Size-Controlled Nanoparticle Clusters of Narrow Size-Polydispersity Formed Using Multiple Particle Types Through Competitive Stabilizer Desorption to a Liquid-Liquid Interface.

A novel colloidal approach is presented for preparing fully dispersed nanoparticle (NP) assemblies (clusters) of narrow size-polydispersity over a wide range of sizes through irreversible depletion of stabilizing ligands onto a liquid-liquid interface. Unusually, the relative monodispersity of the assemblies continuously improves throughout the process. A detailed kinetics study into the assembly of iron oxide NP clusters shows that the assembly rate decreases with NP concentration, pinpointing the role of the interface in size focusing. A new protocol for identifying initial conditions that enable controlled assembly is described, which allows extension of the approach to multiple NP types, opening up a general route to colloidally processed materials. The process uses cheap materials, it is reproducible, robust, and scaleable, and it allows for selection of both particle and cluster size. In the case of assemblies of magnetic iron oxide NPs, these advantages enable tuning of the magnetic properties of the assemblies for applications such as magnetically targetable MRI-trackable agents in biomedicine.

The dynamics and pH-dependence of Ag43 adhesins' self-association probed by atomic force spectroscopy.

Self-associating auto-transporter (SAAT) adhesins are two-domain cell surface proteins involved in bacteria auto-aggregation and biofilm formation. Antigen 43 (Ag43) is a SAAT adhesin commonly found in Escherichia coli whose variant Ag43a has been shown to promote persistence of uropathogenic E. coli within the bladder. The recent resolution of the tri-dimensional structure of the 499 amino-acids' ß-domain in Ag43a has shed light on the possible mechanism governing the self-recognition of SAAT adhesins, in particular the importance of trans-interactions between the L shaped ß-helical scaffold of two α-domains of neighboring adhesins. In this study, we use single-molecule force spectroscopy (SMFS) and dynamic force spectroscopy (DFS) to unravel the dynamics of Ag43-self association under various pH and molecular elongation rate conditions that mimic the situations encountered by E. coli in its natural environment. Results evidenced an important stretchability of Ag43α with unfolding of sub-domains leading to molecular extension as long as 150 nm. Nanomechanical analysis of molecular stretching data suggested that self-association of Ag43 can lead to the formation of dimers and tetramers driven by rapid and weak cis- as well as slow but strong trans-interaction forces with a magnitude as large as 100-250 pN. The dynamics of cis- and trans-interactions were demonstrated to be strongly influenced by pH and applied shear force, thus suggesting that environmental conditions can modulate Ag43-mediated aggregation of bacteria at the molecular level.

Electrodynamics of soft multilayered particles dispersions: dielectric permittivity and dynamic mobility.

We report a theory for the evaluation of the electrodynamics of dispersions of spherical soft multilayered (bio)particles, with microorganisms and polyelectrolyte multilayers-coated particles as illustrative paradigms. These particles generally consist of a hard (ion- and water-impermeable) core component supporting a succession of step-function or diffuse-like concentric soft (permeable) polymeric layers defined by distinct electrostatic, hydrodynamic and structural properties. The formalism is based on a rigorous numerical resolution of the coupled Navier-Stokes-Brinkman equation, continuity equations for the flow and for the ionic species present in solution, and the non-linear Poisson equation corrected for the multilayered nature of the soft interphase. The frequency-dependent dynamic mobility and dielectric permittivity of such soft particles suspensions are discussed as a function of the key electrohydrodynamic features of the constituting particulate peripheral layers and solution salinity. It is shown that the frequency dependent permittivity is mostly affected by the total charge carried by the overall soft interphase. In contrast, the dynamic mobility is mainly determined by the charge and friction characteristics of the layers located within an electrokinetically-active outer particle region whose extension is defined by the electric double layer thickness and the Brinkman length. Results highlight that under particular electrolyte concentration and layer-to-layer thickness ratio conditions, the dynamic mobility may reflect the physico-chemical and structural properties of the only innermost layers of the soft particle coating.

Thermo-regulated adhesion of the Streptococcus thermophilus Δrgg0182 strain.

The physicochemical determinants governing the temperature-dependent adhesion of Streptococcus thermophilus to abiotic surfaces are identified under physiological condition for cells either lacking or not the Rgg0182 transcriptional regulator involved in their thermal adaptation. For that purpose, the wild type LMG18311 strain and Δrgg0182 mutant were imaged using highly resolved atomic force microscopy (AFM) at various cell growth temperatures (42 to 55 °C). The corresponding hydrophobic/hydrophilic balance of the cells was quantitatively addressed via the measurement by chemical force microcopy of their adhesion to a reference hydrophobic surface. Analysis of force-separation distance curves further allowed us to discriminate cell surfaces according to the presence or absence of biopolymers. These results were interpreted in relation to the measured adhesion of the Δrgg0182 mutant onto the hydrophobic wall of microwells in the temperature range from 46 to 52 °C. It is evidenced that the viscoelastic Δrgg0182 cell envelop behaves as a thermo-responsive film whose hydrophobicity increases with increasing temperature, thereby favoring cell attachment to hydrophobic surfaces. Regardless cell growth temperature, wild-type cells do not attach to hydrophobic surfaces and the presence of the Rgg0182 transcriptional regulator is associated with the synthesis of hydrophilic cell surface biopolymers. Throughout, the impact of electrostatics on bioadhesion is ruled out upon examination of electrohydrodynamic cell properties at 50 °C.

Metal speciation in a complexing soft film layer: a theoretical dielectric relaxation study of coupled chemodynamic and electrodynamic interfacial processes.

