Recent advances in electrochemical detection in capillary electrophoresis

Recent progress of on-line sample preconcentration techniques in microchip electrophoresis

This review highlights recent developments and applications of on-line sample preconcentration techniques to enhance the detection sensitivity in microchip electrophoresis (MCE); references are mainly from 2008 and later. Among various developed techniques, we focus on the sample preconcentration based on the changes in the migration velocity of analytes in two or three discontinuous solutions system, since they can provide the sensitivity enhancement with relatively easy experimental procedures and short analysis times. The characteristic features of the on-line sample preconcentration applied to microchip electrophoresis (MCE) are presented, categorized on the basis of "field strength-" or "chemically" induced changes in the migration velocity. The preconcentration techniques utilizing field strength-induced changes in the velocity include field-amplified sample stacking, isotachophoresis and transient-isotachophoresis, whereas those based on chemically induced changes in the velocity are sweeping, transient-trapping and dynamic pH junction.

On-chip electrochemical detection of CdS quantum dots using normal and multiple recycling flow through modes

A flexible hybrid polydimethylsiloxane (PDMS)-polycarbonate (PC) microfluidic chip with integrated screen printed electrodes (SPE) was fabricated and applied for electrochemical quantum dots (QDs) detection. The developed device combines the advantages of flexible microfluidic chips, such as their low cost, the possibility to be disposable and amenable to mass production, with the advantages of electrochemistry for its facility of integration and the possibility to miniaturize the analytical device. Due to the interest in biosensing applications in general and particularly the great demand for labelling alternatives in affinity biosensors, the electrochemistry of cadmium sulfide quantum dots (CdS QDs) is evaluated. Square wave anodic stripping voltammetry (SWASV) is the technique used due to its sensitivity and low detection limits that can be achieved. The electrochemical as well as the microfluidic parameters of the developed system are optimized. The detection of CdS QDs in the range between 50 to 8000 ng mL(-1) with a sensitivity of 0.0009 μA/(ng mL(-1)) has been achieved. In addition to the single in-chip flow through measurements, the design of a recirculation system with the aim of achieving lower detection limits using reduced volumes (25 μL) of sample was proposed as a proof-of-concept.

Recent progress of online sample preconcentration techniques in microchip electrophoresis

Microchip electrophoresis (MCE) has been advanced remarkably by the applications of several separation modes and the integration with several chemical operations on a single planer substrate. MCE shows superior analytical performance, e.g., high-speed analysis, high resolution, low consumption of reagents, and so on, whereas low-concentration sensitivity is still one of the major problems. To overcome this drawback, various online sample preconcentration techniques have been developed in MCE over the past 15 years, which have successfully enhanced the detection sensitivity in MCE. This review highlights recent developments in online sample preconcentration in MCE categorized on the basis of "dynamic" and "static" methods. The dynamic techniques including field amplified stacking, ITP, sweeping, and focusing have been easily applied to MCE, which provide effective enrichments of various analytes. The static techniques such as SPE and filtration have also been combined with MCE. In the static techniques, extremely high preconcentration efficiency can be obtained, compared to the dynamic methods. This review provides comprehensive tables listing the applications and sensitivity enhancement factors of these preconcentration techniques employed in MCE.

Simultaneous determination of flavones and phenolic acids in the leaves of Ricinus communis Linn. by capillary electrophoresis with amperometric detection

Capillary electrophoresis (CE) with amperometric detection (AD) has been developed for the separation and determination of disaccharide glycoside rutin, gentistic acid, quercetin, and gallic acid in the leaves of Ricinus communis Linn. for the first time. The effects of the acidity and the concentration of the running buffer, separation voltage, injection time, and detection potential were investigated to acquire the optimum conditions for the determination of the four analytes. The detection electrode was a 300microm diameter carbon disc electrode at a detection potential of +0.90V (versus saturated calomel electrode (SCE)). The four analytes could be well separated within 10min in a 40cm length fused silica capillary at a separation voltage of 15kV in a 50mM borate buffer (pH 9.0). The relation between peak current and analyte concentration was linear over about 3 orders of magnitude with detection limits (S/N=3) ranging from 0.8 to 2.9microM for all the analytes. The proposed method has been successfully applied to monitor flavones and phenolic acids in the real plant samples with satisfactory assay results.

