Miniaturized thin-layer radial flow cell with interdigitated ring-shaped microarray electrode used as amperometric detector for capillary electrophoresis

Signal amplification in a microchannel from redox cycling with varied electroactive configurations of an individually addressable microband electrode array

Amperometric detection at microelectrodes in lab-on-a-chip (LOAC) devices lose advantages in signal-to-background ratio, reduced ohmic iR drop, and steady-state signal when volumes are so small that diffusion fields reach the walls before flux becomes fully radial. Redox cycling of electroactive species between multiple, closely spaced microelectrodes offsets that limitation and provides amplification capabilities. A device that integrates a microchannel with an individually addressable microband electrode array has been used to study effects of signal amplification due to redox cycling in a confined, static solution with different configurations and numbers of active generators and collectors. The microfabricated device consists of a 22 microm high, 600 microm wide microchannel containing an array of 50 microm wide, 600 microm long gold microbands, separated by 25 microm gaps, interspersed with an 800 microm wide counter electrode and 400 microm wide passive conductor, with a distant but on-chip 400 microm wide pseudoreference electrode. Investigations involve solutions of potassium chloride electrolyte containing potassium ferrocyanide. Amplification factors were as high as 7.60, even with these microelectrodes of fairly large dimensions (which are generally less expensive, easier, and more reproducible to fabricate), because of the significant role that passive and active (instrumentally induced) redox cycling plays in confined volumes of enclosed microchannels. The studies are useful in optimizing designs for LOAC devices.

Simultaneous determination of dihydroxybenzene and phenylenediamine positional isomers using capillary zone electrophoresis coupled with amperometric detection

In general capillary zone electrophoresis (CZE) separation models, o-, m-, and p-phenylenediamine isomers can be separated in a weak acidic running buffer for their pK(a) values being 4.52, 5.64, 6.04, respectively, while o-, m-, and p-dihydroxybenzene isomers can be separated in a weak basic buffer for their pK(a) values being 9.40, 9.40 and 10.04, respectively. So, it is hard to find a suitable running buffer at a fixed pH in normal CZE for simultaneous separation of these two groups of positional isomers. In this paper, a novel method based on alternately running two different pH buffers in CZE coupled with amperometric detection (CZE-AD) was designed to simultaneously determine these two groups of positional isomers. It is found that when two different pH running buffers were employed alternately under appropriate order and time, the six analytes could be separated perfectly in less than 20 min and the detection limits were as low as 10(-7) mol/L. Furthermore, the effects of working electrode potential, pH and concentration of running buffer, separation voltage and injection time on CZE-AD were investigated. Experimental results demonstrated that the introduced method was practical to simultaneously determine two groups of positional isomers with different pK(a) and had some advantages of high sensitivity, good repeatability and small sample requirement.

Simultaneous determination of sugars and ascorbic acid by capillary zone electrophoresis with amperometric detection at a carbon paste electrode modified with polyethylene glycol and Cu2O

In this paper, a kind of novel carbon paste electrode modified with double modifiers-polyethylene glycol (PEG) and Cu(2)O (PEG/Cu(2)O CPME) in capillary zone electrophoresis with amperometric detection (CZE-AD) was applied to simultaneously determine three sugars: glucose, sucrose, fructose and ascorbic acid (AA). The catalytic electrochemical properties of PEG/Cu(2)O CPME could enhance sensitivity obviously compared with carbon paste electrode modified with only PEG or Cu(2)O at a relatively lower detection potential (+0.3 V versus SCE). Especially, the electrochemical detection response of AA increased obviously to the same level of saccharides by adding PEG into the Cu(2)O carbon paste modified electrode. The four analytes could be perfectly separated within 22 min, the linear ranges were from 1.0 x 10 (-6) to 5.0 x 10(-5) mol L(-1) and the detection limits were at 10(-7) mol L(-1) magnitude (S/N=3). The present working electrode was successfully employed to analyse beverage samples with recoveries in the range 93-107% and RSDs less than 4%. Above results demonstrated that capillary zone electrophoresis coupled with the PEG/Cu(2)O carbon paste modified electrode was of convenient preparation, high sensitivity, good repeatability and could be used in the rapid determination of practical samples.

