Control Of Electroosmotic Flow Research Articles

Field-Effect Control of Electroosmotic Pumping Using Porous Silicon–Silicon Nitride Membranes

Field-effect control of electroosmotic (EO) flow, which has been demonstrated on single microchannels, provides a novel method to modulate the zeta potential and, therefore, the EO flow through a porous membrane. To realize field-effect control of EO pumping with a membrane, a SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> -coated porous silicon membrane was designed and fabricated. The heavily doped silicon core was used as a conducting electrode to apply the transverse gate potential, which modulates the zeta potential of the channel walls and, thereby, the EO flow rate with constant externally applied electric fields along the channels. We observed significant electrolytic-rectification effect, i.e., for gate voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) < 0, a substantial current leakage through the SiN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> was observed, whereas negligible leakage current was detected for V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> > 0. Significant EO flow control, nearly 70% reduction in flow velocity, was observed for positive gate bias, while only 15 % flow-velocity enhancement was observed for negative gate bias of similar magnitude. This first demonstration of field-effect control on porous membranes opens the door for making high-flow rate EO pumps with porous membranes of low zeta potential materials.

Thermal control of electroosmotic flow in a microchannel through temperature-dependent properties

A numerical investigation is conducted on the electroosmotic flow and associated heat transfer in a two-dimensional microchannel. The objective of this study is to explore a new conceptual idea that is control of an electroosmotic flow by using a thermal field effect through the temperature-dependent physical properties. Two exemplary problems are examined: a flow in a microchannel with a constant vertical temperature difference between two horizontal walls and a flow in a microchannel with the wall temperatures varying horizontally in a sinusoidal manner. The results of numerical computations showed that a proper control of thermal field may be a viable means to manipulate various non-plug-like flow patterns. A constant vertical temperature difference across the channel produces a shear flow. The horizontally-varying thermal condition results in spatial variation of physical properties to generate fluctuating flow patterns. The temperature variation at the wall with alternating vertical temperature gradient induces a wavy flow.

Numerical simulation of electroosmotic flow in hydrophobic microchannels

Electroosmotic flow (EOF) is a promising way for driving and mixing fluids in microfluidics. For the parallel-plate microchannel with the hydrophobic surface, this paper solved the governing equations using the finite element method (FEM), and the effects of microchannel height, electric strength and ionic concentration on EOF were thus investigated. The simulation indicates that the transient characteristics of EOF are similar in hydrophobic and hydrophilic microchannels, the steady time of EOF is proportional to the square of microchannel height, and the scale is microsecond. EOF velocity is proportional to the electric strength and independent of the channel height, and decreases slowly with the ionic concentration, which is lower than that in hydrophilic microchannel due to the presence of slip length in hydrophobic microchannel. The results can provide valuable insights into the optimal design of microchannel surfaces to achieve accurate EOF control in hydrophobic microchannel.

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.

Continuous and precise particle separation by electroosmotic flow control in microfluidic devices

A new scheme has been described for continuous particle separation using EOF in microfluidic devices. We have previously reported a method for particle separation, called "pinched flow fractionation (PFF)", in which size-dependent and continuous particle separation can be achieved by introducing pressure-driven flows with and without particles into a pinched microchannel. In this study, EOF was employed to transport fluid flows inside a microchannel. By controlling the applied voltage to electrodes inserted in each inlet/outlet port, the flow rates from both inlets, and flow rates distributed to each outlet could be accurately tuned, thus enabling more effective separation compared to the pressure-driven scheme. In the experiment, the particle behaviors were compared between EOF and pressure-driven flow schemes. In addition, micrometer- and submicrometer-sized particles were accurately separated and individually collected using a microchannel with multiple outlet branch channels, demonstrating the high efficiency of the presented scheme.

Plasma processing for polymeric microfluidics fabrication and surface modification: Effect of super-hydrophobic walls on electroosmotic flow

We report on the fabrication and electrokinetic characterization of poly-methyl methacrylate (PMMA) microfluidics by deep O2 plasma etching, utilizing a photosensitive poly-(dimethyl siloxane) (PDMS) as a resist (in situ mask). The mass production amenability, the high throughput without the use of mold, the dry character along with the flexibility to control surface properties towards specific demands are some of the advantages of this method. Intense ion bombardment ensures high etch rates (∼1.5μm/min) and anisotropy. A PMMA lid was thermally bonded to the plasma fabricated microchannel under 1.8kg/cm2 at 120°C. Surface roughness and hydrophilization are some unique features induced by plasma processing, affecting the electrokinetic performance of the microfluidic and resulting in relatively high electroosmotic flow (EOF) mobilities of 2.83×10−4cm2/Vs. Teflon-like coating deposition on the engraved part modified the surface into a super-hydrophobic and resulted in even higher EOF mobility (3.89×10−4cm2/Vs), thus proposing an alternative means in microfluidic surface modification and EOF control.

