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.     

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.     

Development of a universal serial bus-powered mini-high-voltage power supply for microchip electrophoresis

We describe a miniature high-voltage power supply (HVPS) with dimensions of 4.7 x 5.6 x 2.5 cm (W x L x H) powered by universal serial bus (USB) ports. Two strategies were adopted to ensure its efficient power usage. (i) Only two high-voltage converters (one positive and one negative) and two relays were used for power saving, while keeping the sample plug stable and well-defined and avoiding sample leakage for microchip electrophoresis. (ii) The components and their running modes were specially designed to decrease power waste according to the feature of different periods of the microchip electrophoresis process. Performance of this USB-based mini-HVPS was demonstrated using sodium fluorescein analyte with microchip electrophoresis/LIF detection.     

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.     

Versatile method for electroosmotic flow measurements in microchip electrophoresis

A novel versatile method for the determination of low or high electroosmotic mobility values in microdevices of variable microchannel design is presented. The electroosmotic flow (EOF) calculation is based on the difference between the apparent and effective mobilities of a reference compound. The proposed method uses microchip frontal electrophoresis for the determination of these mobilities. This requires simple monochannel microchip design and demonstrates versatile and time-saving procedure when compared to conventional current monitoring method when measuring low EOF. It has been applied successfully to the characterization of different coating procedure in glass and poly(dimethylsiloxane) microchips.     

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.     

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).     

Affinity monolith preconcentrators for polymer microchip capillary electrophoresis

Developments in biology are increasing demands for rapid, inexpensive, and sensitive biomolecular analysis. In this study, polymer microdevices with monolithic columns and electrophoretic channels were used for biological separations. Glycidyl methacrylate-co-ethylene dimethacrylate monolithic columns were formed within poly(methyl methacrylate) microchannels by in situ photopolymerization. Flow experiments in these columns demonstrated retention and then elution of amino acids under conditions optimized for sample preconcentration. To enhance analyte selectivity, antibodies were immobilized on monoliths, and subsequent lysozyme treatment blocked nonspecific adsorption. The enrichment capability and selectivity of these affinity monoliths were evaluated by purifying fluorescently tagged amino acids from a mixture containing green fluorescent protein (GFP). Twenty-fold enrichment and 91% recovery were achieved for the labeled amino acids, with a >25 000-fold reduction in GFP concentration, as indicated by microchip electrophoresis analysis. These devices should provide a simple, inexpensive, and effective platform for trace analysis in complex biological samples.     

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.     

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.     

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.     

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.     

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.     

Low-density lipoprotein analysis in microchip capillary electrophoresis systems

Due to the mounting evidence for altered lipoprotein and cholesterol-lipoprotein content in several disease states, there has been an increasing interest in analytical methods for lipoprotein profiling for diagnosis. The separation of low- and high-density lipoproteins (LDL and HDL, respectively) has been recently demonstrated using a microchip capillary electrophoresis (CE) system [1]. In contrast to this previous study, the present report demonstrates that LDL analysis can be performed in an uncoated glass microchannel. Moreover, by adding sodium dodecyl sulfate (SDS) to the sample at a concentration well below the critical micellar concentration prior to injection, the LDL peak undergoes a focusing effect and exhibits an apparent efficiency of 2.2 x 10(7) plates/m. Laser light scattering experiments demonstrate that the low concentration of SDS used does not significantly alter lipoprotein particle size distribution within the time course that the analysis is performed. It is thus hypothesized that SDS nondisruptively coats LDL particles. The peak sharpening effect, observed only when SDS is added solely to the sample, is probably due to a mobility gradient created between the sample and the running buffer. The chip-based method demonstrated here has the potential for rapid analysis and sensitive detection of different LDL forms of clinical relevance.     

High-sensitivity capillary and microchip electrophoresis using electrokinetic supercharging

Electrokinetic supercharging (EKS) is considered as one of the most powerful online preconcentration techniques in electrophoresis. It combines the efficient preconcentration power of field-amplified sample injection and the exceptional selective nature of transient isotachophoresis. It has a wide range of applications to different types of analytes ranging from small ions to large proteins and DNA fragments. This comprehensive review--up to date--provides listing for all the works, developments, and advances in EKS. The review will pay particular attention to innovations, new methodologies for manipulation, challenges for improving the detection sensitivity, and various applications of EKS in capillaries and microchips.