Direction Of Electroosmotic Flow Research Articles

Novel cationic coating agent for protein separation by capillary electrophoresis(†).

A novel positively charged surfactant N-dodecyl-N,N-dimethyl-(1,2-propandiol) ammonium chloride was used for the dynamic coating of the inner wall of a silica capillary. This paper covers the evaluation of dynamic coating and study of the influence of the analysis conditions for the magnitude and direction of electroosmotic flow as well as for the effective and selective separation of chosen proteins (ribonuclease A, cytochrome c, lysozyme, and myoglobin). The concentration of 0.1 mM of N-dodecyl-N,N-dimethyl-(1,2-propandiol) ammonium chloride enabled the reversal of the electro-osmotic flow, however, to separate basic as well as neutral proteins the higher concentration of the studied surfactant was necessary. The final conditions for the separation of studied proteins were set at 100 mM sodium acetate pH 5.5 with 10.0 mM of the studied surfactant. The results were also compared with those of two commercially available cationic surfactants, cetyltrimethylammonium bromide and dodecyltrimethylammonium bromide. Additionally, the developed method for protein separation was applied for the determination of lysozyme in a cheese sample. The limits of detection and quantification of lysozyme were 0.9 and 3.0 mg/L, respectively. The mean concentration of lysozyme found in the cheese sample was 167.3 ± 10.3 mg/kg.

A facile and versatile approach for controlling electroosmotic flow in capillary electrophoresis via mussel inspired polydopamine/polyethyleneimine co-deposition

Electroosmotic flow (EOF), which reveals the charge property of capillary inner surface, has an important impact on the separation performance and reproducibility of capillary electrophoresis (CE). In this study, a novel, facile and versatile method to achieve diverse and controllable EOF in CE was reported based on the co-deposition of mussel-inspired polydopamine (PDA) and branched polyethyleneimine (PEI) on the capillary inner surface as the hybrid functional coating. After these PDA/PEI co-deposited columns were reinforced by the post-incubation of FeCl3, various magnitude and direction of EOF in CE could be easily achieved by varying a number of preparation parameters, including the mass ratio of DA/PEI and the molecular weight of PEI (including PEI-600, PEI-1800, PEI-10000 and PEI-70000). The separation effectiveness and stability of the hybrid coated columns were verified by the analysis of aromatic acids and aniline derivatives. The results showed that the controllable and diverse EOF was important in enhancing the separation performance of the analytes. The baseline separation of all the five aromatic acids can be achieved in 7min with high separation efficiency by using the PDA/PEI-600 co-deposited column with the mass ratio of 6:1. On the other hand, with the PDA/PEI-70000 co-deposited column with the mass ratio of 6:1, the aniline compounds were easily baseline separated within 10min. By contrast, using the bare and PDA coated columns, the migration of the aromatic acids was very slow and the baseline separation of the aniline compounds cannot be obtained. Moreover, the co-deposited columns showed long lifetime and good stability. The relative standard deviations for intra-day, inter-day and capillary-to-capillary repeatability of the PDA/PEI-600 co-deposited column with the mass ratio of 6:1, which was reinforced by the post-incubation of FeCl3, were all lower than 5%.

Electroosmotic flow reversal outside glass nanopores.

We report observations of a striking reversal in the direction of electroosmotic flow (EOF) outside a conical glass nanopore as a function of salt concentration. At high ionic strengths (>100 mM), we observe EOF in the expected direction as predicted by classical electrokinetic theory, while at low salt concentrations (<1 mM) the direction of the flow is reversed. The critical crossover salt concentration depends on the pore diameter. Finite-element simulations indicate a competition between the EOF generated from the inner and outer walls of the pore, which drives flows in opposite directions. We have developed a simple analytical model which reveals that, as the salt concentration is reduced, the flow rates inside the pore are geometrically constrained, whereas there is no such limit for flows outside the pore. This model captures all of the essential physics of the system and explains the observed data, highlighting the key role the external environment plays in determining the overall electroosmotic behavior.

