Bimodal stringence-mediated response of metal-detecting luminescent whole cell bioreporters: experimental evidence and quantitative theory

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Ultra-slow diffusion of hexacyanoferrate anions in poly(diallyldimethyl ammonium chloride)-poly(acrylic acid sodium salt) multilayer films

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Hetero-interaction between Gouy–Stern double layers: Charge and potential regulation

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Decoding the Time-Dependent Response of Bioluminescent Metal-Detecting Whole-Cell Bacterial Sensors

International audienceThe signal produced by aqueous dispersions of bioluminescent, metal-responsive whole-cell bacterial sensors is indicative of the concentration of bioavailable metal ions in solution. The conventional calibration-based strategy followed for measuring this concentration is however inadequate to provide any quantitative prediction of the cells response over time as a function of e.g. their growth features, their defining metal accumulation properties, or the physicochemical medium composition. Such an evaluation is still critically needed for assessing on a mechanistic level the performance of biosensors in terms of metal bioavailability and toxicity monitoring. Herein we report a comprehensive formalism unraveling how the dependence of bioluminescence on time is governed by the dynamics of metal biouptake, by the activation kinetics of lux-based reporter gene, by the ensuing rate of luciferase production, the kinetics of light emission and quenching. It is shown that bioluminescence signal corresponds to the convolution product between two time-dependent functions, one detailing the dynamic interplay of the above micro-and nanoscale processes, and the other pertaining to the change in concentration of photoactive cell sensors over time. Numerical computations illustrate how the shape and magnitude of the bioluminescence peak(s) are intimately connected to the dependence of the photoactive cells concentration on time and to the magnitudes of Deborah numbers that compare the relevant timescales of the biointerfacial and intracellular events controlling light emission. Explicit analytical expressions are further derived for practical situations where bioluminescence is proportional to the concentration of metal ions in solution. The theory is further quantitatively supported by experiments performed on luminescent cadmium-responsive lux-based Escherichia coli biosensors

Faradaic double layer depolarization in electrokinetics: Onsager relations and substrate limitations

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Coupling between electrokinetics and electrode kinetics by bipolar faradaic depolarisation processes in microfluidic channels

International audienceThis article is concerned with the nature and impact of bipolar faradaic electron transfer processes in the context of measuring electrokinetic parameters at the interface between an electronically conductive substrate such as a solid metal layer, and a liquid medium. More specifically, it analyses the steady state electric current through the electrodic substrate layer in terms of its short-circuiting effect on the system's electrokinetic quantities, such as the streaming potential. Ample attention is paid to the electrodic behaviour of the chosen metal and its electron transfer characteristics with respect to redox functions in the medium. The electrochemical reversibility of redox couple species is expressed in terms of their oxidation and reduction rate constants as compared to their diffusive transport rates under lateral flow conditions. High values for rate constants lead to high reversibilities and large bipolar leaking currents through the metal substrate. In turn, high electron transfer rate constants generate large reductions in measured values for electrokinetic quantities such as streaming potentials that become a non-linear function of the pressure gradient applied through the fluidic chamber. The present article presents an overview of theoretical and experimental approaches of this intricate coupling between bipolar electrode kinetics and electrokinetics and the impact from Hans Lyklema's contributions. It highlights not only the implications of bipolar faradaic depolarisation processes in electrokinetics but also the importance of bipolar electrochemistry principles in various electroanalytical applications reported for e.g. the control of microfluidic flows, for surfaces functionalisation, particles manipulation or for the wireless detection of electroactive analytes

Electrokinetics of soft polymeric interphases with layered distribution of anionic and cationic charges