Analysis of inorganic and small organic ions by CE with amperometric detection

Capillary electrophoresis has become a widely useful analytical technology. Amperometric detection is extensively employed in capillary electrophoresis for its many inherent virtues, such as rapid response, remarkable sensitivity, and low cost of both detectors and instrumentations. Analysis of inorganic and small organic ions by capillary electrophoresis is an important research field. This review focuses on the recent developments of capillary electrophoresis coupled with amperometric detection for analysis of inorganic and small organic ions. Advancements in electrophoresis separation modes, amperometric detection modes, working electrodes, and applications of inorganic ions, amino acids, phenols, and amines are discussed.

In-column fiber-optic laser-induced fluorescence detection for CE

A highly sensitive in-column fiber-optic LIF detector for CE has been constructed and evaluated. In this detection system, a 457-nm diode-pumped solid-state blue laser was used as the excitation light source and an optical fiber (40 mum od) was used to transmit the excitation light. One end of the optical fiber was inserted into the separation capillary and was in situ positioned at the detection window. The other end of the fiber was protruded from the capillary to capture the excitation light beam from the blue laser. Fluorescence emission was collected by a 40 x microscope objective, focused on a spatial filter, and passed through a yellow color filter before reaching the photomultiplier tube. The present CE-fluorescence detection is a simple and compact optical system. It reduces the laser scattering effect from the capillary and fiber as compared to the conventional LIF detection for CE. Its utility was successfully demonstrated by the separation and determination of D-penicillamine labeled with naphthalene-2,3-dicarboxaldehyde. The detection limit was 0.8 nM (S/N = 3). The present detection scheme has been proven to be attractive for sensitive fluorescence detection for CE.

Determination of active constituents in Lonicera confusa DC. by capillary electrophoresis with amperometric detection

A method based on capillary electrophoresis with amperometric detection has been developed for the determination of luteolin, chlorogenic acid, 3,5-dicaffeoylquinic acid and caffeic acid in the dried flower buds, leaves and stems (three medicinal parts) of Lonicera confusa DC., respectively. The effects of several important factors such as detection potential, the concentration of the running buffer, separation voltage and injection time were investigated to acquire the optimum conditions. The detection electrode was a 300 microm diameter carbon disc electrode at a working potential of + 0.90 V (vs saturated calomel electrode). The four analytes can be well separated within 10 min in a 40 cm-long fused silica capillary at a separation voltage of 12 kV in a 50 mM borate-25 mM phosphate buffer (pH 8.0). The relationship between peak current and analyte concentration was linear over about 3 orders of magnitude with detection limits (S/N = 3) ranging from 0.35 to 0.52 microM for all analytes. The proposed method has been successfully applied to the monitoring of bioactive constituents in the real plant samples with satisfactory assay results.

Determination of honokiol and magnolol in Cortex Magnoliae Officinalis by capillary electrophoresis with electrochemical detection

Capillary electrophoresis with electrochemical detection has been employed for the determination of honokiol and magnolol in Cortex Magnoliae Officinalis (i.e. Magnolia Bark) for the first time. Effects of several important factors such as the concentration and the acidity of the running buffer, separation voltage, injection time, and detection potential were investigated to acquire the optimum conditions. The detection electrode was a 300 microm diameter carbon disc electrode at a working potential of +0.90 V (versus saturated calomel electrode (SCE)). The two analytes can be well separated within 6 min in a 40 cm length fused silica capillary at a separation voltage of 18 kV in a 50mM borate buffer (pH 9.2). The relation between peak current and analyte concentration was linear over about three orders of magnitude with the detection limits (S/N=3) of 0.38 and 0.51 microM for honokiol and magnolol, respectively. The proposed method has been successfully applied to monitor the two bioactive constituents in the real plant samples with satisfactory assay results.

