Monitoring environmental pollutants by microchip capillary electrophoresis with electrochemical detection

During the past decade, significant progress in the development of miniaturized microfluidic systems has occurred due to the numerous advantages of microchip analysis. This review focuses on recent advances and the key strategies in microchip capillary electrophoresis (CE) with electrochemical detection (ECD) for separating and detecting a variety of environmental pollutants. The subjects covered include the fabrication of microfluidic chips, ECD, typical applications of microchip CE with ECD in environmental analysis, and future prospects. It is expected that microchip CE-ECD will become a powerful tool in the environmental field and will lead to the creation of truly portable devices.     

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.     

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.     

Detection method for microchip separations

The features of analytical systems utilizing microfluidic devices, especially detection methods, are described. Electrochemical detection (EC), laser-induced fluorescence (LIF), mass spectrometry (MS), and chemical luminescence (CL) methods are covered. EC enables detection without labeling and has been used in recent years because of its low cost and sensitivity. LIF is the most generally used detection method in microchip separations. Use of LED as an excitation source for fluorescence measurement was also developed for the purpose of miniaturization of the entire system, including detection and separation. Although MS enables highly sensitive analysis, the interface between MS and micro channels is still under examination. This review with fifty-two references introduces interesting detection methods for microchip separations. Related separation methods using microfluidic devices are also discussed.     

Performance evaluation of a capillary electrophoresis electrochemical chip integrated with gold nanoelectrode ensemble working and decoupler electrodes

This paper presents a capillary electrophoresis poly(methyl methacrylate) (PMMA) based microchip for electrochemical detection applications featuring embedded gold nanoelectrode ensemble (GNEE) working and decoupler electrodes. In fabricating the microchip, the GNEE films are pressed directly onto the metallic electrode structures using a hot embossing technique, and the microfluidic channels are then sealed using a low-temperature azeotropic solvent bonding method. The detection performance of the microchip is evaluated using dopamine and catechol analytes for illustration purposes. The experimental results show that the GNEE working electrode provides a significantly higher signal response than that obtained from a bulk gold electrode when applied to the detection of dopamine analyte. Compared to a conventional bulk palladium decoupler electrode, the GNEE decoupler electrode reduces both the amplitude of the charge current (3.5 nA vs. 18.7 nA) and the baseline drift at higher separation voltages. The measured baseline current drift for the microchip equipped the proposed GNEE decoupler electrode is around three times smaller than the microchip with the palladium decoupler electrode under the applied separation electric field from 40 V/cm to 240 V/cm. Finally, when detecting a mixture of 1mM dopamine and 1mM catechol, the calculated signal response of the microchip with a GNEE decoupler electrode is approximately five times higher than that obtained from a microchip with a bulk Pd decoupler electrode, resulting in the detection limit of 1 microM for the proposed GNEE-based microchip device. Overall, the results indicate that the proposed capillary electrophoresis-electrochemical detection (CE-ED) microchip with embedded GNEE working and decoupler electrodes provides an ideal solution for sample detection in lab-on-a-chip and micro total analysis applications.     

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.     

Small-angle optical deflection from collinear configuration for sensitive detection in microfluidic systems

This paper describes a novel detection system based on small-angle optical deflection from the collinear configuration of a microfluidic chip. In this system, the incident light beam was focused on the microchannel through the edge of a lens, resulting in a small deflection angle that deviated 20° from the collinear configuration. The emitted fluorescence was collected through the center of the same lens and delivered to a photomultiplier tube in the vertical direction; the reflection light of the chip plate was kept away from the detector. In contrast to traditional confocal and nonconfocal laser-induced fluorescence detection systems, background levels resulting from scattered excitation light, reflection and refraction from the microchip was significantly eliminated. Significant enhancement of the signal-to-noise ratio was obtained by shaping a laser beam that combined an attenuator with a spectral filter to optimize laser power and the dimensions of the laser beam. FITC and FITC-labeled amino acid were used as model analytes to demonstrate the performance sensitivity, separation efficiency, and reproducibility of this detection system by using a hybrid polydimethylsiloxane/glass microfluidic device. The limit of detection of FITC was estimated to be 2 pM (0.55 zmol) (S/N = 3). Furthermore, the single cell analysis for the determination of intracellular glutathione in a single 3T3 mouse fibroblast cell was demonstrated. The results suggest that the proposed optical arrangements will be promising for development of sensitive, low-cost microfluidic systems.     