The transport behavior of As, Cu, Pb, and Zn during electrokinetic remediation of a contaminated soil using electrolyte conditioning

Electrokinetic remediation (also known as electrokinetics) is a promising technology for removing metals from fine-grained soils. However, few studies have been conducted regarding the transport behavior of multi-metals during electrokinetics. We investigated the transport of As, Cu, Pb, and Zn from soils during electrokinetics, the metal fractionation before and after electrokinetics, the relationships between metal transport and fractionation, and the effects of electrolyte conditioning. The main transport mechanisms of the metals were electroosmosis and electromigration during the first two weeks and electromigration during the following weeks. The direction of electroosmotic flow was from the anode to the cathode, and the metals in the dissolved and reducible-oxides fractions were transported to the anode or cathode by electromigration according to the chemical speciation of the metal ions in the pore water. Moreover, a portion of the metals that were initially in the residual fraction transitioned to the reducible and soluble fractions during electrokinetic treatment. However, this alteration was slow and resulted in decreasing metal removal rates as the electrokinetic treatment progressed. In addition, the use of NaOH, H3PO4, and Na2SO4 as electrolytes resulted in conditions that favored the precipitation of metal hydroxides, phosphates, and sulfates in the soil. These results demonstrated that metal removal was affected by the initial metal fractionation, metal speciation in the pore solution, and the physical–chemical parameters of the electrolytes, such as pH and electrolyte composition. Therefore, the treatment time, use of chemicals, and energy consumption could be reduced by optimizing pretreatment and by choosing appropriate electrolytes for the target metals.

CHIRAL SEPARATIONS USING HEPARIN AND POLYELECTROLYTE MULTILAYERS IN CAPILLARY ELECTROPHORESIS

Reports on the use of heparin as a chiral additive show widely different separation conditions and inconsistent results. A systematic evaluation of the separation of pheniramine enantiomers using heparin as a chiral selector suggested that heparin was interacting with the capillary surface at acidic pH. Further testing confirmed the interaction between heparin and the capillary surface at acidic pH causing an increase in the electroosmotic flow (EOF). Polyelectrolyte multilayer (PEM) was chosen as a potential means to eliminate the interaction between heparin and the capillary surface. Generation of the PEMs on a bare capillary was monitored by measuring the changes in EOF following the deposition of each polyelectrolyte layer; deposition provided consistent EOF intensities and facilitated control over EOF direction. Results from this study indicated that a stable PEM system could be generated, which prevents the interaction of heparin with the capillary surface. This PEM system showed excellent reproducibility, with only a slight loss of resolution due to the increased interactions between the analyte and the charged capillary wall.

Cation-selective electropreconcentration.

A cation-selective microfluidic sample preconcentration system is described. The cation sample was electropreconcentrated using a reversed-direction electroosmotic flow (EOF) and an anion-permselective filter, where an electric double layer (EDL) overlap condition existed. The anion-permselective filter between microchannels was fabricated by three different methods: 1) extending a positively charged, nanoporous, polymer membrane by photopolymerization of poly(diallyldimethylammonium chloride) (PDADMAC); 2) etching a nanochannel and then coating it with a positively-charged monomer, N-[3-(trimethoxysilyl)propyl]-N'-(4-vinylbenzyl)ethylenediamine hydrochloride (TMSVE); and, 3) etching a nanochannel and then coating it with a positively-charged, pre-formed polymer, polyE-323. The EOF direction in the microchannel was reversed by both TMSVE and polyE-323 coatings. The cation-selective preconcentration was investigated using charged fluorescent dyes and tetramethylrhodamine isothiocyanate (TRITC)-tagged peptides/proteins. The preconcentration in the three different systems was compared with respect to efficiency, dependence on buffer concentration and pH, tolerable flow rate, and sample adsorption. Both TMSVE- and polyE-323-coated nanochannels showed robust preconcentration at high flow rates, whereas the PDADMAC membrane maintained anion-permselectivity at higher buffer concentrations. The TMSVE-coated nanochannels showed a more stable preconcentration process, whereas the polyE-323-coated nanochannels showed a lower peptide sample adsorption and robust efficiency under a wide range of buffer pHs. The system described here can potentially be used for the preconcentration of cationic peptides/proteins on microfluidic devices for subsequent analyses.