Determination of enkephalin peptides by nonaqueous capillary electrophoresis with electrochemical detection

Nonaqueous capillary electrophoresis with electrochemical detection (NACE-ED) was applied to the analysis of enkephalin peptides. The effect of different buffer compositions on the electrophoretic behavior of methionine enkephalin, leucine enkephalin, and [D-Ala2]-leucine enkephalin was studied. Separation of the protonated and the deprotonated peptides was obtained using ACN/methanol-based electrolyte systems. The electrochemical behavior of the enkephalins was studied by the capillary batch injection analysis technique. NACE-ED yielded well-defined signals in the oxidation mode only for the negatively charged analytes. The optimized BGE for the counterelectroosmotic separation consisted of 10 mM ammonium acetate in ACN/methanol (3:1 v/v). Using a platinum microdisk electrode set to an actual potential of +0.65 V detection limits in the submicromolar range were observed which are about one order of magnitude lower compared to UV detection. Problems concerning EOF instability and electrode fouling caused by water and other neutral sample impurities transported by the EOF can be avoided in the EOF-inverted mode using poly(ethylene glycol)-coated capillaries and an actual working electrode potential of +1.0 V. For the quantification of the enkephalins [D-Ala2]leucine enkephalin was used as internal standard. The practical utility for the determination of enkephalins in spiked plasma samples after SPE was demonstrated.

Sensitive chiral analysis by capillary electrophoresis

In this review, an updated view of the different strategies used up to now to enhance the sensitivity of detection in chiral analysis by CE will be provided to the readers. With this aim, it will include a brief description of the fundamentals and most of the recent applications performed in sensitive chiral analysis by CE using offline and online sample treatment techniques (SPE, liquid-liquid extraction, microdialysis, etc.), on-column preconcentration techniques based on electrophoretic principles (ITP, stacking, and sweeping), and alternative detection systems (spectroscopic, spectrometric, and electrochemical) to the widely used UV-Vis absorption detection.

Determination of glycosides and sugars in Moutan Cortex by capillary electrophoresis with electrochemical detection

A method based on capillary electrophoresis with electrochemical detection has been developed for the separation and determination of paeoniflorin, sucrose, paeonoside, glucose, and fructose in Moutan Cortex for the first time. Effects of several important factors such as the concentration of NaOH, separation voltage, injection time, and detection potential were investigated to acquire the optimum conditions. The detection electrode was a 300 microm diameter copper disc electrode at a working potential of +0.60 V (versus saturated calomel electrode (SCE)). The five analytes can be well-separated within 12 min in a 40 cm length fused silica capillary at a separation voltage of 12 kV in a 75 mM NaOH aqueous solution. The relation between peak current and analyte concentration was linear over about 3 orders of magnitude with detection limits (S/N = 3) ranging from 0.9 to 1.3 microM for all analytes. The proposed method has been successfully applied to monitor glycoside and sugar contents in the real plant samples with satisfactory assay results.

Characterization of etched electrochemical detection for electrophoresis in micron inner diameter capillaries

An enhanced etched electrochemical (EC) detection technique has been developed for CE in micron inner diameter capillaries. The design improvements allow for better alignment between the capillary bore and the electrode. This new method involves utilizing a carbon fiber microelectrode and etching both the carbon fiber and the detection end of a micrometer-sized inner diameter capillary to limit dead volume and analyte diffusion at the amperometric EC detector. To understand the factors affecting enhanced detector efficiency, a detailed examination of the relationship between detector design and performance has been completed by exploring the effects of varying electrode diameter, tip shape, and size, in addition to the etch length of the capillary outlet. The enhanced detection provides peak efficiencies as high as 75000 theoretical plates and estimated detection limits as low as 40 nM for dopamine. This etched detection method should further facilitate volume-limited sample analysis by CE.