Polymer Entrapment in Polymerized Silicate for Preparing Highly Stable Capillary Coatings for CE and CE−MS

An easy-to-implement capillary coating strategy based on polymer entrapment in the network of polymerized silicate is described. In this manner, cationic polymers are tightly fixed onto the inner wall of the capillary for electroosmotic flow control without necessitating complex surface modification chemistries. The resulting coated capillary exhibited good stability over a wide range of pH, good reproducibility, strong endurance in more than 300 electrophoretic runs, and tolerance of commonly employed organic solvent additives in CE. Applications in CE-MS analysis of biologically important anions as well as sample enrichment are shown. Additionally, it was used as a durable base for attachment of multiple layers of charged polymers on the wall, via electrostatic interaction with the preceding layer. Thus, two novel types of highly stable coated capillaries, one with anodic EOF and the other cathodic, were developed.

Control of electroosmotic flow by a cation additive to enhance the separation of amino acids by micellar electrokinetic chromatography

The effect of a divalent cation (Mg2+) and 3 monovalent cations (Na+, Li+, and K+) as buffer additives on the electroosmotic flow (EOF) was investigated in order to improve the separation performance of p-acetamidobenzenesulfonyl fluoride (PAABS-F) derivatives of 20 standard amino acids by micellar electrokinetic chromatography (MEKC). The EOF can be decreased with increasing concentration of cations with the order of cations as Mg2+>K+>Na+>Li+. However, it was found that the resolution cannot be improved easily using a buffer cation which is more capable of decreasing the EOF. Taking the migration time, resolution, and peak shape into account, Na+ is the best buffer additive, although the EOF decreased most using Mg2+. Using 20mmol/L borate at pH 9.3 containing 140mmol/L SDS and 20mmol/L Na+ as electrolyte, 20 standard amino acids were successfully separated within 14min.

Fully Packed Capillary Electrochromatographic Microchip with Self-Assembly Colloidal Silica Beads

A fully packed capillary electrochromatographic (CEC) microchip showing improved solution and chip handling was developed. Microchannels for the CEC microchip were patterned on a cyclic olefin copolymer substrate by injection molding and packed fully with 0.8-microm monodisperse colloidal silica beads utilizing a self-assembly packing technique. The silica packed chip substrate was covered and thermally press-bonded. After fabrication, the chip was filled with buffer solution by self-priming capillary action. The self-assembly packing at each channel served as a built-in nanofilter allowing quick loading of samples and running buffer solution without filtration. Because of a large surface area-to-volume ratio of the silica packing, reproducible control of electroosmotic flow was possible without leveling of the solutions in the reservoirs resulting 1.3% rsd in migration rate. The capillary electrophoretic separation characteristics of the chip were studied using fluorescein isothiocyanate (FITC)-derivatized amino acids as probe molecules. A mixture of FITC and four FITC-derivatized amino acids was successfully separated with 2-mm separation channel length.

Electrodes for microfluidic devices produced by laser induced forward transfer

The laser induced forward transfer (LIFT) process was used to create conductive lines and pads for rapid prototyping and repairing microdevices. Single 0.1–10 μJ pulses from a 120 fs 800 nm titanium:sapphire laser were used to transfer films consisting of 40–80 nm thick gold to create the lines. Experiments were conducted in air ambient. The laser was focused using 4× and 10× microscope objectives and produced 5–20 μm diameter metal spots which were overlapped to produce conductive lines. Electrodes with widths between 10 and 50 μm have been produced and their resistances have been measured. The resistivities of these LIFT produced Au electrodes were found to be approximately (1–4) × 10 −6 Ω m. It has also been shown that the conductivity of the lines can be further improved by electrical curing. The LIFT process was used to repair heaters for microfluidic applications and preliminarily create electrodes for control of electro-osmotic flow in microfluidic devices.

Selective sampling of multiply phosphorylated peptides by capillary electrophoresis for electrospray ionization mass spectrometry analysis