Separation of Proteins and Nanoparticles by Charge and Size with a Tunable Semiconductor Membrane

We study the applicability of tunable nanoporous semiconductor membranes made of the heavily doped silicone for separation of nanoparticles and proteins by their size and charge. We demonstrate that this type of membrane can overcome one of the major shortcomings of membrane applications for particle separation: the compromise between membrane selectivity and permeability. The microscopic computational model that we have developed describes the translocation process of filtered objects with the translational-rotational Brownian Dynamics taking into consideration effects from the dielectrophoresis, the electrolyte solution flow, and the self-consistent electrostatic potential distribution within the continuum Poisson-Nernst-Planck approach. Our results indicate that the tunable local electric field arising inside the membrane can effectively control interaction of filtered objects with the nanopore to either block its passage or increase the translocation rate by modulating the electroosmotic flow direction and magnitude. By extracting the membrane permeability from our microscopic simulations, we compute the macroscopic sieving factors and show that the size and charge selectivity of the membrane can be tuned by the applied voltage in the broad range.View Large Image | View Hi-Res Image | Download PowerPoint Slide

Separation of Ions in Nanofluidic Channels with Combined Pressure-Driven and Electro-Osmotic Flow

Separation of ionic species with the same electrophoretic mobility but different valence in electrolyte systems can occur within nanometer-scale channels with finite electrical double layers (EDLs). This is because EDL thicknesses are a significant fraction of slit height in such channels and can create transverse analyte concentration profiles that allow for unique separation modalities when combined with axial fluid flow. Previous work has shown such separation to occur using either pressure-driven flow or electro-osmotic flow separately. Here, we develop a Poisson-Boltzmann model to compare the separation of such ions using the combination of both pressure-driven and electro-osmotic flow. Applying a pressure gradient in the opposite direction of electro-osmotic flow can allow for zero or infinite retention of analyte species, which we investigate using three different wall boundary conditions. Furthermore, we determine conditions in fused silica nanochannels with which to generate optimal separation between two analytes of different charge but the same mobility. We also give simple rules of thumb to achieve the best separation efficacy in nanochannel systems.

Transport Characteristics of Aromatic Acids in the Capillary Electrochromatography with Mixed Magnetic Nanoparticles Coating as Stationary Phase

In this paper, novel open-tubular electrochromatographic system with core/shell magnetic nanoparticles modified by amino and C18 functional groups as mixed stationary phase by using a series of permanent magnets to immobilize nanoparticles into the capillary were investigated. The new system showed high separation ability and efficiency due to high specific surface area of nanomaterial and mixing separation mechanism. The transport equations of aromatic acids using the open-tubular electrochromatographic column with magnetic nanoparticle coating as stationary phases were derived under isocratic and double stepwise gradient elutions, and the retention characteristics of organic acids separated by the columns were further investigated. The results showed that the electroosmotic flow gradually decreased with the increase of Fe3O4@SiO2-NH2 in the mixed stationary phases, and the direction of electroosmotic flow reversed eventually. The capacity factor was relevant to the ratio of ion-exchange and reversed-phase stationary phases. Furthermore, experiments confirmed that the capacity factor of solute had a good linear relationship with the ratio of mixed stationary phases, and the correlation coefficient was greater than 0.9166 after fitting. Under the double stepwise gradient elution condition, the migration time of solute can be predicted by its elution time under isocratic elution. After the solutes were eluted by phosphate buffer stepwise gradient elution, the relative errors between predicted retention time and actual retention time of solute were less than 8.0%. After they were eluted by organic modifier concentration stepwise gradient elution, the relative errors between two values of solute were less than 10%. After they were eluted by pH value stepwise gradient elution, the relative errors between two val- ues of solute were less than 5.8%. Rhizoma gastrodiae was separated by the constructed system under the optimized chroma- tographic condition, the main components of gastrodin, vanilla alcohol and citric acid were identified based on retention time of standards, and the relative errors between predicted value and actual value of citric acid was 11%. The open-tubular elec- trochromatographic system shows great potential in the method development of analysis of complicated samples. Keywords gradient elution; transport characteristic; mixed stationary phase 1 引言

Tentative Comparison of Tube Radial Distribution Chromatography and CZE

Tube radical distribution chromatography (TRDC) uses an untreated open tubular capillary tube and a ternary mixture of solvents (water and hydrophilic/hydrophobic organic solvents) as a carrier solution. A model analyte mixture comprising 1-naphthol, 1-naphthoic acid, 1-naphthalenesulfonic acid, 2,6-naphthalenedisulfonic acid, and 1,3,6-naphthalenetrisulfonic acid was examined by the TRDC and capillary zone electrophoresis (CZE) systems that comprised mainly a capillary tube and a detector. In the TRDC system the elution order of analytes could be changed by altering the component ratios of the solvents, whereas in the CZE system the elution order was changed by altering the electroosmotic flow direction. The experimental data obtained provide clues about the features and utility of TRDC as a new separation method.

