Capillary electrophoresis and microchip capillary electrophoresis with electrochemical and electrochemiluminescence detection

Recent advances and key strategies in capillary electrophoresis and microchip CE with electrochemical detection (ECD) and electrochemiluminescence (ECL) detection are reviewed. This article consists of four main parts: CE-ECD; microchip CE-ECD; CE-ECL; and microchip CE-ECL. It is expected that ECD and ECL will become powerful tools for CE microchip systems and will lead to the creation of truly disposable devices. The focus is on papers published in the last two years (from 2005 to 2006).     

Recent developments in electrochemical detection for microchip capillary electrophoresis

Significant progress in the development of miniaturized microfluidic systems has occurred since their inception over a decade ago. This is primarily due to the numerous advantages of microchip analysis, including the ability to analyze minute samples, speed of analysis, reduced cost and waste, and portability. This review focuses on recent developments in integrating electrochemical (EC) detection with microchip capillary electrophoresis (CE). These detection modes include amperometry, conductimetry, and potentiometry. EC detection is ideal for use with microchip CE systems because it can be easily miniaturized with no diminution in analytical performance. Advances in microchip format, electrode material and design, decoupling of the detector from the separation field, and integration of sample preparation, separation, and detection on-chip are discussed. Microchip CEEC applications for enzyme/immunoassays, clinical and environmental assays, as well as the detection of neurotransmitters are also described.     

Carbon nanotube and diamond as electrochemical detectors in microchip and conventional capillary electrophoresis

As two important polymorphs of carbon, carbon nanotube (CNT) and diamond have been widely employed as electrode materials for electrochemical sensing. This review focuses on recent advances and the key strategies in the fabrication and application of electrochemical detectors in microchip and conventional capillary electrophoresis (CE) using CNT and boron-doped diamond. The subjects covered include CNT-based electrochemical detectors in microchip CE, CNT-based electrochemical detectors in conventional CE, boron-doped diamond electrochemical detectors in microchip CE, and boron-doped diamond electrochemical detectors in conventional CE. The attractive properties of CNT and boron-doped diamond make them very promising materials for the electrochemical detection in microchip and conventional CE systems and other microfluidic analysis systems.     

Recent developments in amperometric detection for microchip capillary electrophoresis

The interest in microfluidic devices has increased considerably over the past decade due to the numerous advantages of working within a miniature, microfabricated format. This review focuses on recent advances in coupling amperometric detection with microchip capillary electrophoresis (CE). Advances in electrochemical cell design, isolation of the detector from the separation field, and integration of both pre- and postseparation reaction chambers are discussed. The use of microchip CE with amperometric detection for enzyme/immunoassays, clinical and environmental assays, and the determination of neurotransmitters is described.     

Fabrication of calcium fluoride capillary electrophoresis microdevices for on-chip infrared detection

In this paper, we demonstrate microfluidic capillary electrophoresis (CE) devices made in CaF2 , for optical detection in a broad spectral range. We have designed methods for micromachining and enclosing capillaries in CaF2. The utility of these microdevices has been shown through CE analysis of fluorescently labeled amino acids. We have also performed infrared spectroscopy for analyte identification in microfluidic CaF2 channels. These CaF2 microdevices open the door to microchip separations with optical detection in the ultraviolet, visible, and infrared spectral regions.     

Fabrication and performance of a three-dimensionally adjustable device for the amperometric detection of microchip capillary electrophoresis