Determination of aminoheterocycle and azabicycle in gliclazide bulk by capillary zone electrophoresis with amperometric detection

A simple, reliable and reproducible method, based on capillary zone electrophoresis with amperometric detection (CZE-AD), was developed for simultaneous determination of 3-amino-3-azabicyclo[3,3,0]octane (aminoheterocycle) and 3-azabicyclo[3,3,0]octane (azabicycle) in gliclazide bulk drug. The optimal conditions of CZE-AD were 50 mM borate solution (pH 9.0) as running buffer, 14 kV as separation voltage and 0.95 V (versus SCE) as detection potential. Under the selected optimum conditions, the two analytes could be perfectly separated within 9 min. The linearity range of aminoheterocycle was from 1.0 x 10(-6) to 1.0 x 10(-3) M and that of azabicycle was from 2.0 x 10(-6) to 1.0 x 10(-3) M. Their detection limits were 5.0 x 10(-7) and 1.0 x 10(-6) M, respectively, (S/N=3). This proposed method demonstrated long-term stability and reproducibility with relative standard deviations of less than 2% for both migration time and peak current. It has been successively used for the determination of these two analytes in gliclazide bulk drug, and the assay results were satisfactory.

Integrated capillary electrophoresis amperometric detection microchip with replaceable microdisk working electrode. II. Influence of channel cross-sectional area on the separation and detection of dopamine and catechol

The interference of separation high voltage with the electrochemical detection is a major challenge to the microchip capillary electrophoresis-electrochemical detection systems with end-channel detection mode. Using dopamine and catechol as model analytes, the influences of channel cross-sectional area and channel-to-electrode distance on the high-voltage interference, accordingly on the separation and detection performances of the microchip capillary electrophoresis-electrochemical detection system were investigated. With the increase of the channel cross-sectional area from 312 through 450-615 microm2, the apparent half-wave potentials of hydrodynamic voltammetry for dopamine at the field strength of 288 V/cm shifted positively from 285 through 330-400 mV. By using a chip with the smallest channel cross-section (312 microm2 with top width of 37.3 microm and depth of 8.9 microm) the residual high-voltage field in the detection cell was small, so that detection was conducted at a channel-to-electrode distance of 20 microm to achieve better performances of separation and detection.

Effects of the cell geometry and operating parameters on the performance of an external contactless conductivity detector for microchip electrophoresis

Quantitative data on the effect of the electrode geometry on the signal strength and the signal-to-noise ratio is given. The measurements are affected by the unavoidable presence of stray capacitance. Best results are achieved for short and narrow electrodes arranged in an antiparallel configuration and separated by a minimal gap, which determines the dimensions of the actual detection volume. Limits of detection between 150 and 250 microg l(-1) and separation efficiencies from 13,000 to 17,000 theoretical plates were achieved for six inorganic cations (NH(4)(+), K(+), Ca(2+), Na(+), Mg(2+)and Li(+)) with electrodes of 1 mm width and a detection gap of 0.5 mm (separation channel length: 7.5 cm) when operating the detector at 20 V(pp) and 500 kHz. The analyses of all major inorganic cations in tap and rain water samples were demonstrated for the first time in microchip electrophoresis with contactless conductivity detection.

Determination of Three Bioactive Constituents in Moutan Cortex by Capillary Electrophoresis with Electrochemical Detection

Capillary electrophoresis with electrochemical detection has been employed for the separation and determination of the three active constituents (paeonol, benzoyloxypaeoniflorin, and oxypaeoniflorin) in traditional Chinese medicine, Moutan Cortex (root cortex of Paeonia suffruticosa Andrews). The effects of several important factors, such as the concentration of running buffer, the separation voltage, the injection time, and the detection potential, were investigated to determine the optimum conditions. The detection electrode was a 300 microm diameter carbon-disc electrode at a working potential of +0.90 V (versus SCE). The three analytes could be well separated within 7 min in a 40 cm length fused-silica capillary at a separation voltage of 12 kV in a 50 mM borate buffer (pH 9.2). The relation between the peak current and the analyte concentration was linear over 3 orders of magnitude with detection limits (S/N = 3) ranging from 0.4 to 0.7 microM for all analytes. The proposed method has been successfully applied to the determination of paeonol, benzoyloxypaeoniflorin, and oxypaeoniflorin in real plant samples with satisfactory assay results.

