit a strong linear correlation with not only the ion concentration, but also the preconcentration time. The mercury microelectrode array based sensors were found to exhibit a good stability over a period of about 5 h, for Cd concentration as low as 1 μg l −1 and a preconcentration time of 5 min.

In situ FTIR spectroelectrochemistry of a microarray electrode

A thin-layer microdisk array electrode (TLMDAE) was designed for in situ reflectance FTIR spectroelectrochemical studies. A theoretical estimation, cyclic voltammetry, chronoamperometry and in situ IR measurements demonstrate that this novel design of array electrode results in advantages such as reduced ohmic potential drop, small cell constant and facility for diffusional exchange between thin layer and bulk solution. It has been suggested that the enhanced edge diffusion on the TLMDAE leads to a reduced accumulation of species in the thin layer.

Determination of adenine and guanine by capillary zone electrophoresis with end‐column amperometric detection at a carbon fiber microdisk array electrode

AbstractCapillary zone electrophoresis was employed to the analysis of adenine and guanine by using end‐column amperometric detector with a carbon fiber microdisk array electrode. Optimum conditions of separation and detection are 0.01 mol/L H3PO4–0.01 mol/L H3BO3–0.01 mol/L HAc‐NaOH (pH 9.2) for the buffer solution, 20 kV for the separation voltage, 3.0 kV and 5 s for injection voltage and time, respectively, and 1.00 V (vs.SCE) for the detection potential. The limit of detection is 8.2 fmol for adenine and 2.9 fmol for guanine (S/N = 2). The method was applied to determination of adenine and guanine in hydrolytic product of yeast‐RNA and calf thymus‐DNA.

Determination of bovine serum albumin by capillary zone electrophoresis with end-column amperometric detection at the carbon fiber microdisk array electrode

Capillary zone electrophoresis (CZE) was employed to the analysis of bovine serum albumin (BSA) using an end-column amperometric detector with a carbon fiber microdisk array electrode. The optimum conditions of separation and detection are 0.025 mol l −1 KH 2PO 4-0.02 mol l −1 NaOH for the buffer solution, 20 kV for the separation voltage, 5 kV and 10 s for the injection voltage and the injection time, and 0.90 V vs. SCE for the detection potential. The limit of detection is 4 × 10 −7 mol l −1 or 0.7 fmol ( S N = 2 ). The method was applied to determining BSA in bovine serum.

Determination of Hydroxylamine by Capillary Electrophoresis-Electrochemical Detection with a Palladium-Particle Modified Carbon Fiber Microdisk Array Electrode

A highly dispersed ultramicro palladium-particle modified carbon fiber microdisk array electrode (Pd-CFE) was employed for capillary electrophoresis-elecrochemical (CEEC) detection of hydroxylamine (HA). The Pd particles obtained were in the nanometer scale, had a high electrocatalytic activity towards HA and exhibited good reproducibility and stability. A linear relationship between the current and the analyte concentration was found between 5 × 10−6 and 1 × 10−3 mol/1 of HA with a correlation coefficient of 0.9992. The detection limit was 5 × 10−8 mol/1. The applicability of the method for the determination of HA in river water and waste water was investigated.

Selective Determination of Cytochrome c by Capillary Zone Electrophoresis with Amperometric Detection at a L-Cysteine-Modified Gold Microdisk Array Electrode

Capillary zone electrophoresis was employed for the analysis of cytochrome c using an end-column amperometric detector with a L-cysteine-modified gold microdisk array electrode. The optimum conditions of separation and detection are 0.04 mol/L KH2PO4 - 0.06 mol/L Na2HPO4 for the buffer solution, 8 kV for the separation voltage, 4 kV and 10 s for the injection voltage and the injection time, and −0.12 V vs. SCE for the detection potential. When the fused-silica capillary modified by ethylene glycol was used, the adsorption of cytochrome c on the surface of the capillary wall could be overcome. The limit of detection is 3 × 10−5 mol/L or 62 fmol (S/N = 2). The method was applied to determining cytochrome c in pig heart.