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.     

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.     

Use of a Carbon-ink Microelectrode Array for Signal Enhancement in Microchip Electrophoresis with Electrochemical Detection

In this communication, we demonstrate that a carbon ink microelectrode array, where the electrodes are held at the same potential, affords significant signal enhancement in microchip electrophoresis with amperometric detection. The ability to fabricate an array of carbon ink microelectrodes with a palladium decoupler was demonstrated and the resulting electrodes were integrated with a valving microchip design. The use of an 8 electrode array led to a significant improvement in the limits of detection at the expense of separation resolution due to the increased detection zone size. It is also shown that microdialysis sampling can be integrated with the microchip device and a multi-analyte separation achieved.     

Nanoband electrode for high-performance in-channel amperometric detection in dual-channel microchip capillary electrophoresis

A nanoband electrode detector integrated with a dual-channel polydimethylsiloxane microchip is proposed for in-channel amperometric detection in microchip capillary electrophoresis. Gold nanoband electrodes, which were fabricated on SU-8 substrates with a 100-nm-width gold layer, were introduced into the dual-channel microchip to be an electrochemical detector. Due to the nano-sized width of the detector, the noise of the amperometric detection was significantly reduced, and a high separation resolution was achieved for monitoring the analytes. The detection sensitivity of the system was improved by high signal-to-noise ratio, and a low detection limit on microchip was obtained for p-aminophenol (2.09 nM). Because of the high resolution in measuring half-peak width, the plate number that is used to evaluate the separation efficiency was 1.5-fold higher than that using 50-μm-width electrochemical detector. The effect of sample injection time and data acquisition time on separation efficiency was investigated, and an attractive separation efficiency was achieved with a plate number up to 17,500.     

Determination of chloride, chlorate and perchlorate by PDMS microchip electrophoresis with indirect amperometric detection

In this work, chloride, chlorate and perchlorate are fast separated on PDMS microchip and detected via in-channel indirect amperometric detection mode. With PDMS/PDMS microchip treated by oxygen plasma, anions chloride (Cl-), chlorate (ClO3-), and perchlorate (ClO4-) are separated within 35s. Some parameters including buffer salt concentration, buffer pH, separation voltage and detection potential are investigated in detail. The separation conditions using 15 mM (pH 6.12) of 2-(N-morpholino)ethanesulfonic acid (MES)+L-histidine (L-His) as running buffer, -2000 V as separation voltage and 0.7 V as detection potential are optimized. Under this condition, the detection limits of Cl-, ClO3-, and ClO4- are 1.9, 3.6, and 2.8 microM, respectively.     

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.     

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.     

Fast and simultaneous detection of heavy metals using a simple and reliable microchip-electrochemistry route: An alternative approach to food analysis

This paper reports, for the first, the fast and simultaneous detection of prominent heavy metals, including: lead, cadmium and copper using microchip CE with electrochemical detection. The direct amperometric detection mode for microchip CE was successfully applied to these heavy metal ions. The influences of separation voltage, detection potential, as well as the concentration and pH value of the running buffer on the response of the detector were carefully assayed and optimized. The results clearly show that reliable analysis for lead, cadmium, and copper by the degree of electrophoretic separation occurs in less than 3min using a MES buffer (pH 7.0, 25mM) and l-histidine, with 1.2kV separation voltage and -0.8V detection potential. The detection limits for Pb(2+), Cd(2+), and Cu(2+) were 1.74, 0.73 and 0.13microM (S/N=3). The %R.S.D. of each peak current was <6% and migration times <2% for prolonged operation. To demonstrate the potential and future role of microchip CE, analytical possibilities and a new route in the raw sample analysis were presented. The results obtained allow the proposed microchip CE-ED acts as an alternative approach for metal analysis in foods.     