The ionization of phosphorylated peptides in positive ion mode mass spectrometry is generally less efficient compared with the ionization of their non-phosphorylated counterparts. This can make phosphopeptides much more difficult to detect. One way to enhance the detection of phosphorylated proteins and peptides is by selectively isolating these species. Current approaches of phosphopeptide isolation are based on the favorable interactions of phosphate groups with immobilized metals. While these methods can be effective in the extraction, they can lead to incomplete sample recovery, particularly for the most strongly bound multiply phosphorylated components. A non-sorptive method of phosphopeptide isolation using capillary electrophoresis (CE) was recently reported [Zhang et al., Anal. Chem. 77 (2005) 6078]. The relatively low isoelectric points of phosphopeptides cause them to remain anionic at acidic sample pH. Hence, they can be selectively injected into the capillary by an applied field after the electroosmotic flow (EOF) is suppressed. The technique was previously coupled with matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). In this work, the exploitation of selective sampling in conjugation with electrospray ionization mass spectrometry (ESI-MS) is presented. The transition was not immediately straightforward. A number of major alterations were necessary for ESI interfacing. These adaptations include the choice of a suitable capillary coating for EOF control and the incorporation of organic solvent for efficient ESI. As expected, selective injection of phosphopeptides greatly enhanced the sensitivity of their detection in ESI-MS, particularly for the multiply phosphorylated species that were traditionally most problematic. Furthermore, an electrophoretic separation subsequent to the selective injection of the phosphopeptides was performed prior to analysis by ESI-MS. This allowed us to resolve the multiply phosphorylated peptides present in the samples, predominantly based on the number of phosphorylation sites on the peptides.

Control of Electroosmotic Flow by Polymer Coating: Effects of the Electrical Double Layer

We report on the molecular dynamics simulations of electroosmotic flow control by polymer coating. We show that polymer coating modulates the flow by rendering drag to fluids and by changing the ion distribution and ion-surface interactions in the electrical double layer. Because of the latter two effects, the polymer coating can even enhance the flow under certain conditions. Identifying the effects of these processes is crucial for the rational design of polymer coating for electroosmotic flow control.

Water-Vapor Plasma-Based Surface Activation for Trichlorosilane Modification of PMMA

Separation rates and resolutions within capillary electrophoretic (CE) systems can be enhanced when surface zeta potentials are uniform with minimum deviations from ideal pluglike flow. Microfluidic CE devices based on poly(methyl methacrylate) (PMMA) are being developed due to the optical clarity, availability, stability, and reproducible electroosmotic flow (EOF) rates displayed by this polymer. Control of EOF in polymer-based CE systems can be achieved by surface zeta potential alteration through chemical modification. Herein, a method will be presented for the surface functionalization of PMMA with chemistry analogous to formation of trichlorosilane self-assembled monolayers on SiO2. The current method involves two separate steps. First, surface activation with water-vapor plasma introduces surface hydroxylation. Second, treatment of the plasma-treated PMMA with a substituted trichlorosilane solution forms the functional surface layer. The modified surfaces were characterized using several analytical techniques, including water contact angle, X-ray photoelectron spectroscopy, Fourier transform infrared-attenuated total reflection, secondary ion mass spectroscopy, and measurement of EOF velocities within PMMA microchannels.

Field-effect control of electro-osmotic flow in microfluidic networks

This paper describes microfluidic networks based on glass microchannels with integrated insulated gate electrodes that are used to modify the zeta-potential at specific locations on the wall surface of the microchannels. This modification is used to control direction and size of the electro-osmotic flow (EOF) in the microchannels. A new microfabrication process was developed that compared to earlier work in our institute [R.B.M. Schasfoort, S. Schlautmann, J. Hendrikse, A. van den Berg, Science 286 (1999) 942] leads to mechanically more robust structures and better flow visualization in the gate region. In the new process sequence, the microfluidic chips were fabricated using deposition on a Pyrex glass substrate of a thin-film metal electrode, which was covered with a thin insulating film of silicon dioxide, followed by chemical mechanical polishing prior to bonding to a second glass plate to form closed microchannels. Electrical breakdown of the metal–insulator structures was measured and occurred at 9.6 ± 0.3 MV/cm. Experiments using fluorescent beads to visualize the flow patterns in the microchannel showed that the EOF is linearly dependent on the applied gate voltage. The average EOF could be stopped completely for longitudinal fields of 150 V/cm by applying a gate field of 1.7 MV/cm.

Quantitative study on selective stacking of zwitterions in large‐volume sample matrix by moving reaction boundary in capillary electrophoresis

The paper advanced the theoretical procedures for quantitative design on selective stacking of zwitterions in full capillary sample matrix by a cathodic-direction moving reaction boundary (MRB) in capillary electrophoresis (CE) under control of electroosmotic flow (EOF). With the procedures, we conducted the theoretical computations on the selective stacking of two test analytes of L-histidine (His) and L-tryptophan (Trp) by the MRB created with 30 mM pH 3.0 formic acid-NaOH buffer and 2-80 mM sodium formate. The results revealed the following three predictions. At first, the MRB cannot stack His and Trp plugs if less than 12.5 mM sodium formate is used to form the MRB and prepare the sample matrix. Second, the MRB can stack His and/or Trp sample plugs completely if higher than 50 mM sodium formate is chosen to form the MRB. Third, the MRB can only focus His plug completely, but stack Trp plug partially if 20-50 mM sodium formate is used; this implied the complete MRB-induced selective stacking to His rather than Trp. All the three predictions were quantitatively proved by the experiments. With great dilution of sample matrix and control of EOF, controllable, simultaneous and MRB-induced selective stacking and separation of zwitterions were achieved. The theoretical results hold evident significances to the quantitative design of selective stacking conditions and the increase of detection sensitivity of zwitterions in CE. In addition, the control of EOF by cetyltrimethylammonium bromide (CTAB) can evidently improve the stacking efficiency to both His and Trp.