Physisorbed surface coatings for poly(dimethylsiloxane) and quartz microfluidic devices

Surface modifications of microfluidic devices are of essential importance for successful bioanalytical applications. Here, we investigate three different coatings for quartz and poly(dimethylsiloxane) (PDMS) surfaces. We employed a triblock copolymer with trade name F(108), poly(L-lysine)-g-poly(ethylene glycol) (PLL-PEG), as well as the hybrid coating n-dodecyl-β-D-maltoside and methyl cellulose (DDM/MC). The impact of these coatings was characterized by measuring the electroosmotic flow (EOF), contact angle, and prevention of protein adsorption. Furthermore, we investigated the influence of static coatings, i.e., the incubation with the coating agent prior to measurements, and dynamic coatings, where the coating agent was present during the measurement. We found that all coatings on PDMS as well as quartz reduced EOF, increased reproducibility of EOF, reduced protein adsorption, and improved the wettability of the surfaces. Among the coating strategies tested, the dynamic coatings with DDM/MC and F(108) demonstrated maximal reduction of EOF and protein adsorption and simultaneously best long-term stability concerning EOF. For PLL-PEG, a reversal in the EOF direction was observed. Interestingly, the static surface coating strategy with F(108) proved to be as effective to prevent protein adsorption as dynamic coating with this block copolymer. These findings will allow optimized parameter choices for coating strategies on PDMS and quartz microfluidic devices in which control of EOF and reduced biofouling are indispensable.

Open-access and multi-directional electroosmotic flow chip for positioning heterotypic cells

We propose a novel method of cell positioning using electroosmotic flow (EOF) to analyze cell-cell interactions. The EOF chip has an open-to-air configuration, is equipped with four electrodes to induce multi-directional EOF, and allows access of tools for liquid handling and of physical probes for cell measurements. Evaluation of the flow within this chip indicated that it controlled hydrodynamic transport of cells, in terms of both speed and direction. We also evaluated cell viability after EOF application and determined appropriate conditions for cell positioning. Two cells were successively positioned in pocket-like microstructures, one in each micropocket, by controlling the EOF direction. As an experimental demonstration, we observed contact interactions between two individual cells through gap junction channels. The EOF chip should provide ways to elucidate various cell-cell interactions between heterotypic cells.

Open Tubular Capillary Electrochromatographic Separation of Proteins and Peptides Using a TiO2 Nanoparticle-Deposited Capillary by Liquid Phase Deposition

A titanium dioxide (TiO2) nanoparticles (NP)-deposited column by the liquid phase deposition (LPD) method was exploited for open tubular capillary electrochromatographic (OTCEC) separation of proteins and peptides. With the phosphate buffer, the direction of electroosmotic flow (EOF) for the TiO2 NP-deposited capillary was the same as that for the bare fused silica capillary, but the magnitude of EOF was much smaller at pH≧7. Its OTCEC performance was then evaluated with four proteins (bovine serum albumin, ovalbumin, hemoglobin, and myoglobin) by varying parameters of the background electrolyte (BGE), such as pH value, composition, and concentration. Good separation was achieved with the phosphate buffer (40 mM, pH 9.0) and an applied voltage of 15 kV. In comparison with the bare fused silica capillary, the relatively high resolving power of the TiO2 NP-deposited capillary would mainly result from the ligand exchange interaction between analytes and the phosphate adsorbed on the TiO2. Finally, separation of the proteolytic digest mixture was also performed with the TiO2 NP-deposited capillary at well defined conditions, indicating its potential applicability for biological analysis.