带有可更换半自准工作电极的集成化毛细管电泳安培检测芯片

自Woolley等首次报道集成于玻璃芯片上的微型毛细管电泳-安培检测(Chip-based capillary electrophoresis with amperometric detection,CE-AD)系统以来,CE-AD以其高效、高速、高灵敏度以及易微型化集成化等特点引起研究者的关注．在芯片上实现柱端安培检测可用直接制作在芯片上的喷(镀)膜工作电极,或采用外置的壁喷式电极。前者集成化程度高,后者的工作电极可以更换,大大提高了芯片的重复利用率。     

Capillary electrophoresis microchip coupled with on-line chemiluminescence detection

In the present work, chemiluminescence detection was integrated with capillary electrophoresis microchip. The microchip was designed on the principle of flow-injection chemiluminescence system and capillary electrophoresis. It has three main channels, five reservoirs and a detection cell. As model samples, dopamine and catechol were separated and detected using a permanganate chemiluminescent system on the prepared microchip. The samples were electrokinetically injected into the double-T cross section, separated in the separation channel, and then oxidized by chemiluminescent reagent delivered by a home-made micropump to produce light in the detection cell. The electroosmotic flow could be smoothly coupled with the micropump flow. The detection limits for dopamine and catechol were 20.0 and 10.0 μM, respectively. Successful separation and detection of dopamine and catechol demonstrated the distinct advantages of integration of chemiluminescent detection on a microchip for rapid and sensitive analysis.     

Design of separation length and electric field strength for high-speed DNA electrophoresis

Gel-based DNA separation on microchip will play an important role in future genomic analysis due to its potential for high-efficiency and high-speed. Optimal design of microchip and separation condition is essential to take full advantage of high-speed separation on microchip. Separation length L and electric field strength E, which are crucial for design of microchip system, are focused on in this paper. Simultaneous optimization of L and E was carried out to achieve the most rapid separation. It was shown that the condition of L and E and the shortest separation time is closely related to the shape of resolution Rs surface in a three-dimensional space with axes E, L, and Rs. This surface was investigated, taking sample injection, detector, diffusion, and Joule heating into account. Thermal gradient broadening due to Joule heating helps to produce camber or ridge shape of Rs surface, which is essential for the shortest separation length and separation time. Sample plug length and detection volume should be more carefully controlled in microchip. The property of diffusion coefficient was shown to play a key role in determining Rs surface.     

In-channel amperometric detection for microchip electrophoresis using a wireless isolated potentiostat

The combination of microchip electrophoresis with amperometric detection leads to a number of analytical challenges that are associated with isolating the detector from the high voltages used for the separation. While methods such as end-channel alignment and the use of decouplers have been employed, they have limitations. A less common method has been to utilize an electrically isolated potentiostat. This approach allows placement of the working electrode directly in the separation channel without using a decoupler. This paper explores the use of microchip electrophoresis and electrochemical detection with an electrically isolated potentiostat for the separation and in-channel detection of several biologically important anions. The separation employed negative polarity voltages and tetradecyltrimethylammonium bromide (as a buffer modifier) for the separation of nitrite (NO??), glutathione, ascorbic acid, and tyrosine. A half-wave potential shift of approximately negative 500 mV was observed for NO?? and H?O? standards in the in-channel configuration compared to end-channel. Higher separation efficiencies were observed for both NO?? and H?O? with the in-channel detection configuration. The limits of detection were approximately two-fold lower and the sensitivity was approximately two-fold higher for in-channel detection of nitrite when compared to end-channel. The application of this microfluidic device for the separation and detection of biomarkers related to oxidative stress is described.     

Determination of cationic neurotransmitters and metabolites in brain homogenates by microchip electrophoresis and carbon nanotube-modified amperometry

An electrophoretic method for simultaneous determination of catecholamines and their O-methoxylated metabolites on the microchip as well as in the capillary is presented. A complex separation system employing sodium dodecyl sulfate (SDS) micelles, dendrimers forming a second pseudostationary phase and borate complexation is needed for the satisfactory separation of the selected compounds on the short migration length. A carbon nanotube-modified working electrode has been applied for the sensitive amperometric detection with submicromolar detection limits. The applicability of this new method for the analytics of real samples is demonstrated by analysis of mouse brain homogenate on the microchip and human urine by capillary electrophoresis.     

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.     

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.     

