Adamantyl-functionalized polymer monolith for capillary electrochromatography

An adamantyl (ADM)-functionalized monolithic stationary phase was newly synthesized by a single-step copolymerization of 1-adamantyl-(α-trifluoromethyl) acrylate, ethylene dimethacrylate, and 2-acrylamido-2-methyl-1-propanesulfonic acid in order to prevent the peak tailing of basic solutes in capillary electrochromatography and was compared with butyl methacrylate (BMA)-based one. The ADM structure shields the negatively charged groups on the surface of monolith from basic solutes, resulting in better peak shapes than BMA-based monolithic stationary phase. As the monomers ratio decreased, the monolithic column had lower retention and higher column efficiency which was likely due to lower phase ratio and smaller globule size of monolith, respectively. The ADM-functionalized monolithic columns exhibited a good repeatability and reproducibility of column preparation with relative standard deviation values below 9% in the studied chromatographic parameters.     

Voltage control for microchip capillary electrophoresis analyses

This paper presents an inexpensive and easy-to-implement voltage sequencer instrument for use in microchip capillary electrophoresis (MCE) actuation. The voltage sequencer instrument takes a 0–5 V input signal from a microcontroller and produces a reciprocally proportional voltage signal with the capability to achieve the voltages required for MCE actuation. The unit developed in this work features four independent voltage channels, measures 105 × 143 × 45 mm (width × length × height), and the cost to assemble is under 60 USD. The system is controlled by a peripheral interface controller and commands are given via universal serial bus connection to a personal computer running a command line graphical user interface. The performance of the voltage sequencer is demonstrated by its integration with a fluorescence spectroscopy MCE sensor using pinched sample injection and electrophoretic separation to detect ciprofloxacin in samples of milk. This application is chosen as it is particularly important for the dairy industry, where fines and health concerns are associated with the shipping of antibiotic-contaminated milk. The voltage sequencer instrument presented represents an effective low-cost instrumentation method for conducting MCE, thereby making these experiments accessible and affordable for use in industries such as the dairy industry.     

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.     

Carbon nanotubes in capillary electrophoresis, capillary electrochromatography and microchip electrophoresis

Carbon nanotubes are among the plethora of novel nanostructures developed since the 1980s. Nanotubes have attracted considerable interest by the scientific community thanks to their extraordinary physical and chemical properties. Research areas have flourished in recent years and now include the nano-electronic, (bio)sensor and analytical field along with many others. This review covers applications of carbon nanotubes in capillary electrophoresis, capillary electrochromatography and microchip electrophoresis. First, carbon nanotubes and a range of electrophoretic techniques are briefly introduced and key references are mentioned. Next, a comprehensive survey of achievements in the field is presented and critically assessed. The merits and downsides of carbon nanotube addition to the various capillary electrophoretic modes are addressed. The different schemes for fabricating electrochromatographic stationary phases based on carbon nanotubes are discussed. Finally, some future perspectives are offered.      

An end-channel amperometric detector for microchip capillary electrophoresis

An end-channel amperometric detector with a guide tube for working electrode was designed and integrated on a home-made glass microchip. The guide tube was directly patterned and fabricated at the end of the detection reservoir, which made the fixation and alignment of working electrode relatively easy. The fabrication was carried out in a two-step etching process. A 30 μm carbon fiber microdisk electrode and Pt cathode were also integrated onto the amperometric detector. The characteristics and primary performance of the home-made microchip capillary electrophoresis (MCCE) were investigated with neurotransmitters. The baseline separation of dopamine (DA), catechol (CA) and epinephrine (EP) was achieved within 80 s. Separation parameters such as injection time, buffer components, pH of the buffer were studied. Relative standard deviations of not more than 6.0% were obtained for both peak currents and migration times. Under the selected separation conditions, the response for DA was linear from 5 to 200 μM and from 20 to 800 μM for CA. The limits of detection of DA and CA were 0.51 and 2.9 μM, respectively (S/N=3).     

Applications of capillary electrophoresis on microchip

CE on microchip is an emerging separation technique that has attracted wide attention and gained considerable popularity. Because of miniaturization of the separation format, CE on chip typically offers shorter analysis time and lower reagent consumption with potential development of portable analytical instrumentation. This review with 143 references is focused on proteins and peptides analysis, DNA separation including fragment sizing, genotyping, mutation detection and sequencing, and also the analysis of low-molecular-weight compounds, namely explosive residues and warfare agents, pharmaceuticals and drugs of abuse, and various small molecules in body fluids.     

