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.     

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.     

Polystyrene/graphene composite electrode fabricated by in situ polymerization for capillary electrophoretic determination of bioactive constituents in Herba Houttuyniae

A novel graphene/polystyrene composite electrode was developed for the enhanced amperometric detection of CE in this work. The composite electrode was fabricated on the basis of the in situ polymerization of a mixture of graphene and prepolymerized styrene in the bore of a piece of fused‐silica capillary under heat. SEM, XRD, and FTIR offered insights into the nature of the composite. The results indicated that graphenes were well dispersed and embedded throughout the PS matrix to form an interconnected conducting network. The performance of this unique graphene‐based detector has been demonstrated by separating and detecting rutin, isoquercitrin, quercitrin, and chlorogenic acid in Herba Houttuyniae (a traditional Chinese medicine) in combination with CE. The prepared graphene‐based CE detector offered significantly lower detection potential, yielded enhanced signal‐to‐noise characteristics, and exhibited high resistance to surface fouling and enhanced stability. It showed long‐term stability and reproducibility with a relative standard deviation of 3.1% for the peak current (n=15).     

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.     

Carbon-nanotube/copper composite electrodes for capillary electrophoresis microchip detection of carbohydrates

The preparation of carbon nanotube (CNT)/copper composite electrodes, based on co-mixing CNT and Cu powders within mineral oil, is described. The new composite electrode is used for improved amperometric detection of carbohydrates following their capillary electrophoresis (CE) microchip separations. The CNT/Cu composite electrode detector displays enhanced sensitivity compared to detectors based on copper or CNT alone. The marked catalytic action of the CNT/Cu composite material permits effective low potential (+0.5 V vs. Ag/AgCl) amperometric detection, and is coupled to the renewability, bulk modification and versatility advantages of composite electrodes. The CNT/Cu composite surface also leads to a greater resistance to surface fouling compared to that observed at the copper electrode. Factors affecting the electrocatalytic activity and the CE microchip detection are examined and optimized. The CNT/Cu composite electrode is also shown to be useful for the detection of amino acids as indicated from preliminary results. While the present work has focused on the enhanced CE microchip detection of carbohydrates and amino acids, the CNT/metal-composite electrode route should benefit the detection of other important groups of analytes.     

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.     

Graphene/poly(ethylene‐co‐vinyl acetate) composite electrode fabricated by melt compounding for capillary electrophoretic determination of flavones in Cacumen platycladi

In this report, a graphene/poly(ethylene‐co‐vinyl acetate) composite electrode was fabricated by melt compounding for the amperometric detection of capillary electrophoresis. The composite electrode was fabricated by packing a mixture of graphene and melted poly(ethylene‐co‐vinyl acetate) in a piece of fused silica capillary under heat. The structure of the composite was investigated by scanning electron microscopy and Fourier transform infrared spectroscopy. The results indicated that graphene sheets were well dispersed in the composite to form an interconnected conducting network. The performance of this unique graphene‐based detector has been demonstrated by separating and detecting rutin, quercitrin, kaempferol, and quercetin in Cacumen platycladi in combination with capillary electrophoresis. The four flavones have been well separated within 9 min in a 50‐cm‐long capillary at a separation voltage of 12 kV using a 50 mM sodium borate buffer (pH 9.2). The graphene‐based detector offered significantly lower operating potentials, substantially enhanced signal‐to‐noise characteristics, lower expense of operation, high resistance to surface fouling, and enhanced stability. It showed long‐term stability and repeatability with relative standard deviations of <5% for the peak current (n = 15).     

Wall-jet conductivity detector for microchip capillary electrophoresis

A new end-column ‘hybrid’ contactless conductivity detector for microchip capillary electrophoresis (CE) was developed. It is based on a “hybrid” arrangement where the receiving electrode is insulated by a thin layer of insulator and placed in the bulk solution of the detection reservoir of the chip, whereas the emitting electrode is in contact with the solution eluted from the channel outlet in a wall-jet arrangement. The favorable features of the new detector including the high sensitivity and low noise, can be attributed to both the direct contact of the ‘emitting’ electrode with the analyte solution as well as to the insulation of the detection electrode from the high DC currents in the electrophoretic circuit. Such arrangement provides a 10-fold sensitivity enhancement compared to currently used on-column contactless conductivity CE microchip detector as well as low values of noise and easy operation. The new design of the wall-jet conductivity detector was tested for separation of explosive-related methylammonium, ammonium, and sodium cations. The new detector design reconsiders the wall-jet arrangement for microchip conductivity detection in scope of improved peak symmetry, simplified study of inter-electrode distance, isolation of the electrodes, position of the wall-jet electrode to the separation channel, baseline stability and low limits of detection.     