Surface modification of poly(dimethylsiloxane) microchips using a double-chained cationic surfactant for efficiently resolving fluorescent dye adsorption

This paper described a double-chained cationic surfactant, didodecyldimethylammonium bromide (DDAB), for dynamic surface modification of poly(dimethylsiloxane) (PDMS) microchips to reduce the fluorescent dyes adsorption onto the microchannel. When DDAB with a high concentration was present as the dynamic modification reagent in the running and sample buffer, it not only reversed the direction of electroosmotic flow, but also efficiently suppressed fluorescent dyes pyronine Y (PY) or rhodamine B (RB) adsorption onto the chip surface. In addition, vesicles formed by DDAB in the buffer with higher surface charge density and electrophoretic mobility could provide wider migration window and potential for the separation of compounds with similar hydrophobicity. Factors affecting modification, such as pH and concentrations of the buffer, DDAB concentration in the buffer were investigated. Compared with commonly used single-chained cetyltrimethylammonium bromide, DDAB provided a better modification performance. Furthermore, PY and RB were separated successfully on a PDMS microchip at the appropriate conditions with DDAB.

Electrokinetic remediation of contaminated soil with waste-lubricant oils and zinc

The feasibility of electrokinetic technology on the remediation of mixed-waste-contaminated railroad soil, contaminated by lubricant oil and zinc, was investigated. To enhance the removal efficiency, catholyte purging with 0.1 M HNO 3 and a supply of non-ionic surfactant, secondary alcohol ethoxylate, was applied to the anode to remove Zn and to solubilize the lubricant oil. The catholyte purging maintained the soil pH as acidic and enhanced desorption of zinc from the soil, where the zeta potential of the acidic soil became positive. Thereafter, the direction of electro-osmotic flow was changed from the cathode to anode and the flow rate was reduced. The lesser in magnitude reverse electro-osmotic flow inhibited the migration of zinc and the lubricant oil was removed by the electro-osmotic flow. The removal of zinc and lubricant oil was enhanced with an increase in voltage gradient; however, a higher voltage gradient resulted in higher energy expenditure. After electrokinetic operation over 17 days, the removal efficiency of zinc was 22.1–24.3%, and that of lubricant oil was 45.1–55.0%. Although the removal of lubricant oil was quite high, the residual concentration did not meet Korean regulation levels.

Manipulation and charge determination of proteins in photopatterned solid supported bilayers

This work demonstrates the use of deep UV micropatterned chlorotrimethylsilane (TMS) monolayers to support lipid membranes on SiO(2) surfaces. After immersing such a patterned surface into a solution containing small unilamellar vesicles of egg PC, supported bilayer lipid membranes were formed on the hydrophilic, photolyzed regions and lipid monolayer over the hydrophobic, non-photolyzed regions. A barrier between the lipid monolayer and bilayer regions served to stop charged lipids migrating between the two. This allows the system to be used to separate charged lipids or proteins by electrophoresis. Either oppositely charged fluorescence labeled lipids [Texas Red DHPE (negative charge) and D291 (positive charge)] or lipids with different charge numbers [Texas Red DHPE (one negative charge) and NBD PS (two negative charges)] can be separated. We have also studied the migration of streptavidin attached to a biotinylated lipid. Negatively charged streptavidin responds to the applied electric field by moving in the direction of electroosmotic flow, i.e. towards the negative electrode. However the direction of streptavidin movement can be controlled by altering the difference in zeta potential between that of the streptavidin (zeta(1)) and the lipid membrane (zeta(2)). If zeta(1) > zeta(2), streptavidin moves to the negative electrode, while if zeta(1) < zeta(2), streptavidin moves to the positive electrode. This balance was manipulated by adding positively charged lipid DOTAP to the membrane. After measuring the average drift velocity of streptavidin as a function of DOTAP concentration, the point where zeta(1) approximately zeta(2) was found. At this point zeta(1) was calculated to be -9.8 mV which is in good agreement with the value of -13 mV from force measurements and corresponds to a charge of -2e per streptavidin, thus demonstrating the applicability of this method for determining protein charge.

