Application of a high-pressure electro-osmotic pump using nanometer silica in capillary liquid chromatography

Electroosmosis Dominates Electrophoresis of Antibiotic Transport Across the Outer Membrane Porin F

We report that the dynamics of antibiotic capture and transport across a voltage-biased OmpF nanopore is dominated by the electroosmotic flow rather than the electrophoretic force. By reconstituting an OmpF porin in an artificial lipid bilayer and applying an electric field across it, we are able to elucidate the permeation of molecules and their mechanism of transport. This field gives rise to an electrophoretic force acting directly on a charged substrate but also indirectly via coupling to all other mobile ions, causing an electroosmotic flow. The directionality and magnitude of this flow depends on the selectivity of the channel. Modifying the charge state of three different substrates (norfloxacin, ciprofloxacin, and enoxacin) by varying the pH between 6 and 9 while the charge and selectivity of OmpF is conserved allows us to work under conditions in which electroosmotic flow and electrophoretic forces add or oppose. This configuration allows us to identify and distinguish the contributions of the electroosmotic flow and the electrophoretic force on translocation. Statistical analysis of the resolvable dwell times reveals rich kinetic details regarding the direction and the stochastic movement of antibiotics inside the nanopore. We quantitatively describe the electroosmotic velocity component experienced by the substrates and their diffusion coefficients inside the porin with an estimate of the energy barrier experienced by the molecules caused by the interaction with the channel wall, which slows down the permeation by several orders of magnitude.

Advancement of electroosmotic pump in microflow analysis: A review

This review (with 152 references) covers the progress made in the development and application of electroosmotic pumps in a period from 2009 through 2018 in microflow analysis. Following a short introduction, the review first categorizes various electroosmotic pumps into five subclasses based on the materials used for pumping: i) open channel EOP, 2) packed-column EOP, iii) porous monolith EOP, iv) porous membrane EOP, and v) other types of EOP. Pumps in each subclass are discussed. A next section covers EOP applications, primarily the applications of EOPs in micro flow analysis and micro/nano liquid chromatography. Other scattered applications are also examined. Perspectives, trends and challenges are discussed in the final section.

Impact of ionization equilibrium on electrokinetic flow of weak electrolytes in nanochannels

Weak electrolyte transport in nanochannels or nanopores has been actively explored in recent experiments. In this paper, we establish a new electrokinetic model where the ionization balance effect of weak electrolytes is outlined, and performed numerical calculations for H₃PO₄ concentration-biased nanochannel systems. By considering the roles of local chemical equilibrium in phosphorous acid ionization, the simulation results show quantitative agreement with experimental observations. Based on the model, we predict that enhanced energy harvesting capacity could be accomplished by utilizing weak electrolytes compared to the conventional strong electrolyte approaches in a concentration gradient-based power-generating system.

Electroosmotic Flow in Microchannel with Black Silicon Nanostructures

Although electroosmotic flow (EOF) has been applied to drive fluid flow in microfluidic chips, some of the phenomena associated with it can adversely affect the performance of certain applications such as electrophoresis and ion preconcentration. To minimize the undesirable effects, EOF can be suppressed by polymer coatings or introduction of nanostructures. In this work, we presented a novel technique that employs the Dry Etching, Electroplating and Molding (DEEMO) process along with reactive ion etching (RIE), to fabricate microchannel with black silicon nanostructures (prolate hemispheroid-like structures). The effect of black silicon nanostructures on EOF was examined experimentally by current monitoring method, and numerically by finite element simulations. The experimental results showed that the EOF velocity was reduced by 13 ± 7%, which is reasonably close to the simulation results that predict a reduction of approximately 8%. EOF reduction is caused by the distortion of local electric field at the nanostructured surface. Numerical simulations show that the EOF velocity decreases with increasing nanostructure height or decreasing diameter. This reveals the potential of tuning the etching process parameters to generate nanostructures for better EOF suppression. The outcome of this investigation enhances the fundamental understanding of EOF behavior, with implications on the precise EOF control in devices utilizing nanostructured surfaces for chemical and biological analyses.