Applications of a versatile sol-gel derived renewable electrode for capillary electrophoresis

A new electrode has been developed and applied for amperometric detection in capillary electrophoresis (CE), comprised of carbon sol-gel composite material. The versatility of the sol-gel technique permits the flexible configuration of the electrode. The performance of such a sol-gel carbon composite electrode (CCE) is first evaluated in a typical CE application for the detection of purine-based compounds. Application of the CCE is also demonstrated for the detection of phenolic compounds in a micellar system. Separation resolution for non-ionic phenolic compounds can significantly be enhanced by introducing sodium dodecyl sulfate (SDS) at a concentration above its critical micelle concentration (cmc) to the buffer. Another design of the CCE incorporating the electrocatalyst Cu2O is employed for the analysis of sugars and organic acids based on dynamic modification with cetyltrimethylammonium bromide (CTAB). It has been found that the presence of surfactant in the separation buffer does not adversely influence the electrochemical detection using a sol-gel derived carbon electrode.     

Applications of a versatile sol-gel derived renewable electrode for capillary electrophoresis

A new electrode has been developed and applied for amperometric detection in capillary electrophoresis (CE), comprised of carbon sol-gel composite material. The versatility of the sol-gel technique permits the flexible configuration of the electrode. The performance of such a sol-gel carbon composite electrode (CCE) is first evaluated in a typical CE application for the detection of purine-based compounds. Application of the CCE is also demonstrated for the detection of phenolic compounds in a micellar system. Separation resolution for non-ionic phenolic compounds can significantly be enhanced by introducing sodium dodecyl sulfate (SDS) at a concentration above its critical micelle concentration (cmc) to the buffer. Another design of the CCE incorporating the electrocatalyst Cu₂O is employed for the analysis of sugars and organic acids based on dynamic modification with cetyltrimethylammonium bromide (CTAB). It has been found that the presence of surfactant in the separation buffer does not adversely influence the electrochemical detection using a sol-gel derived carbon electrode.     

Carbon paste-based electrochemical detectors for microchip capillary electrophoresis/electrochemistry

The first reported use of a carbon paste electrochemical detector for microchip capillary electrophoresis (CE) is described. Poly(dimethylsiloxane) (PDMS)-based microchip CE devices were constructed by reversibly sealing a PDMS layer containing separation and injection channels to a separate PDMS layer that contained carbon paste working electrodes. End-channel amperometric detection with a single electrode was used to detect amino acids derivatized with naphthalene dicarboxaldehyde. Two electrodes were placed in series for dual electrode detection. This approach was demonstrated for the detection of copper(II) peptide complexes. A major advantage of carbon paste is that catalysts can be easily incorporated into the electrode. Carbon paste that was chemically modified with cobalt phthalocyanine was used for the detection of thiols following a CE separation. These devices illustrate the potential for an easily constructed microchip CE system with a carbon-based detector that exhibits adjustable selectivity.     

Amperometric detection of cytochrome c by capillary electrophoresis at a sol–gel carbon composite electrode

Capillary electrophoresis (CE) was employed for the determination of cytochrome c using a wall-jet amperometric detector consisting a copper(I) oxide-modified sol–gel carbon composite electrode (CCE), which exhibits a sensitive electrocatalytic response for the oxidation of cytochrome c. The optimum conditions of separation and detection are 0.08 M NaOH for the separation solution, 12 kV for separation voltage and +0.60 V versus saturated calomel electrode (SCE) for the detection potential. Calibration was linear over the concentration range 1–600 μM with the limit of detection of 3.4 μM, based on a signal-to-noise ratio (S/N) of 3.     