We report a comprehensive formalism for the dynamics of metal speciation across an interphase formed between a complexing soft film layer and an electrolyte solution containing indifferent ions and metal ions that form complexes with charged molecular ligands distributed throughout the film. The analysis integrates the intricate interplay between metal complexation kinetics and diffusive metal transfer from/toward the ligand film, together with the kinetics of metal electrostatic partitioning across the film/solution interphase. This partitioning is determined by the settling dynamics of the interfacial electric double layer (EDL), as governed by time-dependent conduction-diffusion transports of both indifferent and reactive metal ions. The coupling between such chemodynamic and electrodynamic processes is evaluated via derivation of the dielectric permittivity increment for the ligand film/electrolyte interphase that is perturbed upon application of an ac electric field (pulsation ω) between electrodes supporting the films. The dielectric response is obtained from the ω-dependent distributions of all ions across the ligand film, as ruled by coupled Poisson-Nernst-Planck equations amended for a chemical source term involving the intra-film complex formation and dissociation pulsations (ω(a) and ω(d) respectively). Dielectric spectra are discussed for bare and film coated-electrodes over a wide range of field pulsations and Deborah numbers De = ω(a,d)/ω(diff), where ω(diff) is the electric double layer relaxation pulsation. The frequency-dependent dynamic or inert character of the formed metal complexes is then addressed over a time window that ranges from transient to fully relaxed EDL. The shape and magnitude of the dielectric spectra are further shown to reflect the lability of dynamic complexes, i.e. whether the overall speciation process at a given pulsation ω is primarily rate-limited either by complexation kinetics or by ion-transport dynamics. The limits, strengths and extensions of the approach are further discussed within the context of metal speciation dynamics at soft planar and particulate complexing interphases.

Bacterial surface appendages strongly impact nanomechanical and electrokinetic properties of Escherichia coli cells subjected to osmotic stress.

The physicochemical properties and dynamics of bacterial envelope, play a major role in bacterial activity. In this study, the morphological, nanomechanical and electrohydrodynamic properties of Escherichia coli K-12 mutant cells were thoroughly investigated as a function of bulk medium ionic strength using atomic force microscopy (AFM) and electrokinetics (electrophoresis). Bacteria were differing according to genetic alterations controlling the production of different surface appendages (short and rigid Ag43 adhesins, longer and more flexible type 1 fimbriae and F pilus). From the analysis of the spatially resolved force curves, it is shown that cells elasticity and turgor pressure are not only depending on bulk salt concentration but also on the presence/absence and nature of surface appendage. In 1 mM KNO(3), cells without appendages or cells surrounded by Ag43 exhibit large Young moduli and turgor pressures (∼700-900 kPa and ∼100-300 kPa respectively). Under similar ionic strength condition, a dramatic ∼50% to ∼70% decrease of these nanomechanical parameters was evidenced for cells with appendages. Qualitatively, such dependence of nanomechanical behavior on surface organization remains when increasing medium salt content to 100 mM, even though, quantitatively, differences are marked to a much smaller extent. Additionally, for a given surface appendage, the magnitude of the nanomechanical parameters decreases significantly when increasing bulk salt concentration. This effect is ascribed to a bacterial exoosmotic water loss resulting in a combined contraction of bacterial cytoplasm together with an electrostatically-driven shrinkage of the surface appendages. The former process is demonstrated upon AFM analysis, while the latter, inaccessible upon AFM imaging, is inferred from electrophoretic data interpreted according to advanced soft particle electrokinetic theory. Altogether, AFM and electrokinetic results clearly demonstrate the intimate relationship between structure/flexibility and charge of bacterial envelope and propensity of bacterium and surface appendages to contract under hypertonic conditions.

Electrostatic interactions between diffuse soft multi-layered (bio)particles: beyond Debye-Hückel approximation and Deryagin formulation.

We report a steady-state theory for the evaluation of electrostatic interactions between identical or dissimilar spherical soft multi-layered (bio)particles, e.g. microgels or microorganisms. These generally consist of a rigid core surrounded by concentric ion-permeable layers that may differ in thickness, soft material density, chemical composition and degree of dissociation for the ionogenic groups. The formalism allows the account of diffuse interphases where distributions of ionogenic groups from one layer to the other are position-dependent. The model is valid for any number of ion-permeable layers around the core of the interacting soft particles and covers all limiting situations in terms of nature of interacting particles, i.e. homo- and hetero-interactions between hard, soft or entirely porous colloids. The theory is based on a rigorous numerical solution of the non-linearized Poisson-Boltzmann equation including radial and angular distortions of the electric field distribution within and outside the interacting soft particles in approach. The Gibbs energy of electrostatic interaction is obtained from a general expression derived following the method by Verwey and Overbeek based on appropriate electric double layer charging mechanisms. Original analytical solutions are provided here for cases where interaction takes place between soft multi-layered particles whose size and charge density are in line with Deryagin treatment and Debye-Hückel approximation. These situations include interactions between hard and soft particles, hard plate and soft particle or soft plate and soft particle. The flexibility of the formalism is highlighted by the discussion of few situations which clearly illustrate that electrostatic interaction between multi-layered particles may be partly or predominantly governed by potential distribution within the most internal layers. A major consequence is that both amplitude and sign of Gibbs electrostatic interaction energy may dramatically change depending on the interplay between characteristic Debye length, thickness of ion-permeable layers and their respective protolytic features (e.g. location, magnitude and sign of charge density). This formalism extends a recent model by Ohshima which is strictly limited to interaction between soft mono-shell particles within Deryagin and Debye-Hückel approximations under conditions where ionizable sites are completely dissociated.