Versatile programmable eight-path-electrode power supply for automatic manipulating microfluids of a microfluidic chip

A versatile programmable eight-path-electrode power supply (PEPS) system for manipulating microfluids of a complex microfluidic chip has been developed. The PEPS system consisted of a single chip microprocessor as a central control unit and a personal computer (PC) as an upper computer, and the program could be operated under Windows98/2000/XP. The voltage output of each electrode was in the range of 0 to +8000 V (0.1% precision) while the current output was in the range of 0 to +999 μA. The voltage of eight electrodes could be operated either independently or synchronously by random combination of any electrodes through switching. The voltage output modes were “switch-off/floating”, “switch-on” and “grounded” and fast switched at ms-level between these modes, and run time (0.1 s precision) of these modes could be controlled as desired. The PEPS system was conveniently for controlling flow rate and direction of electroosmotic flow (EOF) in a chip network. Six electrodes were chosen to control the repeated ‘injection and separation’ of 1.0 × 10 −5 M fluorescein isothiocyanate (FITC) in a six-reservoir glass-based chip. The relative standard deviation (R.S.D., n = 4, S/N = 10) of the repeated operation was 0.9% for the reservation time ( t R) and 2.3% for the peak height, respectively.

Magnetically Immobilized Beds for Capillary Electrochromatography

Fritless packed beds comprised of magnetically responsive octadecylsilane bonded silica particles have been constructed for reversed-phase electrochromatography. The magnetic particles were immobilized in the capillary by applying an external magnetic field transverse to the direction of electroosmotic flow. Being subjected to the interplay of fluid dragging and magnetic forces, the initial loosely packed particle assembly was compacted into a uniform packing structure. The magnetically immobilized beds obtained were used as stationary phases for separation of neutral compounds, with retention behavior and column efficiency similar to those of slurry-packed columns. The results suggest that the magnetic attraction approach to fritless column packing may be used for construction of advanced chip-based chromatography, especially in complex architectures comprising curved and intersecting channels.

Extending chromosomal DNA in microstructures using electroosmotic flow

The extension of chromosomal DNA is a key technique in high-resolution gene locationsuch as extended fibre fluorescent in situ hybridization. In this paper, we propose a newmicro-device designed for on-chip integration of three functions, (1) positioningof cells, (2) extending chromosomal DNA from the cells, and (3) anchoring theextended DNA fibres. The device has a flow chamber equipped with (a) multi-phaseelectrodes to create electroosmotic flow (EOF) of variable direction, (b) a set ofmicro-fabricated pocket-like structures for cell positioning, and (c) a micro-pillar arraywhose surface is chemically modified to anchor DNA fibres. First, unidirectional EOF isinduced, by which cells are carried into the micro-pockets and positioned, one cellat each pocket. After rupturing the cells, chromosomal DNA from each cell ishydrodynamically extended by EOF. When the EOF direction is rotated through90°, the fibres are made to contact with the pillars to be anchored. Because the velocity of theEOF is controllable, the breakage of DNA during the process can be minimized. Thusimmobilized DNA fibres are suspended a few microns above the surface, allowing freeaccess of probe molecules.

Analysis of double-stranded DNA by capillary electrophoresis using poly(ethylene oxide) in the presence of hexadecyltrimethylammonium bromide

The impact of hexadecyltrimethylammonium bromide (CTAB) on the separation of ds-DNA by capillary electrophoresis in conjunction with laser-induced fluorescence (CE–LIF) detection using poly(ethylene oxide) (PEO) solution is described. The use of CTAB for improved separation reproducibility and efficiency of DNA has not been demonstrated although it is widely used for controlling the magnitude and direction of electroosmotic flow in CE. With increasing CTAB concentration, the interactions of DNA with ethidium bromide (EtBr) and with the capillary wall decrease. For the separation of DNA fragments with the sizes ranging from several base pairs (bp) to 2176 bp, a polymer solution consisting of 0.75% poly(ethylene oxide), 100 mM TB buffer (pH 8.0), 25 μg/mL EtBr, and 0.36 μg/mL CTAB is proper. Using the PEO solution, we separated a mixture of DNA markers V (pBR 322 /HaeIII digest) and VI (pBR 328/ BglI digest and pBR 328/ HinfI digest) within 8 min at −375 V/cm, with the limit of detection of 2.0 ng/mL based on the peak height for the 18-bp DNA fragment. The method is highly efficient (>10 6 plate/m), repeatable (RSD of the migration times <1.5%), and sensitive. In addition, it is convenient to fill a capillary (75 μm in diameter) with such a low-viscosity PEO solution by syringe pushing.