An electrochemical detector cell for open tubular liquid chromatography and capillary electrophoresis

Summary An electrochemical detector cell has been developed for micro-flow separation systems (OTLC, CE). The cell contains two electrodes, a disk-shaped working electrode made from a carbon fiber bundle, and a tubular Ag/AgCl quasi-reference electrode. The effective cell volume and the coulometric yield have been determined, for different electrode diameters and at different flow rates, in an OTLC system. An effective cell volume of less than 1 nl was observed. The applicability of the cell was demonstrated with the detection of OPA-derivatized amino acids. For use in CE, the cell was equipped with an additional compartment, housing a semi-permeable joint for the decoupling of the high electric field used for the electrophoretic separation. Results are shown on the determination of catecholamines by CE with electrochemical detection. Detection limits with both OTLC and CE were well below 1 fmole.     

Fabrication of a carbon nanotube-polyurethane composite electrode by in situ polyaddition for use in amperometric detection in capillary electrophoresis

The article describes the preparation of an electrode for amperometric detection in capillary electrophoresis (CE). It consists of a copper wire that was coated with a composite consisting of carbon nanotubes and polyurethane that was fabricated by in-situ polyaddition from a mixture of polyurethane prepolymer, curing agent, and carbon nanotubes (CNTs) inside a fused silica capillary. The structure of the composite was characterized by scanning electron microscopy, X-ray diffraction, thermogravimetry and FT-IR. The results indicated that CNTs were well dispersed and embedded throughout the composite to form an interconnected conducting network. The performance and advantages of the detection electrode are demonstrated by the separation and detection of standard mixtures of the hesperidin, hesperetin, naringenin and naringin by CE. The four flavanones are well separated within 12 min in a 40 cm long capillary at a separation voltage of 12 kV using a 50 mM sodium borate buffer (pH 9.2). The CNT-based electrode offers lower detection potential (0.8 V), enhanced detection limits (0.22–0.31 μM), lower costs of operation, high resistance to surface fouling, and improved stability. It shows long-term stability and repeatability, and relative standard deviations are <5 % for the peak current (for n = 15). The method was applied to the determination of flavanone in the peels of citrus fruits.

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Graphical abstract A copper electrode modified with a composite consisting of carbon nanotubes and polyurethane for amperometric detection in capillary electrophoresis

     

Facile preparation of carbon nanotube/poly(ethyl 2-cyanoacrylate) composite electrode by water-vapor-initiated polymerization for enhanced amperometric detection

A carbon nanotube/poly(ethyl 2-cyanoacrylate) (CNT/PECA) composite electrode was developed for enhanced amperometric detection. The composite electrode was fabricated on the basis of water-vapor-initiated polymerization of a mixture of CNTs and ethyl 2-cyanoacrylate in the bore of a piece of fused silica capillary. The morphology and structure of the composite were investigated by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. The results indicate that the CNTs were well dispersed and embedded throughout the PECA matrix to form an interconnected CNT network. The analytical performance of this unique CNT-based detector has been demonstrated by separating and detecting six flavones in combination with capillary electrophoresis. The advantages of the CNT/PECA composite detector include lower operating potential, higher sensitivity, low expense of fabrication, satisfactory resistance to surface fouling, and enhanced stability; these properties indicate great promise for a wide range of applications.     

Miniaturized capillary electrophoresis system with a carbon nanotube microelectrode for rapid separation and detection of thiols

Multi-walled carbon nanotube (CNT) was mixed with epoxy to fabricate microdisc electrode used as a detector for a specially designed miniaturized capillary electrophoresis (CE)-amperometric detection system for the separation and detection of several bioactive thiols. The end-channel CNT amperometric detector offers favourable signal-to-noise characteristics at a relatively low potential (0.8 V) for detecting thiol compounds. Factors influencing the separation and detection processes were examined and optimized. Four thiols (homocysteine, cysteine, glutathione, and N-acetylcysteine) have been separated within 130 s at a separation voltage of 2000 V using a 20 mM phosphate running buffer (pH 7.8). Highly linear response is obtained for homocysteine, cysteine, glutathione, and N-acetylcysteine over the range of 5-50 μM with detection limits of 0.75, 0.8, 2.9, and 3.3 μM, respectively. Good stability and reproducibility (R.S.D. < 5%) are obtained reflecting the minimal adsorption of thiols at the CNT electrode surface. The new microchip protocol should find a wide range of bioanalytical applications involving assays of thiol compounds.     