A microchip CE-amperometric detection (AD) system has been fabricated by integrating a two-dimensionally adjustable CE microchip and an AD cell containing a one-dimensionally adjustable disk detection electrode in a Plexiglas holder. It facilitates the precise 3-D alignment between the channel outlet and the detection electrode without a complicated 3-D manipulator. The performance of this unique system was demonstrated by separating five aromatic amines (1,4-phenyldiamine, aniline, 2-methylaniline, 4-chloroaniline, and 1-naphthylamine) of environmental concern. Factors influencing their separation and detection processes were examined and optimized. The five analytes have been well separated within 140 s in a 74 cm long separation channel at a separation voltage of +2500 V using a 10 mM phosphate buffer (pH 3.5). Highly linear response is obtained for the five analytes over the range 20-200 microM with the detection limits ranging from 0.46 to 1.44 microM, respectively. The present system demonstrated long-term stability and reproducibility with RSDs of less than 5% for the peak current (n = 9). The new approach for the microchannel-electrode alignment should find a wide range of applications in CE, flowing injection analysis, and other microfluidic analysis systems.     

Analysis of thrombin‐antithrombin complex formation using microchip electrophoresis and mass spectrometry

Preterm birth (PTB) related health problems take over one million lives each year, and currently, no clinical analysis is available to determine if a fetus is at risk for PTB. Here, we describe the preparation of a key PTB risk biomarker, thrombin‐antithrombin (TAT), and characterize it using dot blots, MS, and microchip electrophoresis (µCE). The pH for fluorescently labeling TAT was also optimized using spectrofluorometry and spectrophotometry. The LOD of TAT was measured in µCE. Lastly, TAT was combined with six other PTB risk biomarkers and separated in µCE. The ability to make and characterize TAT is an important step toward the development of an integrated microfluidic diagnostic for PTB risk.     

Recent developments in electrokinetically driven analysis on microfabricated devices

This review is devoted to the rapid developments in the field of microfluidic separation devices in which the flow is electrokinetically driven, and where the separation element forms the heart of the system, in order to give an overview of the trends of the last three years. Examples of microchip layouts that were designed for various application areas are given. Optimization of mixing and injection strategies, designs for the handling of multiple samples, and capillary array systems show the enormous progress made since the first proof-of-concept papers about lab-on-a-chip devices. Examples of functional elements for on-chip preconcentration, filtering, DNA amplification and on-chip detection indicate that the real integration of various analytical tasks on a single microchip is coming into reach. The use of materials other than glass, such as poly(dimethylsiloxane) and polymethylmethacrylate, for chip fabrication and detection methods other than laser-induced fluorescence (LIF) detection, such as mass spectrometry and electrochemical detection, are described. Furthermore, it can be observed that the separation modes known from capillary electrophoresis (CE) in fused-silica capillaries can be easily transferred to the microchip platform. The review concludes with an overview of applications of microchip CE and with a brief outlook.     

Carbon-nanotube-alginate composite modified electrode fabricated by in situ gelation for capillary electrophoresis

This report describes the development and the application of a novel carbon-nanotube (CNT)-alginate composite modified electrode as a sensitive amperometric detector for capillary electrophoresis (CE). The composite electrode was fabricated on the basis of in situ gelation of a mixture of CNTs and sodium alginate on the surface of a carbon disc electrode in aqueous calcium chloride solution. SEM, energy-dispersive spectroscopy, XRD, and FTIR spectroscopy offered insights into the nature of the novel composite. The results indicated that the CNTs were well dispersed and embedded throughout the alginate matrix to form an interconnected carbon-nanotube network on the base electrode. The performance of this unique CNT-based detector has been demonstrated, in conjunction with CE, by separating and detecting five caffeic acid derivatives. The new CNT-based CE detector offered significantly lower operating potentials, substantially enhanced signal-to-noise characteristics, and a lower expense of operation. The simplicity and significant performance exhibited by the CNT-alginate composite modified electrode also indicate great promise for the use of this electrode in microchip CE, flowing-injection analysis, and other microfluidic analysis systems.     