Rapid Determination of Paeoniflorin and Three Sugars in Radix Paeoniae Alba by Capillary Electrophoresis

A method based on capillary electrophoresis with electrochemical detection has been developed for the separation and determination of paeoniflorin, sucrose, glucose, and fructose in traditional Chinese medicine, Radix Paeoniae Alba. The effects of several important factors, such as the concentration of NaOH, the separation voltage, the injection time, and the detection potential, were investigated to determine the optimum conditions. The detection electrode was a 300-microm diameter copper disc electrode at a working potential of +0.60 V (versus SCE). The four analytes can be well separated within 8 min in a 40 cm length fused-silica capillary at a separation voltage of 12 kV in a 75 mM NaOH aqueous solution. The relation between the peak current and the analyte concentration was linear over about 3 orders of magnitude with detection limits (S/N = 3) ranging from 1 to 2 microM for all analytes. The proposed method has been successfully applied for the determination of the paeoniflorin and sugars in real plant samples with satisfactory assay results.

Conductivity detection for conventional and miniaturised capillary electrophoresis systems

Since the introduction of capillary electrophoresis (CE), conductivity detection has been an attractive means of detection. No additional chemical properties are required for detection, and no loss in sensitivity is expected when miniaturising the detector to scale with narrow-bore capillaries or even to the microchip format. Integration of conductivity and CE, however, involves a challenging combination of engineering issues. In conductivity detection the resistance of the solution is most frequently measured in an alternating current (AC) circuit. The influence of capacitors both in series and in parallel with the solution resistance should be minimised during conductivity measurements. For contact conductivity measurements, the positioning and alignment of the detection electrodes is crucial. A contact conductivity detector for CE has been commercially available, but was withdrawn from the market. Microfabrication technology enables integration and precise alignment of electrodes, resulting in the popularity of conductivity detection in microfluidic devices. In contactless conductivity detection, the alignment of the electrodes with respect to the capillary is less crucial. Contactless conductivity detection (CCD) was introduced in capillary CE, and similar electronics have been applied for CCD using planar electrodes in microfluidic devices. A contactless conductivity detector for capillaries has been commercialised recently. In this review, different approaches towards conductivity detection in capillaries and chip-based CE are discussed. In contrast to previous reviews, the focus of the present review is on the technological developments and challenges in conductivity detection in CE.

Electrophoresis microchip fabricated by a direct-printing process with end-channel amperometric detection

We describe the development of an electrophoresis microchip fabricated by a direct-printing process, based on lamination of printed polyester films with end-channel amperometric detection. The channel structures are defined by polyester (base and cover) and by a toner layer (walls). The polyester-toner devices presented an electroosmotic flow (EOF) magnitude of approximately 10(-5) cm2 V(-1) s(-1), which is generated by a polymeric mixture of the toner and polyester composition. The microelectrodes used for detection were produced combining this laser-printer technology to compact discs. The performance of this device was evaluated by amperometric detection of iodide and ascorbate. The detection limits found were 500 nmol.L(-1) (135 amol) and 1.8 micromol.L(-1) (486 amol) for iodide and ascorbate, respectively.