Control and Quenching of Electroosmotic Flow with End-Grafted Polymer Chains

Control and Quenching of Electroosmotic Flow with End-Grafted Polymer Chains

Rapid separation of protein isoforms by capillary zone electrophoresis with new dynamic coatings

Many cellular functions are regulated through protein isoforms. Changes in the expression level or regulatory dysfunctions of isoforms often lead to developmental or pathological disorders. Isoforms are traditionally analyzed using techniques such as gel- or capillary-based isoelectric focusing. However, with proper electro-osmotic flow (EOF) control, isoforms with small pI differences can also be analyzed using capillary zone electrophoresis (CZE). Here we demonstrate the ability to quickly resolve isoforms of three model proteins (bovine serum albumin, transferrin, alpha1-antitrypsin) in capillaries coated with novel dynamic coatings. The coatings allow reproducible EOF modulation in the cathodal direction to a level of 10(-9) m2V(-1)s(-1). They also appear to inhibit protein adsorption to the capillary wall, making the isoform separations highly reproducible both in peak areas and apparent mobility. Isoforms of transferrin and alpha1-antitrypsin have been implicated in several human diseases. By coupling the CZE isoform separation with standard affinity capture assays, it may be possible to develop a cost-effective analytical platform for clinical diagnostics.

Separation of fatty alcohol polyethoxylates by capillary electrophoresis through easy electroosmotic flow control with a quaternary diammonium salt

Separation of fatty alcohol polyethoxylates by capillary electrophoresis through easy electroosmotic flow control with a quaternary diammonium salt

Separation of fatty alcohol polyethoxylates by capillary electrophoresis through easy electroosmotic flow control with a quaternary diammonium salt

An optimised protocol for the fast and reliable analysis of fatty alcohol polyethoxylates (FAEs), with resolution between the alkyl chain and ethylene oxide (EO) oligomers, is reported. Phthalic and maleic anhydrides were used to quickly form the hemiesters, which were separated by capillary electrophoresis. Effective reduction of the electroosmotic flow to very low values was achieved by previously rinsing the capillary with a quaternary diammonium salt, which remains strongly adsorbed to the silica wall. The phthalic hemiesters of a FAE mixture with 10 carbon atoms in the alkyl chain, and an average EO number of 6, were separated in a background electrolyte (BGE) containing 25 mM borate buffer of pH 9.0 in water. The same capillary pre-treatment was used to separate mixtures of the maleic hemiesters of hydrophobic FAEs, having 12–16 carbon atoms in the alkyl chain and an average EO number of 3. Full resolution of all the oligomers was achieved by using a finely tuned BGE in which a 35% borate buffer was mixed with 65% acetonitrile.

Control of electroosmotic flow and wall interactions in capillary electrophoresis capillaries by photografted zwitterionic polymer surface layers.

A novel capillary with covalently bonded zwitterionic surface modification was prepared by photograft polymerization of the zwitterionic monomer N,N-dimethyl-N-methacryloxyethyl-N-(3-sulfopropyl)ammonium betaine, onto the inner surface of a UV-transparent fused-silica capillary. Although the zwitterionic moieties in the resulting polymeric "tentacles" comprise both a positive quaternary ammonium group and a negative sulfonate group, the coating has a net zero charge. The electroosmotic flow (EOF) was therefore extensively suppressed on the grafted capillary compared to the native silica capillary and to the silica capillary that had been activated for graft polymerization by reaction with 3-(methacryloyl)oxypropyltrimethoxysilane. It was also found that the EOF can be varied by adding chaotropic anions or divalent cations such as perchlorate ion and magnesium ion to the running buffer, due to the interaction between these ions and zwitterionic functional group. This provides a new way of altering the EOF and the wall interaction without changing the pH or the overall ionic strength of the separation buffer. The influence of pH and ionic strength of separation buffer on the EOF were also investigated to optimize the separation conditions. Good separations of a mixture containing eight inorganic anions were achieved within 5 min under optimal conditions by capillary zone electrophoresis. The newly prepared capillary was also well suited for the separation of peptides or proteins.