Carbon nanotube/polystyrene composite electrode for microchip electrophoretic determination of rutin and quercetin in Flos Sophorae Immaturus

In this report, carbon nanotube/polystyrene (CNT/PS) composite electrodes have been fabricated as sensitive amperometric detectors of microchip capillary electrophoresis (CE) for the determination of rutin and quercetin in Flos Sophorae Immaturus. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of CNT and styrene in the microchannel of a piece of fused silica capillary under heat. The surface morphologies of the composite in the electrodes were observed by using a scanning electron microscope. The performance of this unique system has been demonstrated by separating and detecting rutin and quercetin. The new CNT-based CE detector offered significantly lower detection potentials, yielded substantially enhanced signal-to-noise characteristics, and exhibited resistance to surface fouling and hence enhanced stability. It demonstrated long-term stability and reproducibility with a relative standard deviation of less than 5% for the peak current (n = 20) and should also find a wide range of applications in conventional CE, flowing injection analysis, and other microfluidic analysis systems.     

Integrating amperometric detection with electrophoresis microchip devices for biochemical assays: recent developments

Recent advances in microfluidic systems, particularly in the Micro Total Analysis System (μTAS) or Lab On a Chip (LOC), drive the current analysis tools and equipment towards miniaturization, rapid at-line testing and mobility. The state-of-the-art microfluidic technology targets a wider range but smaller volumes of analytes, making the analytical procedure relatively easier and faster. This trend together with faster electronics and modern instrumentation systems will make real-time and in situ analysis a definite possibility. This review focuses on microchip capillary electrophoresis with amperometric detection (MCE-AD) for the detection of DNA and other electroactive analytes. The problems associated with the microchip design, in particular the choice of materials and the configuration of electrodes are discussed thoroughly and solutions are proposed. Significant developments in the related areas are also covered and reviewed critically.     

Carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode fabricated by in situ polymerization for microchip capillary electrophoresis

We describe the development and application of a novel carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode as a sensitive amperometric detector of microchip capillary electrophoresis (CE). The composite electrode was fabricated by the in situ polymerization of a mixture of CNTs and prepolymerized methyl methacrylate in the microchannel of a piece of fused silica capillary under heat. The performance of this unique system was demonstrated by the separation and detection of phenolic pollutants and purines. The new CNT-based CE detector offered significantly lower operating potentials, yielded substantially enhanced signal-to-noise characteristics, and exhibited resistance to surface fouling and, hence, enhanced stability. Long-term stability and reproducibility with relative standard deviations of less than 5 % for the peak current (n=20) were also demonstrated. The simplicity and significant performance exhibited by the CNT/PMMA composite electrode indicate great promise for conventional CE, flowing-injection analysis, and other microfluidic analysis systems.     

Carbon nanotube/poly(methyl methacrylate) composite electrode for capillary electrophoretic measurement of honokiol and magnolol in Cortex Magnoliae Officinalis

This paper describes the development and the application of a novel carbon nanotube/poly(methyl methacrylate) (CNT/PMMA) composite electrode as a sensitive amperometric detector of CE. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of CNT and prepolymerized methylmethacrylate in the microchannel of a piece of fused-silica capillary under heat. The performance of this unique system has been demonstrated by separating and detecting honokiol and magnolol in traditional Chinese medicine, Cortex Magnoliae Officinalis. Factors influencing their separation and detection processes were examined and optimized. Honokiol and magnolol were well separated within 7 min in a 40 cm long capillary at a separation voltage of 15 kV using a 50 mM borate buffer (pH 9.2). The new CNT-based CE detector offered significantly lower operating potentials, yielded substantially enhanced S/N characteristics, and exhibited resistance to surface fouling and hence enhanced stability. It demonstrated long-term stability and reproducibility with RSDs of less than 5% for the peak current (n = 9) and should also find a wide range of applications in microchip CE, flowing injection analysis, and other microfluidic analysis systems.     

