Jacques Josserand

Ion current rectification and rectification inversion in conical nanopores: a perm-selective view

Ionic transport in charged conical nanopores is known to give rise to ion current rectification. The present study shows that the rectification direction can be inverted when using electrolyte solutions at very low ionic strengths. To elucidate these phenomena, electroneutral conical nanopores containing a perm-selective region at the tip have been investigated and shown to behave like classical charged nanopores. An analytical model is proposed to account for these rectification processes.

Protein adsorption in static microsystems: effect of the surface to volume ratio{

A numerical model for the adsorption kinetics of proteins on the walls of a microchannel has been developed using the finite element method (FEM) to address the coupling with diffusion phenomena in the restricted microchannel volume. Time evolutions of the concentration of one species are given, both in solution and on the microchannel walls. The model illustrates the adsorption limitation sometimes observed when the microdimensions of these systems induce a global depletion of the bulk solution. A new non-dimensional parameter is introduced to predict the final value of the coverage of any microsystem under static adsorption. A working curve and a criteria (h/K[Gamma](max) > 10) are provided in order to choose, for given adsorption characteristics, the value of the volume-to-surface ratio (i.e. the channel height h) avoiding depletion effects on the coverage (relative coverage greater than 90% of the theoretical one). Simulations were compared with confocal microscopy measurements of IgG antibody adsorption on the walls of a PET microchannel. The fit of the model to the experimental data show that the adsorption is under apparent kinetic control.

Iontophoretic Fraction Collection for Coupling Capillary Zone Electrophoresis with Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

An automated fraction collection interface has been developed for coupling capillary electrophoresis (CE) with matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). This fraction collection approach is based on electromigration and diffusion and does not rely on the presence of a liquid junction, sheath-liquid, electro-osmotic flow, or a superimposed hydrodynamic flow. Neutrally coated capillary with negligible electro-osmosis can thus be used to provide high-efficiency separations of biological compounds. The in-capillary separation resolution is totally independent from the spotting process. CE-separated species can be collected either in a multiwell plate or directly on a MALDI target. In the present work, an eight-protein mixture, submitted to trypsin proteolysis, has been used as a sample test and separations have been conducted in 50 microm i.d. neutrally coated capillaries. As compared to direct MALDI MS analysis, the integration of CE improved the number of detected peptides from 36 to 87 and the average sequence coverage from 24% to 38%. Internal calibration was used, and an average mass accuracy of 16.1 ppm is reported. Finally, diffusion-migration numerical simulations of the iontophoretic fraction collection process have been carried out.

Finite element simulation of Off-Gel™ buffering

The protonation of an aqueous solution of two ampholytes AH and BH next to a gel buffered by immobilized acid moieties IH has been studied by finite element simulation in an iterative scheme. A ten species model has been formulated, taking into account transient diffusion and equilibrium kinetics of the two amphoteric species AH and BH, of water and of the immobilized species IH. This model has been developed to illustrate the pH evolution between an ampholyte solution and an Immobiline gel, and to study the influence of the Immobiline concentration on protons and ampholyte distributions. It has been demonstrated that a minimum initial Immobiline concentration of 10−2 M is necessary to maintain the pH in the gel in contact with a closed chamber, when the difference between the isoelectric points of AH and BH is 4 and when the initial concentration of the ampholytes in solution is in the micromolar range. This approach provides a first theoretical framework for the recently developed Off‐Gel™ electrophoresis.

Contact Galvani potential differences at liquidjliquid interfaces Part II. Contact diffusion potentials in microsystems

The purpose of this work is to study diffusion potentials (i.e., liquid junction potentials) established between two static or flowing solutions in microsystems. One of the motivations of investigating the diffusion potential distribution is to be able to establish a potential gradient in a cell without introducing electrodes and using a potentiostat. By using finite element simulations, different geometries (i.e., microhole, Y-channel mixing and microtube injection) have been studied numerically. The calculations have allowed systematic studies of the influence of concentration ratio, flow rate and detector position. It is shown that the diffusion potential can be a useful way to quantify the degree of mixing or filling of solutions in microsystems. The theoretical part has been corroborated by experimental measurements of potential differences across a parallel flow channel, taking into account the diffusion potential and differences in the electrode potentials. It is apparent that the theoretical model gives a good fit to the experimental results.