Food analysis on microchip electrophoresis: An updated review

From 2008 to date, basically, single-cross microchip electrophoresis (ME) design has been used for food analysis with electrochemicaland laser-induced fluorescencedetection being the most commonprinciples coupled. In the last 4 years, the main outlines were: (i) the exploration of new analytes such as heavy metals, nitrite, micotoxins, microorganisms, and allergens; (ii) the development of electrokinetic microfluidic (bio-) sensors into microchip format for the detection of toxins; and interestingly (iii) although sample preparation is still performed off-chip, an important increase in works dealing with complicated food samples has been clearly noticed. Although microchip technology based on electrokinetics is emerging from important fields such as authentication of foods, detection of frauds, toxics, and allergens; the marriage between micro- and nanotechnologies and total integration approaches has not reached the expected impact in the field but it is still a great promise for the development of ME of new generations for food analysis.     

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 advances in surface-enhanced Raman scattering detection technology for microfluidic chips

Microfluidic chip devices and their application to sensitive chemical and biological analyses have attracted significant attention over the past decade. The miniaturization of reaction systems offers practical advantages over conventional benchtop systems. In this case, however, a highly sensitive on-chip detection method is important for the monitoring of chemical reactions as well as for the detection of analytes inside the channel because the detection volume in a micrometer-size channel is extremely small. Recently, a surface-enhanced Raman scattering (SERS) technique is being regarded as a potential candidate for the highly sensitive detection of analytes in a microfluidic chip. This review provides a general survey and an in-depth look at recent developments in SERS techniques for the biological/environmental analysis of minute analytes in a microfluidic chip.     

Recent advances in microchip enantioseparation and analysis

This review summarizes recent developments (over the past decade) in the field of microfluidics-based solutions for enantiomeric separation and detection. The progress in various formats of microchip electrodriven separations, such as MCE, microchip electrochromatography, and multidimensional separation techniques, is discussed. Innovations covering chiral stationary phases, surface coatings, and modification strategies to improve resolution, as well as integration with detection systems, are reported. Finally, combinations with other microfluidic functional units are also presented and highlighted.     

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).     

Floating resistivity detector for microchip electrophoresis

A newly developed conductivity detector, the floating resistivity detector (FRD), for microchip electrophoresis was introduced in this work. The detector design permits decoupling of the detection circuit from the high separation voltage without compromising separation efficiency. This greatly simplifies the integration of microchip electrophoresis systems. Its method of detection relies on platinum electrodes being dipped in two buffer-filled branched detection probe reservoirs on the microchip device. In this way, analytes passing through the detection window will not pass through and subsequently adsorb onto the electrodes, alleviating problems of electrode fouling due to analyte contamination and surface reactions. A customized microchip design was proposed and optimized stepwise for the new FRD system. Each branched detection probe was determined to be 4.50 mm long with a 0.075 mm detection window gap between them. The distance between the detection window and buffer waste reservoir was determined to be 1.50 mm. The optimized microchip design was subsequently used in the analysis of four groups of analytes - inorganic cations, amino acids, aminoglycosides antibiotics, and biomarkers. Based on the preliminary results obtained, the detection limits were in the range of 0.4-0.7 mg/L for the inorganic cations and 1.5-15 mg/L for the amino compounds.     

Label-free fluorescence detection in capillary and microchip electrophoresis

Herein, we summarize the current status of native fluorescence detection in microchannel electrophoresis, with a strong focus on chip-based systems. Fluorescence detection is a powerful technique with unsurpassed sensitivity down to the single-molecule level. Accordingly fluorescence detection is attractive in combination with miniaturised separation techniques. A drawback is, however, the need to derivatize most analytes prior to analysis. This can often be circumvented by utilising excitation light in the UV spectral range in order to excite intrinsic fluorescence. As sensitive absorbance detection is challenging in chip-based systems, deep-UV fluorescence detection is currently one of the most general optical detection techniques in microchip electrophoresis, which is especially attractive for the detection of unlabelled proteins. This review gives an overview of research on native fluorescence detection in capillary (CE) and microchip electrophoresis (MCE) between 1998 and 2008. It discusses material aspects of native fluorescence detection and the instrumentation used, with particular focus on the detector design. Newer developments, featured techniques, and their prospects in the future are also included. In the last section, applications in bioanalysis, drug determination, and environmental analysis are reviewed with regard to limits of detection.     

Microfluidic chips for biological and medical research

Advantages of devices on a microchip platform are discussed in comparison with traditional systems. Stages and processes of creation of microfluidic chips are considered. The basic technologies of formation micro- and nanostructures on a substrate from various materials and techniques for microchip sealing are introduced. Special attention is given to microfluidic chips for separation and analysis of nucleic acids and proteins, as well as to microchips for PCR. Examples of integrated systems on the basis of microfluidic technique are considered. Data on the commercialization of devices based on microfluidic chips are presented.     

