Observing life at the nanoscale

International audienc

Atomic Force Microscopy on microbial surfaces

International audienceDans ce workshop/atelier de la conférence Mifobio de 2021, nous présentons l'utilisation de l'AFM pour imager la surface des micro-organismes et pour sonder les propriétés hydrophobes avec des sondes AFM fonctionnalisées. Un exemple est donné sur une microalgue

Supported enzymes for antibiofilm protection

National audienc

Probing Bacterial Adhesion at the Single-Molecule and Single-Cell Levels by AFM-Based Force Spectroscopy

International audienc

Supported enzymes for antibiofilm protection

National audienc

Microbial adhesion and ultrastructure from the single-molecule to the single-cell levels by Atomic Force Microscopy

International audienc

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

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

The microbial adhesive arsenal deciphered by atomic force microscopy

International audienc

Electrostatic interactions between soft nanoparticles beyond the Derjaguin approximation: effects of finite size of ions and charges, dielectric decrement and ion correlations

International audienceHypothesis. Electrostatic interactions between colloids are governed by the overlap of their electric double layers (EDLs) and the ionic screening of the structural charges distributed at their core surface and/or in their peripheral ion-permeable shell, relevant to soft particles like polymer colloids and microorganisms. Whereas ion size-mediated effects on the organization of isolated EDLs have been analysed, their contribution to the electrostatic energy of interacting soft particles has received less attention. Theory and simulations. Herein, we elaborate a formalism to evaluate the electrostatic interaction energy profile between spherical core/shell particles, building upon a recent Poisson-Boltzmann theory corrected for the sizes of ions and particle structural charges, for ion correlations and dielectric decrement. Interaction energy is derived from pairwise disjoining pressure and exact Surface Element Integration method, beyond the Derjaguin approximation. The theory is sufficiently flexible to tackle homoand hetero-interactions that involve weakly to highly charged hard, porous or core/shell nano-to micro-sized particles in asymmetric multivalent electrolytes. Findings. Results illustrate how ion steric effects, ion correlations and dielectric decrement impact the sign, magnitude and range of the interactions depending on the particle size, the Debye length, and the geometric and electrostatic properties of the particle core and shell components