Review: Electric field driven pumping in microfluidic device

Pumping of fluids with precise control is one of the key components in a microfluidic device. The electric field has been used as one of the most popular and efficient nonmechanical pumping mechanism to transport fluids in microchannels from the very early stage of microfluidic technology development. This review presents fundamental physics and theories of the different microscale phenomena that arise due to the application of an electric field in fluids, which can be applied for pumping of fluids in microdevices. Specific mechanisms considered in this report are electroosmosis, AC electroosmosis, AC electrothermal, induced charge electroosmosis, traveling wave dielectrophoresis, and liquid dielectrophoresis. Each phenomenon is discussed systematically with theoretical rigor and role of relevant key parameters are identified for pumping in microdevices. We specifically discussed the electric field driven body force term for each phenomenon using generalized Maxwell stress tensor as well as simplified effective dipole moment based method. Both experimental and theoretical works by several researchers are highlighted in this article for each electric field driven pumping mechanism. The detailed understanding of these phenomena and relevant key parameters are critical for better utilization, modulation, and selection of appropriate phenomenon for efficient pumping in a specific microfluidic application.

Ionic Origin of Electro-osmotic Flow Hysteresis

Electro-osmotic flow, the driving of fluid at nano- or micro-scales with electric field, has found numerous applications, ranging from pumping to chemical and biomedical analyses in micro-devices. Electro-osmotic flow exhibits a puzzling hysteretic behavior when two fluids with different concentrations displace one another. The flow rate is faster when a higher concentration solution displaces a lower concentration one as compared to the flow in the reverse direction. Although electro-osmotic flow is a surface phenomenon, rather counter intuitively we demonstrate that electro-osmotic flow hysteresis originates from the accumulation or depletion of pH-governing minority ions in the bulk of the fluid, due to the imbalance of electric-field-induced ion flux. The pH and flow velocity are changed, depending on the flow direction. The understanding of electro-osmotic flow hysteresis is critical for accurate fluid flow control in microfluidic devices, and maintaining of constant pH in chemical and biological systems under an electric field.

High-performance liquid chromatography separation of unsaturated organic compounds by a monolithic silica column embedded with silver nanoparticles

The optimization of a porous structure to ensure good separation performances is always a significant issue in high-performance liquid chromatography column design. Recently we reported the homogeneous embedment of Ag nanoparticles in periodic mesoporous silica monolith and the application of such Ag nanoparticles embedded silica monolith for the high-performance liquid chromatography separation of polyaromatic hydrocarbons. However, the separation performance remains to be improved and the retention mechanism as compared with the Ag ion high-performance liquid chromatography technique still needs to be clarified. In this research, Ag nanoparticles were introduced into a macro/mesoporous silica monolith with optimized pore parameters for high-performance liquid chromatography separations. Baseline separation of benzene, naphthalene, anthracene, and pyrene was achieved with the theoretical plate number for analyte naphthalene as 36,000 m(-1). Its separation function was further extended to cis/trans isomers of aromatic compounds where cis/trans stilbenes were chosen as a benchmark. Good separation of cis/trans-stilbene with separation factor as 7 and theoretical plate number as 76,000 m(-1) for cis-stilbene was obtained. The trans isomer, however, is retained more strongly, which contradicts the long- established retention rule of Ag ion chromatography. Such behavior of Ag nanoparticles embedded in a silica column can be attributed to the differences in the molecular geometric configuration of cis/trans stilbenes.

Lab on a chip packing of submicron particles for high performance EOF pumping

The packing of submicrometer sized silica beads inside a microchannel was enabled by a novel method which avoids the complication and limitations of generating a frit using conventional approaches and the restriction of flow using a submicrometer sized weir. A micrometer sized weir and two short columns of 5 μm and 800 nm silica beads packed in succession behind the weir together functioned as a high pressure frit to allow the construction of a primary packed bed of 390 nm silica beads. This packed bed microchannel was tested as an EOF pump, wherein it exhibited superior performance with regards to pressure tolerance, i.e., sustaining good flow rate under extremely high back pressure, and maximal pressure generation. Under a modest applied electric field strength of 150 V/cm, the flow rate against a back pressure of 1200 psi (∼8.3 MPa) was 40 nL/min, and the maximal pressure reached 1470 psi (∼10 MPa). This work has demonstrated that it is possible to create a high performance packed bed microchannel EOF pump using nanometer sized silica beads, as long as proper care is taken during the packing process to minimize the undesirable mixing of two different sized particles at the boundaries between particle segments and to maximize the packing density throughout the entire packed bed.

