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Concept: Electroosmotic flow


By use of synchrotron X-ray fluorescence and Rutherford backscattering spectrometry, we show the SU-8 soft lithographic process contaminates PDMS. Residues of the antimony containing photoinitiator are transferred from the master mold to the surface of PDMS, uncontrollably intensifying the surface potential, leading to electroosmotic flow variability in PDMS microfluidic devices.

Concepts: Electron, Spectroscopy, Fluid dynamics, Microfluidics, Lithography, Mold, Auger electron spectroscopy, Electroosmotic flow


An improved method for the concurrent determination and separation of cations and anions by microchip electrophoresis with capacitively coupled contactless conductivity detection (ME-C4 D) is described. Two kinds of microchip structures were designed. The first microchip has a long bent separation channel. And for the defects of the first microchip, the second microchip with a Y-type separation channel has been proposed. The background electrolyte (BGE) composed of 20 mm His/MES and 0.01 mm CTAB was optimized for inhibiting the electroosmotic flow (EOF). Due to the low electroosmotic flow, the cations and anions migrate in opposite directions and can be separated from each other. With the precisely controlled high-voltage, cations and anions can be migrated in microchannels according to our requirements and sequentially detected by a C4 D detector built in-house. Samples containing K+ , Na+ , Li+ , Cl- , F- and PO43- were analyzed simultaneously in a single run (within 140 s) by both methods. The reproducibility obtained by both methods remained below 5% for migration time and within 3.5 - 9.1% for peak areas. The proposed concurrent determination methods are inexpensive, simple, fast, ease of operation, high degree of integration. This article is protected by copyright. All rights reserved.

Concepts: Cathode, Ion, Human migration, Physical chemistry, All rights reserved, Copyright, Legal separation, Electroosmotic flow


Plastic microchips have been broadly used as disposable microfluidic devices, but the poorly defined surface properties limit their application. Herein, we proved that an anionic polymer could be used as the background electrolyte (BGE) to provide a strong and stable cathodic electroosmotic flow (EOF) and modulate the electrophoretic behavior for efficient separation in relative thicker microchannels (∼75μm id). A cathodic EOF of ∼3.3×10(-4)cm(2)V(-1)s(-1) was maintained using sodium polystyrene sulfonate (PSSNa) with a molecular weight of 5×10(5) as the BGE, which ensured fluorescein isothiocyanate labeled biogenic amines (BAs) appeared ahead of other components in the electropherograms obtained with microchips of cyclic olefin copolymer. Four selected BAs appeared within 50s and theoretical plate numbers of 8.0×10(5)/m were achieved. The role of PSSNa was evaluated with streaming potential, dynamic light scattering, contact angle and atomic force microscopy. Its functionalities as surface modifier, viscosity regulator and pseudostationary phase were also confirmed. The proposed electrophoretic method was applied in the fast determination of BAs in fish meat samples.

Concepts: Polymer, Capillary electrophoresis, Fluorescein, Colloidal chemistry, Light scattering, Dynamic light scattering, Sodium polystyrene sulfonate, Electroosmotic flow


We present a novel electrokinetic stacking (ES) method based on field amplification on a simple paper device for sample preconcentration. With voltage application, charged probe ions in a solution of lower conductivity stack and form a narrow band at the boundary between the sample and the background electrolyte of higher conductivity. The stacking band appears quickly and stabilizes in a few minutes. With this ES method, three orders of magnitude signal improvement was successfully achieved for both a fluorescein probe and a double-stranded DNA within 300 s. This enhanced stacking efficiency is attributed to a focusing effect due to the balance between electromigration and counter electroosmotic flow. We also applied this ES method to other low-cost fiber substrates such as cloth and thread. Such a simple and highly efficient ES method will find wide applications in the development of sensitive paper-based analytical devices (PADs), especially for low-cost point-of-care testing (POCT).

Concepts: DNA, Base pair, Device, Physical chemistry, Efficiency, Stacking, Stack, Electroosmotic flow