Microchip Capillary Electrophoresis with a Single-Wall Carbon Nanotube/Gold Electrochemical Detector for Determination of Aminophenols and Neurotransmitters

A new SWCNT modified gold detector for microchip capillary electrophoresis–electrochemistry is described. SWCNT modified gold electrode displays greatly improved sensitivity and separation resolution compared to bare gold electrode, reflecting the electrocatalytic activity of SWCNT. The SWCNT/Au electrode exhibits low background noise levels. Parameters such as separation voltage and detection potential of the microchip electrophoresis–electrochemistry with SWCNT modified gold electrode were optimized.     

Microchannel-electrode alignment and separation parameters comparison in microchip capillary electrophoresis by scanning electrochemical microscopy

The end of separation channel in a microchip was electrochemically mapped using the feedback imaging mode of scanning electrochemical microscopy (SECM). This method provides a convenient way for microchannel-electrode alignment in microchip capillary electrophoresis. Influence of electrode-to-channel positions on separation parameters in this capillary electrophoresis-electrochemical detection (CE-ED) was then investigated. For the trapezoid shaped microchannel, detection in the central area resulted in the best apparent separation efficiency and peak shape. In the electrode-to-channel distance ranging from 65 to 15mum, the limiting peak currents of dopamine increased with the decrease of the detection distance due to the limited diffusion and convection of the sample band. Results showed that radial position and axial distance of the detection electrode to microchannel was important for the improvement of separation parameters in CE amperometric detection.     

Microchip capillary electrophoresis with amperometric detection for rapid separation and detection of phenolic acids

A microchip capillary-electrophoresis protocol for rapid and effective measurements of food-related phenolic acids (including chlorogenic, gentisic, ferulic, and vanillic acids) is described. Relevant parameters of the chip separation and amperometric detection are examined and optimized. Under optimum conditions, the analytes could be separated and detected in a 15 mM borate buffer (pH 9.5, with 10% of methanol) within 300 s using a separation voltage of 2000 V and a detection voltage of +1.0 V. Linear calibration plots are observed for micromolar concentrations of the phenolic acid compounds. The negligible sample volumes used in the microchip procedure obviates surface fouling common to amperometric measurements of phenolic compounds. The new microchip protocol offers great promise for a wide range of food applications requiring fast measurements and negligible sample consumption. An application on a commercial red wine was performed with minimal sample preparation and promising results.     

A microfabricated micropillar liquid chromatographic chip monolithically integrated with an electrospray ionization tip

We present the first monolithically integrated silicon/glass liquid chromatography-electrospray ionization microchip for mass spectrometry. The microchip is fabricated by bonding a silicon wafer, which has deep reactive ion etched micropillar-filled channels, together with a glass lid. Both the silicon channel and the glass lid have a through-wafer etched sharp tip that produces a stable electrospray. The microchip is also compatible with laser induced fluorescence (LIF) detection, due to the glass lid. Separation of drugs in less than 5 minutes using either SiO(2) (normal phase) or C(18) coated (reversed-phase) pillars with good sensitivity was demonstrated with mass spectrometric detection as well as separation of fluorescent compounds with LIF detection.     

Rapid separation of antimicrobial metabolites by microchip electrophoresis with UV linear imaging detection

This research examines microchip electrophoresis with linear imaging UV detection for the analysis of antimicrobial metabolites, monoacetylphloroglucinol (MAPG) and 2,4-diacetylphloroglucinol (2,4-DAPG) from Pseudomonas fluorescens F113. Initial results show the separation of MAPG, 2,4-DAPG and resorcinol in less than 20 s. This was achieved using a quartz microchip with a separation channel length of 25 mm. In order to quantitate the amount of MAPG and 2,4-DAPG in a microbial cultured supernatant sample, on-chip sample introduction in a methanol/buffer matrix was investigated. Sample introduction/injection parameters were optimized to improve sensitivity and thus decrease the limit of detection (LOD). The amount of antimicrobial metabolites present was quantitated with a separation time of 15 s. A previously developed capillary electrophoretic method was compared to the microchip method in relation to speed, efficiency, precision, linear range and limit of detection. This investigation shows the fastest separation so far of these antimicrobial metabolites with high efficiency.