Improved separation efficiency of neurotransmitters on a native printed capillary electrophoresis microchip simply by manipulating electroosmotic flow

Separation and determination of dopamine and epinephrine with end-channel electrochemical (EC) detection integrated on a native printed microchip capillary electrophoresis (CE) system was investigated. Factors influencing the separation and detection were investigated and optimized. Results show manipulating EOF, which can be easily achieved by adjusting buffer pH, is a simple and effective way to achieve the baseline separation of dopamine and epinephrine in native polymeric microchips. Without surface modification of microchannel, printed microchips with advantages of low cost and easy preparation can achieve high performance like other microfluidic devices.     

A three-dimensionally adjustable amperometric detector for microchip electrophoretic measurement of nitroaromatic pollutants

A microchip capillary electrophoresis (CE)–amperometric detection (AD) system has been fabricated by integrating a two-dimensionally adjustable CE microchip and an amperometric detection cell containing a one-dimensionally adjustable disc detection electrode in a Plexiglas holder. It facilitates the precise three-dimensional alignment between the channel outlet and the detection electrode without a complicated three-dimensional manipulator. The performance of this unique system was demonstrated by separating four nitroaromatic pollutants (nitrobenzene, 2,4-dinitrotoluene, 2,4,6-trinitrotoluene, and p-nitrobenzene). Factors influencing their separation and detection processes were examined and optimised. The four analytes have been well-separated within 120 s in a 75 cm long separation channel at a separation voltage of +2000 V using an electrophoretic separation medium containing 15 mM borax and 15 mM sodium dodecyl sulfate (pH 9.2). Highly linear response is obtained for the four analytes over the range of 0–5 ppm with the detection limits ranging from 12 to 52 ppb. The present system demonstrated long-term stability and reproducibility with relative standard deviations of less than 5% for the peak current (n = 9). The new approach for the microchannel–electrode alignment should find a wide range of applications in other microfluidic analysis systems.     

Acupuncture injection for field amplified sample stacking and glass microchip‐based capillary gel electrophoresis

Acupuncture sample injection is a simple method to deliver well‐defined nanoliter‐scale sample plugs in PDMS microfluidic channels. This acupuncture injection method in microchip CE has several advantages, including minimization of sample consumption, the capability of serial injections of different sample solutions into the same microchannel, and the capability of injecting sample plugs into any desired position of a microchannel. Herein, we demonstrate that the simple and cost‐effective acupuncture sample injection method can be used for PDMS microchip‐based field amplified sample stacking in the most simplified straight channel by applying a single potential. We achieved the increase in electropherogram signals for the case of sample stacking. Furthermore, we present that microchip CGE of ΦX174 DNA‐HaeⅢ digest can be performed with the acupuncture injection method on a glass microchip while minimizing sample loss and voltage control hardware.     

Electrochemical Detection for Capillary Electrophoresis Microchips: A Review

Electrochemistry detection offers considerable promise for capillary‐electrophoresis (CE) microchips, with features that include remarkable sensitivity, portability, independence of optical path length or sample turbidity, low cost and power requirements, and high compatibility with modern micromachining technologies. This article highlights key strategies in controlled‐potential electrochemical detectors for CE microchip systems, along with recent advances and directions. Subjects covered include the design of the electrochemical detection system, its requirements and operational principles, common electrode materials, isolation from the separation voltage, derivatization reactions, typical applications, and future prospects. It is expected that electrochemical detection will become a powerful tool for CE microchip systems and will lead to the creation of truly portable (and possibly disposable) devices.     

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.     

Integrated circuit microchip system with multiplex capillary electrophoresis module for DNA analysis

In this paper, we describe the use of an integrated circuit (IC) microchip system as a detector in multiplex capillary electrophoresis (CE). This combination of multiplex capillary gel electrophoresis and the IC microchip technology represents a novel approach to DNA analysis on the microchip platform. Separation of DNA ladders using a multiplex CE microsystem of four capillaries was monitored simultaneously using the IC microchip system. The IC microchip-CE system has advantages such as low cost, rapid analysis, compactness, and multiplex capability, and has great potential as an alternative system to conventional capillary array gel electrophoresis systems based on charge-coupled device (CCD) detection.