In this work, a new sulfonated polystyrene based porous layer was synthesized on the wall of a capillary by a single step in situ polymerization process. To obtain a capillary suited for electrochromatography, vinylsulfonic acid (VSA) was, next to divinylbenzene (DVB), copolymerized to induce charges for the electroosmotic flow (EOF) generation. The VSA ratio in the monomer mixture and the polymerization time were optimized while the chromatographic characteristics of the obtained open tubular columns were investigated in electrochromatography. To allow unambiguous study of only chromatographic processes, evaluations were performed with a mixture of sufficiently retained and electrophoretically neutral parabens. Comparison of SEM pictures and chromatograms revealed that the polymerization time had a great influence on the polymer layer morphology and on the chromatographic performance. An increase in the VSA ratio, led to an increase in the mobile phase velocity but simultaneously lowered paraben retention. The novel optimized stationary phase could generate a stable and significant electro-osmotic flow (EOF) of 1.1mm/s over a wide pH range which could be produced in a reproducible manner. Minimal plate heights of 10μm, equivalent to the capillary internal diameter, were obtained. The open-tubular character of this optimized porous layer column allowed successful analyses at elevated temperature, resulting in a maximum efficiency of 85,500 plates for a 75cm capillary and linear velocities up to 1.4mm/s. Finally, a thermal gradient was successfully applied, leading to artificial sharpened peaks with a peak capacity of 55 in a 20min time span.

Concepts: Protein, Polymer, Chromatography, Analytical chemistry, Gas chromatography, Polymerization, Van Deemter equation, Electroosmotic flow


In this paper, we unravel new scaling regimes for streaming potential and electroviscous effects in a nanocapillary with thick overlapping Electric Double Layers (EDLs). We observe that the streaming potential, for a given value of the capillary zeta (ζ) potential, varies with the EDL thickness and a dimensionless parameter R, quantifying the conduction current. Depending on the value of R, variation of the streaming potential with the EDL thickness demonstrates distinct scaling regimes: one can witness a Quadratic Regime where the streaming potential varies as the square of the EDL thickness, a Weak Regime where the streaming potential shows a weaker variation with the EDL thickness, and a Saturation Regime where the streaming potential ceases to vary with the EDL thickness. Effective viscosity, characterizing the electroviscous effect, obeys the variation of the streaming potential for smaller EDL thickness values; however, for larger EDL thickness the electroosmotic flow profile dictates the electroviscous effect, with insignificant contribution of the streaming potential.

Concepts: Electricity, Effect, Effectiveness, Effects unit, Electroosmotic flow


Trans-scleral iontophoresis, i.e. the application of small electric current to enhance drug transport across sclera is an option for non-invasive delivery of corticosteroids to the posterior segment of the eye. In this paper, in vitro trans-scleral iontophoresis of methylprednisolone hemisuccinate was investigated using concentrated drug solutions and short application times to mimic the iontophoretic conditions of in vivo studies. The drug at the donor concentration of 45mg/ml was delivered through isolated porcine sclera under passive and iontophoretic conditions (cathodal, 2.4mA) for 2-15minutes. In a second set of experiments, the drug was delivered for 5minutes at current intensities of 0.9-7.2mA. After donor removal, drug release was followed up to 24hours. The exposure of concentrated solutions to sclera for 2-15minutes under passive conditions caused a notable accumulation of drug up to 0.8mg/cm(2), the release of which was successively followed for 24hours. In cathodal iontophoresis, the amount of accumulated drug increased proportionally to the charge between 0.3 and 1.44 Coulomb. When the charge was increased to 2.16C by increasing the application time or current intensity, no further enhancement was recorded. This behaviour can be ascribed to substantial drug adsorption on the scleral tissue, as demonstrated through streaming potential studies, with the consequent increase of the electroosmotic flow that opposes drug transport. The set up suggested here could help in defining the optimal conditions for in vivo studies with animal models and reducing the number of in vivo experiments.

Concepts: Electron, Electric charge, Electricity, Electric current, In vivo, Corticosteroid, In vitro, Electroosmotic flow


Polystyrene (PS) nanoparticles coated by bovine serum albumin (BSA), hereafter denoted as PS/BSA, were prepared and chemically immobilized for the first time onto a capillary inner wall for open tubular capillary electrochromatography (OTCEC). Electroosmotic flow and scanning electron micrography were used to characterize the prepared nanoparticle-coated capillaries. To investigate the performance of the prepared columns in OTCEC, chiral separation of d,l-tryptophan (dl-Trp) was performed in monolayer BSA-modified capillary and PS/BSA nanoparticle-coated columns. The results indicated that the nanoparticle-modified column afforded a higher resolution compared with the monolayer type. Rapid enantioseparation of dl-Trp (within 3 min) was achieved with the PS/BSA-immobilized column using an electroosmotic pump-assisted CEC. Enantiomer separations of other compounds like dl-tyrosine and warfarin were also achieved with the column. Besides, run-to-run and column-to-column repeatabilities of the PS/BSA-coated column in the chiral separation were systematically introduced.

Concepts: Blood, Capillary, Serum albumin, Bovine serum albumin, Separation, Column, Structural system, Electroosmotic flow