Affinity capillary electrophoresis on microchips

The present study shows that the application of the method of affinity capillary electrophoresis (ACE) to investigate interactions between ligands and their substrates can be realized on microchips. With ACE it is possible to characterize non-covalent molecular interactions (complexation and partition equilibria). Binding constants (K(B)) provide a measured value of the affinity of a ligand molecule to a substrate, which is basic information for the understanding of hormones, drugs and their targets, e.g. receptors in the human body. A microchip electrophoresis instrument equipped with a UV-detector and a home-built chip-station with electrochemical detection were used. ACE could be achieved with model solutions of neurotransmitters using sulfated beta-cyclodextrin (sCD) as substrate in a background buffer. This paper describes the advantages of microchip-ACE (MC-ACE) to traditional affinity capillary electrophoresis on a capillary. The results show that MC-ACE has great potential as a tool for fast scanning of interactions and to calculate binding constants of ligands with their substrates.     

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.     

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.     

Analysis of double-stranded DNA by microchip capillary electrophoresis using polymer solutions containing gold nanoparticles

The impact of gold nanoparticles (GNPs) on the microchip electrophoretic separation of double-stranded (ds) DNA using poly(ethylene oxide) (PEO) is described. Coating of the 75-microm separation channel on a poly(methyl methacrylate) (PMMA) plate in sequence with poly(vinyl pyrrolidone), PEO, and 13-nm GNPs is effective to improve reproducibility and resolution. In this study, we have also found that adding 13-nm GNPs to 1.5% PEO is extremely important to achieve high resolution and reproducibility for DNA separation. In terms of the stability of the GNPs, 100 mM glycine-citrate buffer at pH 9.2 is a good buffer system for preparing 1.5% PEO. The separation of DNA markers V and VI ranging in size from 8 to 2176 base pairs has been demonstrated using the three-layer-coated PMMA microdevice filled with 1.5% PEO containing the GNPs. Using these conditions, the analysis of the polymerase chain reaction products of UGT1A7 was complete in 7 min, with the relative standard deviation values of the peak heights and migration times less than 2.3% and 2.0%, respectively. In conjunction with stepwise changes of the concentrations of ethidium bromide (0.5 and 5 microg/ml), this method allows improved resolution and sensitivity for DNA markers V and VI.     

A viscosity-tunable polymer for DNA separation by microchip electrophoresis

A thermo-responsive separation matrix, consisting of Pluronic F127 tri-block copolymers of poly(ethylene oxide) and poly(propylene oxide), was used to separate DNA fragments by microchip electrophoresis. At low temperature, the polymer matrix was low in viscosity and allowed rapid loading into a microchannel under low pressure. With increasing temperatures above 25°C, the Pluronic F127 solution forms a liquid crystalline phase consisting of spherical micelles with diameters of 17–19 nm. The solution can be used to separate DNA fragments from 100 bp to 1500 bp on poly(methyl methacrylate) (PMMA) chips. This temperature-sensitive and viscosity-tunable polymer provided excellent resolution over a wide range of DNA sizes. Separation is based on a different mechanism compared with conventional matrices such as methylcellulose. To illustrate the separation mechanism of DNA in a Pluronic F127 solution, DNA molecular imaging was performed by fluorescence microscopy with F127 polymer as the separation matrix in microchip electrophoresis. Figure Temperature dependence of the viscosity of 20% w/w Pluronic F127 solution in 1xTBE buffer. Dotted approximates resultant curve.     

Capillary-assembled microchip as an on-line deproteinization device for capillary electrophoresis

A capillary-assembled microchip (CAs-CHIP), prepared by simply embedding square capillaries in a lattice polydimethylsiloxane (PDMS) channel plate with the same channel dimensions as the outer dimensions of the square capillaries, has been used as a diffusion-based pretreatment attachment in capillary electrophoresis (CE). Because the CAs-CHIPs employ square-section channels, diffusion-based separation of small molecules from sample solutions containing proteins is possible by using the multilayer flow formed in the square section channel. When a solution containing high-molecular-weight and low-molecular-weight species makes contact with a buffer solution, the low-molecular-weight species, which have larger diffusion coefficients than the high-molecular-weight species, can be collected in a buffer-solution phase. The collected solution containing the low-molecular-weight species is introduced into the separation capillary to be analyzed by CE. This type of system can be used for CE analysis in which pretreatment is required to remove proteins. In this work a fluorescently labeled protein and rhodamine-based molecules were chosen as model species and a feasibility study was performed.      

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.     

Versatile 3-channel high-voltage power supply for microchip capillary electrophoresis

The fabrication of a battery operated 3-channel high voltage power supply for microchip capillary electrophoresis is described. The power supply consists of two positive and one negative DC-DC converters, a microprocessor controlled timer, a battery and a transformer to recharge the battery and feed the high voltage relays. This arrangement allows the possibility to control the potentials applied in the 0 to +/-4000 V range to a variety of microchip setups. It can also be easily adapted to perform either gated or pinched injection. The inclusion of a rechargeable battery was adopted to feed the DC-DC converters to reduce noise levels and achieve portability.     