Ion exchange resin bead decoupled high-pressure electroosmotic pump

We describe an electroosmotic pump (EOP) that utilizes a cation exchange resin bead as the electric field decoupler. The resin bead serves as a electrical grounding joint without fluid leakage, thus eliminating electrolytic gas interference from the flow channels. The arrangement is easy to practice from readily available components, displays a very low electrical resistance, and is capable of bearing high backpressure (at least 3200 psi). We use a silica xerogel column as the EOP element to pump water and demonstrate a complete capillary ion chromatograph (CIC), which uses a similar bead based microelectrodialytic generator (micro-EDG) to generate a KOH eluent from the pumped water. We observed good operational stability of the complete arrangement over long periods.

A fritless, EOF microchip pump for high pressure pumping of aqueous and organic solvents

A fritless, microchip electroosmotic flow (EOF) pump is microfabricated and demonstrated on a planar soda lime glass substrate to be capable of supplying reasonable flow rates under high back pressures, such as that required for micro-high pressure liquid chromatography (micro-HPLC). The microchip EOF pump is composed of a densely packed microchannel containing 800 nm silica particles and was capable of generating a maximum pressure > 1000 psi ( approximately 7 MPa) and a maximum flow rate of 282 nL/min (aqueous cyclohexylamino alkyl sulfonate (CHES) buffer, 10 mM, pH 9.0, 200 V/cm). Other pumping fluids, such as CHES buffer-acetonitrile mixture (50%, v/v), CHES buffer-methanol mixture (50%, v/v), and pure acetonitrile were also used in a characterization of pump performance that included determinations of the maximum flow rate, maximum pressure, and resulting flow rate against an applied, downstream back pressure. The flow rate under a 200 psi ( approximately 1.4 MPa) back pressure at an applied electric field strength of 250 V/cm ranged from 285 nL/min for aqueous CHES buffer to 44 nL/min for CHES buffer-acetonitrile mixture (50%, v/v), indicating that this EOF pump will meet the future requirements of a micro-HPLC system.

Electroosmotic pumps and their applications in microfluidic systems

Electroosmotic pumping is receiving increasing attention in recent years owing to the rapid development in micro total analytical systems. Compared with other micropumps, electroosmotic pumps (EOPs) offer a number of advantages such as creation of constant pulse-free flows and elimination of moving parts. The flow rates and pumping pressures of EOPs matches well with micro analysis systems. The common materials and fabrication technologies make it readily integrateable with lab-on-a-chip devices. This paper reviews the recent progress on EOP fabrications and applications in order to promote the awareness of EOPs to researchers interested in using micro- and nano-fluidic devices. The pros and cons of EOPs are also discussed, which helps these researchers in designing and constructing their micro platforms.

Application of an in-situ Thermo-polymerized Porous Polymer: Creation of an On-column Frit for a Packed Capillary HPLC Column

A 3-mm length of a porous monolithic polymer was prepared in a 0.32-mm inner-diameter fused-silica capillary by an in-situ thermo-polymerization method and used as an on-column frit for a packed capillary HPLC column. The on-column frit can resist high pressure up to 400 bar. A 5-microm packing material was packed in the capillary with the on-column frit by a slurry method. At pressure driving mode, separation of samples was performed using the capillary HPLC column. The in-situ frit preparation method has the advantages of easy preparation, easy control of the location of the frit and a mild preparing reaction condition.

Applications of nanomaterials in liquid chromatography: Opportunities for separation with high efficiency and selectivity

During recent decades, great efforts have been made to improve the chemical stability, selectivity, and separation efficiency of stationary phases in liquid chromatography. Significant progress has been achieved, especially after the introduction of nanomaterials into separation science. This review covers the applications of nanomaterials playing various roles in liquid chromatography. Future possibilities for developing nanomaterial-based stationary phases are also discussed.