Hydro-voltaic cells. Part I. Concentration cells

The diffusion potential differences established between two laminar flows of electrolyte solutions of different concentrations are exploited to generate an electric current. It is shown that when a single redox couple is present in two flowing electrolyte solutions, the diffusion potential difference can induce a redox reaction at electrodes placed on the opposite walls of the cell, the electrical current being extracted in an external circuit. This hydro-voltaic cell operates solely on a salt concentration gradient and a hydrodynamic flow. In this work, we have investigated the influence of the salt concentration gradient and the concentration of the redox couple on the generated power.

Contact Galvani potential differences at liquid liquid interfaces. Part I: Experimental studies on single salt distribution at liquid liquid interfaces using a streaming technique

The distribution potential established when two liquids are placed in contact has been measured using a streaming technique. In particular the contributions from the diffusion potentials have been quantified. On the basis of the experimental results, the concept of distribution potentials upon the partition of a salt between two phases is revisited. We also compare Galvani potential differences for solutions in equilibrium and for situations where two liquids are placed in contact, as encountered in micro-TAS systems and micro-reactors. Finally, it is shown that in potentiometry we can define, as in traditional amperometry, a half-wave potential that takes into account the mass transfer of the salt to the interface.

Electrochemical multi-tagging of cysteinyl peptides during microspray mass spectrometry: numerical simulation of consecutive reactions in a microchannel

On-line electrogeneration of mass tags in a microspray emitter is used to quantify the number of cysteine groups in a given peptide. A finite-element simulation of the multi-step process yields the relative distribution and concentration of tags, untagged and tagged species in the microchannel before the spray event. The work focuses on the tagging of cysteine moieties in peptides or proteins by electrogenerated quinone mass probes. The main chemical parameters determining the kinetics of the labelling are assessed and discussed considering the microfluidic aspects of the process. The control of the tagging extent allows the simultaneous MS analysis of both the unmodified and modified peptide(s). The number of cysteine groups corresponds to the number of characteristic mass shifts observed from the unmodified peptide. The present theoretical work establishes the range of optimum conditions for the determination of the number of cysteine groups in peptides containing up to five cysteine groups.

Coplanar interdigitated band electrodes for electrosynthesis. Part 5: finite element simulation of paired reactions

Paired reactions on interdigitated anode/cathode were studied by finite element simulations and compared to those occurring in classical parallel plate electrochemical cells. The results clearly show that coplanar interdigitated electrode systems provide an advantageous geometry for paired reactions between two products electrochemically generated on both anode and cathode. The influence of the reaction rate constant, the inter-electrode gap and the flow rate, on the concentration profile and on the conversion rate is investigated. Due to the alignment of the electrodes on the same flow line, diffusion layer overlapping is more easily obtained with this geometry.

Ring magnets for magnetic beads trapping in a capillary

This paper introduces the concept of ring magnets for magnetic beads (MBs) trapping in a capillary. Such magnets enable an easy insertion of a capillary simply like a pearl on a string. With this system, high magnetic forces are obtained thanks to the proximity between the magnet and the capillary, giving the opportunity to work at higher flow rates than with classical setups using two magnets with their magnetization perpendicular to the capillary. Moreover, by alternating magnets and non-magnetic spacers either in attraction or repulsion configuration, it is possible to form a chain and as a consequence to adapt the number of magnets to the desired number of plugs, thus controlling the surface available for molecule binding. Magnetic force mapping was first carried out by numerical simulations for a single ring magnet. The usefulness of this concept was then demonstrated with the achievement of an immunoassay and an online preconcentration experiment. To study the formation of multiplugs, the magnetic force was first simulated for a chain of four magnets in repulsion. This force was then introduced into a convection-diffusion model to understand the influence of the flow velocity on their size and position. The numerical simulations were qualitatively corroborated by microscopic visualizations, carried out in a capillary placed between rectangular magnets having a magnetization parallel to the capillary, and quantitatively by bead capture efficiency experiments.