Confinement effects on the morphology of photopatterned porous polymer monoliths for capillary and microchip electrophoresis of proteins

We find that the morphology of porous polymer monoliths photopatterned within capillaries and microchannels is substantially influenced by the dimensions of confinement. Porous polymer monoliths were prepared by UV-initiated free-radical polymerization using either the hydrophilic or hydrophobic monomers 2-hydroxyethyl methacrylate or butyl methacrylate, cross-linker ethylene dimethacrylate and different porogenic solvents to produce bulk pore diameters between 3.2 and 0.4 microm. The extent of deformation from the bulk porous structure under confinement strongly depends on the ratio of characteristic length of the confined space to the monolith pore size. The effects are similar in cylindrical capillaries and D-shaped microfluidic channels. Bulk-like porosity is observed for a confinement dimension to pore size ratio >10, and significant deviation is observed for a ratio <5. At the extreme limit of deformation a smooth polymer layer 300 nm thick is formed on the surface of the capillary or microchannel. Surface tension or wetting also plays a role, with greater wetting enhancing deformation of the bulk structure. The films created by extreme deformation provide a rapid and effective strategy to create robust wall coatings, with the ability to photograft various surface chemistries onto the coating. This approach is demonstrated through cationic films used for electroosmotic flow control and neutral hydrophilic coatings for electrophoresis of proteins.     

Integrated CMOS microchip system with capillary array electrophoresis

A complementary metal oxide semiconductor (CMOS)-capillary array electrophoresis (CAE) system has been used for DNA analysis. Because of its compactness and multiplex capability, the CAE-CMOS microchip is very suitable for the construction of a miniaturized high-throughput system for bioassays. Use of simultaneous laser-beam focusing on to the capillary array and a microscope objective contributed to the construction of the compact CMOS microchip-CAE system. To test the constructed system 100-base-pair (bp) DNA ladders and Hind III digest lambda DNA were separated in poly(vinylpyrrolidone) (PVP) sieving matrix. The miniaturized and integrated CMOS microchip system used in this work had great potential for combination with a variety of microfabricated devices for biomedical research.     

Protein-aptamer binding studies using microchip affinity capillary electrophoresis

The use of traditional CE to detect weak binding complexes is problematic due to the fast-off rate resulting in the dissociation of the complex during the separation process. Additionally, proteins involved in binding interactions often nonspecifically stick to the bare-silica capillary walls, which further complicates the binding analysis. Microchip CE allows flexibly positioning the detector along the separation channel and conveniently adjusting the separation length. A short separation length plus a high electric field enables rapid separations thus reducing both the dissociation of the complex and the amount of protein loss due to nonspecific adsorption during the separation process. Thrombin and a selective thrombin-binding aptamer were used to demonstrate the capability of microchip CE for the study of relatively weak binding systems that have inherent limitations when using the migration shift method or other CE methods. The rapid separation of the thrombin-aptamer complex from the free aptamer was achieved in less than 10 s on a single-cross glass microchip with a relatively short detection length (1.0 cm) and a high electric field (670 V/cm). The dissociation constant was determined to be 43 nM, consistent with reported results. In addition, aptamer probes were used for the quantitation of standard thrombin samples by constructing a calibration curve, which showed good linearity over two orders of magnitude with an LOD for thrombin of 5 nM at a three-fold S/N.     

High-voltage power supplies to capillary and microchip electrophoresis

Over the past years, the development of capillary electrophoresis (CE) and microchip electrophoresis (ME) systems has grown due to instrumental simplicity and wide application. In both CE and ME, the application of a high voltage (HV) is a crucial step in the electrokinetic (EK) injection and separation processes. Particularly on ME devices, EK injection is often performed with three different modes: gated, pinched, and unpinched. In all these cases, different potential values may be applied to one or multiple channels to control the injection of small sample volumes as well as the separation process. For this reason, the construction of reliable HV power supplies (HVPS) is required. This review covers the advances of the development of commercial and laboratory-built HVPS for CE and ME. Moreover, it intends to be a guide for new developers of electrophoresis instrumentation.     

Polymer solutions and entropic-based systems for double-stranded DNA capillary electrophoresis and microchip electrophoresis

We give an overview of recent development of low-viscosity polymer solutions and entropic trapping networks for double-stranded DNA (dsDNA) separations by conventional capillary electrophoresis and microchip electrophoresis. Theoretical models for describing separation mechanisms, commonly used noncross-linked polymer solutions, thermoresponsive (viscosity-adjustable) polymer solutions, and novel entropic trapping networks are included. The thermoresponsive polymer solutions can be loaded at one temperature into microchannels at lower viscosities, and used in separation at another temperature at entanglement threshold concentrations and higher viscosities. The entropic-based separations use only arrays of regular obstacles acting as size-separations and do not need viscous polymer solutions. These progresses have potential in integration to automated capillary and microfluidic chip systems, enabling better reusability of separation microchannels, much shorter DNA separation times, and higher reproducibility due to less matrix degradation.