International audienceSoft surface coatings attract increasing attention due to the versatile options they provide in numerous applications e.g. in the flourishing nanomedicine and nanobiotechnology areas. Optimisation of the performance of such ion- and solvent-permeable polyelectrolytic materials requires a detailed understanding of their electrostatic properties. This task is rendered difficult by the inherent non-uniform distribution of their structural charges. In this article, we review recent advances made in the measurement and theory of the electrokinetics (electrophoresis/streaming current) of soft surface coatings that carry spatially-separated cationic and anionic charges. Examples of such charge-stratified systems are polyelectrolyte-coated particles, polyelectrolyte multilayers, particles with zwitterionic interfacial functionality, microbial cells or hard–soft composite interfaces. It is shown here that the electrokinetic features of such colloidal systems are remarkably different from those of their counterparts with homogeneously distributed cationic and anionic charges. In particular, the interplay between electrostatic and hydrodynamic flow fields developed under electrokinetic conditions in the bulk and interfacial compartments of charge-stratified colloids/films are shown to induce a reversal of their electrokinetic response (electrophoretic mobility/streaming current) that depends on the concentration of monovalent electrolyte in solution. The prerequisites for occurrence of such spectacular behaviour are theoretically identified in terms of the Debye length, the spatial length scales defining charge layering, and the typical length for flow penetration within the colloids/films. Electrophoresis and streaming current results recently reported for poly(amidoamine) carboxylated nanodendrimers, natural rubber colloids and poly(ethyleneimine)-supported lipid bilayers are further discussed to illustrate the generic electrokinetic properties of soft interfaces defined by a given stratification of their anionic and cationic structural charges

Conditional existence of Donnan potential in soft particles and surfaces: Dependence on steric effects mediated by electrolyte ions and structural charges

International audienceWhen a charged layer decorating a particle or a macroscopic surface is equilibrated with an electrolyte solution,a constant Donnan potential is established through that layer due to charge-driven accumulation of counterionsand companion exclusion of coions. This situation arises when the thickness of the surface layer well exceeds thescreening Debye length, a condition derived from mean-field Poisson-Boltzmann theory within point-like chargeapproximation. Herein, we revisit this condition underlying the applicability of Donnan electrostatic represen-tation with the account of steric effects mediated by the sizes of the electrolyte ions and structural layer charges.A transcendental equation is derived for the Donnan potential as a function of sizes and valences of anions andcations, electrolyte concentration and size of the layer charges, and a closed-form expression is provided forsymmetrical electrolytes. Therefrom we evidence that the existence of a Donnan potential is conditioned not onlyto large values of the layer thickness compared to a here-defined Debye length operative within the shell, but toadditional verification of a criterion that involves space charge density of the layer, solution ionic strength andelectrolyte nondiluteness parameter. Illustrative computational examples show how the existence and magnitudeof the Donnan potential depend on the key molecular descriptors of the electrolyte and soft interface, and theyfurther quantify the deviations from predictions based on classical Donnan potential expression valid for diluteelectrolytes

Electrostatics of soft (bio)interfaces: Corrections of mean-field Poisson-Boltzmann theory for ion size, dielectric decrement and ion-ion correlations

International audienceElectrostatics of soft (ion-permeable) (bio)particles (e.g. microorganisms, core/shell colloids) in aqueous electrolytes is commonly formulated by the mean-field Poisson-Boltzmann theory and integration of the charge contributions from electrolyte ions and soft material. However, the effects connected to the size of the electrolyte ions and that of the structural charges carried by the particle, to dielectric decrement and ion-ion correlations on soft interface electrostatics have been so far considered at the margin, despite the limits of the Gouy theory for condensed and/or multivalent electrolytes.Accordingly, we modify herein the Poisson-Boltzmann theory for core/shell (bio)interfaces to include the aforementioned molecular effects considered separately or concomitantly. The formalism is applicable for poorly to highly charged particles in the thin electric double layer regime and to unsymmetrical multivalent electrolytes.Computational examples of practical interests are discussed with emphasis on how each considered molecular effect or combination thereof affects the interfacial potential distribution depending on size and valence of cations and anions, size of particle charges, length scale of ionic correlations and shell-to-Debye layer thickness ratio. The origins of here-evidenced pseudo-harmonic potential profile and ion size-dependent screening of core/shell particle charges are detailed. In addition, the existence and magnitude of the Donnan potential when reached in the shell layer are shown to depend on the excluded volumes of the electrolyte ions

On the analysis of ionic surface conduction to unravel charging processes at macroscopic soft and hard solid-liquid interfaces

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