Microbead-based electrochemical immunoassay with interdigitated array electrodes

The objective of this study was to develop a sensitive and miniaturized immunoassay by coupling a microbead-based immunoassay with an interdigitated array (IDA) electrode. An IDA electrode amplifies the signal by recycling an electrochemically redox-reversible molecule. The microfabricated platinum electrodes had 25 pairs of electrodes with 1.6-microm gaps and 2.4-microm widths. An enzyme-labeled sandwich immunoassay on paramagnetic microbeads with mouse IgG as the analyte and beta-galactosidase as the enzyme label was used as the model system. beta-Galactosidase converted p-aminophenyl beta-D-galactopyranoside to p-aminophenol (PAP). This enzyme reaction was measured continuously by positioning the microbeads near the electrode surface with a magnet. Electrochemical recycling occurred with PAP oxidation to p-quinone imine (PQI) at +290 mV followed by PQI reduction to PAP at -300 mV vs Ag/AgCl. Dual-electrode detection amplified the signal fourfold compared to single-electrode detection, and the recycling efficiency reached 87%. A calibration curve of PAP concentration vs anodic current was linear between 10(-4) and 10(-6)M. A signal from 1000 beads in a 20-microL drop was detectable and the immunoassay was complete within 10 min with a detection limit of 3.5x10(-15)mol mouse IgG.

In-channel dual-electrode amperometric detection in electrophoretic chips with a palladium film decoupler

An electrophoretic microchip integrated with a Pd-film decoupler and a series-dual electrode was proven practical (200-800 V/cm) for routine amperometric detection. In fluidic systems, amperometric enhancement of parallel-opposed dual-electrode detection is due to redox cycling of analytes between the electrodes. We, however, found that the oxidation current of catecholamines was enhanced significantly (1.9-3.8 folds) by switching from the single electrode mode to dual-series mode. This novel finding was unexpected because the unidirectional flow characteristic of the microfluidic system should eliminate the possibility for analytes physically migrating back and forth between the upstream and downstream electrodes. We attribute the enhancement to turbulence generated by impinging of the flow onto the edge of the downstream electrode. The linear range, sensitivity, limit of detection (S/N = 3) and number of theoretical plates for DA and CA are, respectively, 0.5-50 microM, 47 pA/microM, 0.25 microM, 7000 m(-1) and 1.0-100 microM, 28 pA/microM, 0.49 microM, 15,000 m(-1).

Chiral analysis of neurotransmitters using cyclodextrin-modified capillary electrophoresis equipped with microfabricated interdigitated electrodes

We present cyclodextrin-modified capillary electrophoresis equipped with a microfabricated chip consisting of an array of eight interdigitated microband platinum electrodes (IDs) for simultaneous analysis of three chiral models: epinephrine, norepinephrine and isoproterenol. The IDE chip, positioned very close to the capillary outlet, served as an amplification/detection system. Emerging neurotransmitters at the IDE surface were oxidized at +1.1 V by seven electrodes of the array and then detected by the remaining electrode, poised at +0.0 V. There was an amplification effect on the detecting electrode owing to the recycle between the reduced and oxidized forms of the optical isomers at the electrode surface. The detecting "amplification" current response was governed by the applied potential, the detecting electrode position, the number of adjacent electrodes used for recycling and the distance between the oxidative and reductive electrodes. The six chiral forms of the three neurotransmitters were resolved using 25 mM heptakis(2,6,di-o-methyl)-beta-cyclodextrin with a detection limit of approximately 5 microM. The scheme detected a reduced compound at a reducing potential instead of conventional oxidation detection to alleviate electrode fouling and electroactive interferences. The concurrent oxidation/reduction detection of compounds also facilitated and ascertained peak identification as interfering compounds were unlikely to have the same oxidative/reductive characteristics and mobilities as the analytes of interrogation.

Amperometric and voltammetric detection for capillary electrophoresis

The focus of this article is amperometric and voltammetric detection coupled with capillary electrophoresis. Fundamental concepts and progress in the field of capillary electrophoresis with electrochemical detection (CEEC) that have occurred within the past three years, including new methodologies and unique applications, are highlighted. This review contains 95 references.

Advances in the voltammetric analysis of small biologically relevant compounds

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