Determination of hydroxyl radical by capillary zone electrophoresis with amperometric detection

Hydroxyl radical (OH*) can cause severe damage to cells and tissues. However, its analysis is very difficult for its high reactivity and very short half-life. In this paper, a simple and highly sensitive method, capillary zone electrophoresis with amperometric detection (CZE-AD) was introduced indirectly to determine OH* by determining its reaction products with salicylic acid (SAL), 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-dihydroxybenzoic acid (2,5-DHBA). The optimum conditions of CZE-AD for the determination of 2,3-DHBA and 2,5-DHBA were explored. Under the optimum conditions, SAL, 2,3-DHBA and 2,5-DHBA could be perfectly separated within 15 min, and the linearity ranges of 2,3-DHBA and 2,5-DHBA were between 1.0 x 10(-7) and 1.0 x 10(-4) mol l(-1). Their detection limits were as low as 2 x 10(-8) mol l(-1), which were much lower than that in CE-UV method. The method was also applied to study the free OH* scavenging activity of angelica polysaccharide. The experimental results showed that this CZE-AD method was very sensitive and practical in both the determination of free OH* and the evaluation of free OH* scavenging activities of antioxidants.

Capillary Electrophoresis Chips with a Sheath-Flow Supported Electrochemical Detection System

Microfabricated capillary electrophoresis chips containing an integrated sheath-flow electrochemical detector are developed with the goal of minimizing the influence of separation voltages on end-column detection while maintaining optimum performance. The microdevice consists of an upper glass wafer carrying the etched separation, injection, and sheath-flow channels and a lower glass wafer on which gold- and silver-plated electrodes have been fabricated. The sheath-flow channels join the end of the separation channel from each side, and gravity-driven flow carries the analytes to the electrochemical detector placed at working distances of 100, 150, 200, and 250 microm from the separation channel exit. The performance of this detector is evaluated using catechol and a detection limit of 4.1 microM obtained at a working distance of 250 microm. Detection of DNA restriction fragments and PCR product sizing is demonstrated using the electroactive intercalating dye, iron phenanthroline. Additionally, an allele-specific, PCR-based single-nucleotide polymorphism typing assay for the C282Y substitution diagnostic for hereditary hemochromatosis is developed and evaluated using ferrocene-labeled primers. This study advances the feasibility of high-speed, high-throughput chemical and genetic analysis using microchip electrochemical detection.

On-chip potential gradient detection with a portable capillary electrophoresis system

A portable chip-CE system with potential gradient detection (PGD) was developed and applied to the determinations of alkali metals and alkaloids. The separation efficiency appeared to be satisfactory and nonaqueous capillary electrophoresis (NACE) proved to be applicable to PGD or conductivity detection. The power supplies, separation and detection were built on a device of 3 kg in weight. A branch channel near the end of the separation channel was designed to perform PGD and make the application of relatively high field strength possible. The study is the first report on the application of PGD on the microchip platform. The design of the chip-CE system shows several advantages, such as simplicity, miniaturization and wide applicability.

Characterization of Electrode Fouling and Surface Regeneration for a Platinum Electrode on an Electrophoresis Microchip

A method of electrochemically cleaning noble metal electrodes is presented and characterized for electrophoresis microchips with electrochemical detection. First, the loss of sensitivity due to electrode fouling by serotonin is characterized as a function of injection number and analyte concentration. Signal attenuation is observed to be greater at high concentrations (100 microM) and negligible at very low concentrations (approximately 1 microM). Next, an electrochemical treatment procedure is optimized to yield sensitive and reproducible amperometric detection of the highly adsorptive compounds, serotonin and histamine. Thus, the performance of the electrode is reproducibly regenerated following as much as a approximately 99% reduction in surface activity. Utilizing the optimized three-level waveform, derived from that used for pulsed amperometric detection, detection limits as low as 78 nM and 17 microM have been obtained for serotonin and histamine, respectively. In the case of serotonin, this represents the lowest detection limit for a neurotransmitter by microchip electrophoresis with amperometric detection and the first report of amperometric detection of histamine detection at an unmodified platinum electrode. Repeated use of the electrode and application of electrochemical treatment did not appear to measurably affect the noise, longevity, metal adhesion, or physical appearance of the electrode.

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.