Unconventional detection methods for microfluidic devices

The direction of modern analytical techniques is to push for lower detection limits, improved selectivity and sensitivity, faster analysis time, higher throughput, and more inexpensive analysis systems with ever-decreasing sample volumes. These very ambitious goals are exacerbated by the need to reduce the overall size of the device and the instrumentation - the quest for functional micrototal analysis systems epitomizes this. Microfluidic devices fabricated in glass, and more recently, in a variety of polymers, brings us a step closer to being able to achieve these stringent goals and to realize the economical fabrication of sophisticated instrumentation. However, this places a significant burden on the detection systems associated with microchip-based analysis systems. There is a need for a universal detector that can efficiently detect sample analytes in real time and with minimal sample manipulation steps, such as lengthy labeling protocols. This review highlights the advances in uncommon or less frequently used detection methods associated with microfluidic devices. As a result, the three most common methods - LIF, electrochemical, and mass spectrometric techniques - are omitted in order to focus on the more esoteric detection methods reported in the literature over the last 2 years.     

Impact of reservoir potentials on the analyte behavior in microchip electrophoresis: computer simulation and experimental validation for DNA fragments

Fundamental understanding of the impact of reservoir potentials on the analyte behavior on the microfluidic chips is an important issue in microchip electrophoresis (MCE) for suitable injection and separation of analytes, since the applied potentials may significantly affect the shape of sample plug, sample leakage from the injection channel to the separation channel, injected sample amount, and separation efficiency. This study addressed this issue for the case of a conventional cross-geometry microchip with four reservoirs using computer simulations, the results of which were verified by the analysis of DNA fragments. For the microchip with a definite structure and migration distance, the injected sample amount was shown to be the vital parameter for improving the limit of detection and resolution. During injection, the shape of the sample plug could be adjusted by varying the reservoir potentials. It was demonstrated that a "magnified injection" (applying high voltage on the three reservoirs to the sample reservoir) is useful to enhance the detection sensitivity depending on the analyte composition, although such injection was previously avoided because of introducing too large amounts of the analyte in comparison with two established modes, floating and pinched injection. Optimal magnified injection was proved to improve the sensitivity for about 4 times over that of pinched injection for the analysis of DNA step ladders using microchip gel electrophoresis (MCGE). Sample leakage of DNA fragments could be suppressed by applying a high positive voltage on injection channel during separation, but the voltage degraded the injected amount and resolution.     

Two‐photon excitation in chip electrophoresis enabling label‐free fluorescence detection in non‐UV transparent full‐body polymer chips

One of the most commonly employed detection methods in microfluidic research is fluorescence detection, due to its ease of integration and excellent sensitivity. Many analytes though do not show luminescence when excited in the visible light spectrum, require suitable dyes. Deep‐ultraviolet (UV) excitation (<300 nm) allows label‐free detection of a broader range of analytes but also mandates the use of expensive fused silica glass, which is transparent to UV light. Herein, we report the first application of label‐free deep UV fluorescence detection in non‐UV transparent full‐body polymer microfluidic devices. This was achieved by means of two‐photon excitation in the visible range (λ_ex = 532 nm). Issues associated with the low optical transmittance of plastics in the UV range were successfully circumvented in this way. The technique was investigated by application to microchip electrophoresis of small aromatic compounds. Various polymers, such as poly(methyl methacrylate), cyclic olefin polymer, and copolymer as well as poly(dimethylsiloxane) were investigated and compared with respect to achievable LOD and ruggedness against photodamage. To demonstrate the applicability of the technique, the method was also applied to the determination of serotonin and tryptamine in fruit samples.     

Plastified poly(ethylene terephthalate) (PET)-toner microfluidic chip by direct-printing integrated with electrochemical detection for pharmaceutical analysis

Rapid separation and determination of acetaminophen and its hydrolysate with end-channel electrochemical (EC) detection integrated on a plastified poly(ethylene terephthalate) (PET)-toner microchip capillary electrophoresis (CE) system was investigated. In this separation and detection system, a Pt ultramicroelectrode integrated on a three-dimensional adjustor was used as working electrode. Factors influencing the separation and detection were investigated and optimized. Results show that acetaminophen and p-aminophenol can be well separated within 84 s with R.S.D. < 1% for migration time and R.S.D. < 3.6% for detection current for both analytes. Detection limits for both analytes are determined to be 5.0 μM (S/N = 3). This method has been successfully applied to the detection of trace p-aminophenol in paracetamol tablets. The results demonstrate that the PET-toner microchips can obtain better performance than PDMS microfluidic devices but at much lower cost.     

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.