Microchip capillary electrophoresis with a boron-doped diamond electrochemical detector for analysis of aromatic amines

The attractive features of a boron-doped diamond (BDD) thin-film detector for microchip capillary electrophoretic (CE) separations of dye-related amino-substituted aromatic compounds are described. The diamond electrode was employed in the end-column amperometric detection of 4-aminophenol (4-AP), 1,2-phenylenediamine (1,2-PDA), 2-aminonaphthalene (2-AN), 2-chloroaniline (2-CA), and o-aminobenzoic acid (o-ABA), and its attractive behavior was compared to commonly used screen-printed carbon and glassy-carbon electrodes. These conventional electrode materials exhibit a significant degree of passivation and low sensitivity to the above-mentioned environmental pollutants. The diamond-based electrochemical detection system displayed a favorable analytical performance, including lower noise levels, higher peak resolution with enhanced sensitivity, and improved resistance against electrode passivation. Factors influencing the on-chip analysis were assessed and optimized. The diamond detector displayed detection limits of 2.0 and 1.3 microM for 4-AP and 2-AN, respectively, and a wide linear response for these compounds over the 2-50 microM range. The enhanced stability was demonstrated by relative standard deviation (RSD) values of 1.4% and 4.7% for 100 microM 1,2-PDA and 200 microM 2-CA, respectively, for repetitive detections (n = 7). Besides, the simultaneously observed current decrease was 2.4 and 9.1% for 1,2-PDA and 2-CA, respectively (compared to 21.8 and 41.0% at the screen-printed carbon electrode and 28.3 and 34.1% at the glassy carbon electrode, respectively). The favorable properties of the diamond electrode indicate great promise for environmental applications in CE and other microchip devices.     

Microchip capillary electrophoresis with a boron-doped diamond electrode for rapid separation and detection of purines

Microchip capillary electrophoresis (CE) coupled with a boron-doped diamond (BDD) electrode has been employed for the separation and detection of several purines and purine-containing compounds. The BDD end-channel amperometric detector offers favorable signal-to-noise (S/N) characteristics at the high detection potential (+ 1.3 V) essential for detecting purine-related compounds. Factors influencing the separation and detection processes were examined and optimized. Five purines (guanine, hypoxanthine, guanosine, xanthine, and uric acid) have been separated within 6 min at a separation voltage of 1000V using a borate/phosphate run buffer (pH 8.2). Linear calibration plots are observed for micromolar concentrations of the purine compounds. Good stability and reproducibility (R.S.D. < 5%) are obtained reflecting the minimal adsorption of purines at the BDD surface. Applicability for the detection of nucleosides, nucleotides, and oligonucleotides is illustrated. The new microchip protocol offers great promise for a wide range of bioanalytical applications involving assays of purines and purine-containing compounds.     

Amperometric detector designs for capillary electrophoresis microchips

Electrochemical (EC) detection is a sensitive and miniaturisable detection mode for capillary electrophoresis (CE) microchips. Detection cell design is very important in order to ensure electrical isolation from the high separation voltage. Amperometric detectors with different designs have been developed for coupling EC detection to CE-microchips. Different working electrode alignment: in-channel or end-channel has been tested in conjunction with several materials: gold, platinum or carbon. The end-channel detector was based on a platinum or gold wire manually aligned at the exit of the separation channel. Thick- (screen-printed carbon electrode) and thin-film (sputtered gold film) electrodes have also been employed with this configuration, but with a different design that allowed the rapid replacement of the electrode. The in-channel detector was based on a gold film within the separation channel. A gold-based dual electrode detector, which combined for the first time in- and end-channel detection, has been also tested. These amperometric detectors have been evaluated in combination to poly(methylmethacrylate) (PMMA) and Topas (thermoplastic olefin polymer of amorphous structure) CE-microchips. Topas is a new and promising cyclic olefin copolymer with high chemical resistance. Relevant parameters of the polymer microchip separation such as precision, efficiency or resolution and amperometric detection were studied with the different detector designs using p-aminophenol and L-ascorbic acid as model analytes in Tris-based buffer pH 9.0.     

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.