The safe use and design of nanoparticles (NPs) ask for a comprehensive interpretation of their potentially adverse effects on (micro)organisms. In this respect, the prior assessment of the ...interactions experienced by NPs in the vicinity of - and in contact with - complex biological surfaces is mandatory. It requires the development of suitable techniques for deciphering the processes that govern nano-bio interactions when a single organism is exposed to an extremely low dose of NPs. Here, we used atomic force spectroscopy (AFM)-based force measurements to investigate at the nanoscale the interactions between carboxylate-terminated polyamidoamine (PAMAM) nanodendrimers (radius ca. 4.5 nm) and two bacteria with very distinct surface properties, Escherichia coli and Lactococcus lactis. The zwitterionic nanodendrimers exhibit a negative peripheral surface charge and/or a positive intraparticulate core depending on the solution pH and salt concentration. Following an original strategy according to which a single dendrimer NP is grafted at the very apex of the AFM tip, the density and localization of NP binding sites are probed at the surface of E. coli and L. lactis mutants expressing different cell surface structures (presence/absence of the O-antigen of the lipopolysaccharides (LPS) or of a polysaccharide pellicle). In line with electrokinetic analysis, AFM force measurements evidence that adhesion of NPs onto pellicle-decorated L. lactis is governed by their underlying electrostatic interactions as controlled by the pH-dependent charge of the peripheral and internal NP components, and the negatively-charged cell surface. In contrast, the presence of the O-antigen on E. coli systematically suppresses the adhesion of nanodendrimers onto cells, may the apparent NP surface charge be determined by the peripheral carboxylate groups or by the internal amine functions. Altogether, this work highlights the differentiated roles played by surface polysaccharides in mediating NP attachment to Gram-positive and Gram-negative bacteria. It further demonstrates that the assessment of NP bioadhesion features requires a critical analysis of the electrostatic contributions stemming from the various structures composing the stratified cell envelope, and those originating from the bulk and surface NP components. The joint use of electrokinetics and AFM provides a valuable option for rapidly addressing the binding propensity of NPs to microorganisms, as urgently needed in NP risk assessments.
Due to the complexity of the humic substances (HS), mathematical models have often been employed to understand their roles in the environment. Since no consensus exists with respect to the structure ...and conformation of the HS, models have alternatively given them properties corresponding to impermeable hard spheres or fully permeable polyelectrolytes. In this study, the hydrodynamic permeability of standard HS (Suwannee River fulvic, humic, and peat humic acids) are evaluated as a function of pH and ionic strength. A detailed theoretical model is used to determine the softness parameter (λo), which characterizes the degree of flow penetration into the HS on the basis of measured values of electrophoretic mobilities, diffusion coefficients, and electric charge densities. Their motion in an electric field is evaluated by a rigorous numerical evaluation of the governing electrokinetic equations for soft particles. The hydrodynamic impact of the polyelectrolyte chains is accounted for by a distribution of Stokes resistance centers and partial dissociation of the hydrodynamically immobile ionogenic groups distributed throughout the polyelectrolyte. The results demonstrate that the studied HS are small (radius ca. 1 nm), highly charged (500−650 C g-1 when all sites are dissociated), and very permeable (typical flow penetration length of 25−50% of the radius, depending on pH). The HS also coagulate slightly when lowering the pH of the solution. Modeling of the HS as hard spheres with a charge and slip plane located at the surface is thus physically inappropriate, as are a number of analytical theories for soft particles that hold for low to moderate electrostatic potentials and large colloids. The shortcomings of these simpler approaches, when interpreting the electrophoretic mobilities of HS, are highlighted by comparison with rigorous theoretical predictions.
A framework is presented for understanding the reactivity of nanoparticulate reactants with ions and small molecules. Without loss of generality, the formalism is developed for the case of ...nanoparticles in contact with environmentally relevant metal ions. In addition to reactive sites, nanoparticles generally carry indifferent electric charge distributed over either their surface (hard particles) or volume (soft particles). The ensuing structure and composition of the electric double layer formed within and/or outside the nanoparticulate reactants substantially govern the dynamics of their association and dissociation with ions in aquatic media. A defining feature of permeable nanoparticles is that their charges and reactive sites are spatially confined inside a particle body with an inner medium whose properties may be substantially different from those of the bulk solution. Consequently, the chemodynamic properties of nanoparticulate complexants may differ significantly from those of simple molecular ligands that are homogeneously dispersed in solution. The various physicochemical processes underlying the dynamic reactivity of nanoparticles toward metal ions are here identified, with a focus on the key role played by conductive-diffusion of both metal ions and nanoparticles, the partitioning of ions within the reactive nanoparticulate volume, and the dynamics of the local association/dissociation processes with the reactive sites. The nature of the rate-limiting step in the overall formation/dissociation of the nanoparticulate complexes is shown to depend on the size of the nanoparticle, its charge density, and the ionic strength of the bulk medium. The consequences of these features are further elaborated within the context of dynamics of metal partitioning at a macroscopic consuming biological interphase in the presence of metal complexing nanoparticles.
Colloidal particles have been prepared from polyanionic and polycationic recombinant spider silk protein. The amino acid sequences of these spider silk proteins are identical except for 16 residues ...bearing either a cationic or an anionic ionizable group. Electrophoretic titration showed that protonation of the acidic and basic amino acids had significant impact on the electrophoretic mobility of the protein particles and, in particular, on their point of zero mobility (PZM). The experimentally determined PZMs are in good agreement with the theoretical values evaluated on the basis of the relevant amino acid sequences. A comprehensive description of the electrokinetic properties of the recombinant spider silk protein particles as a function of pH and solution ionic strength was provided from adequate application of electrokinetic theory for soft particles. Within the framework of this formalism, spider silk protein particles are viewed as porous colloids penetrable for ions and characterized by a finite penetration length for the electroosmotic flow. The differentiated electrokinetic properties of the particles were shown to be solely governed by the electrohydrodynamic features of their poorly charged outer peripheral layer with a thickness of about 10–20 nm. This finding was further corroborated experimentally by demonstrating that electrokinetics of particles bearing an additional outer layer consisting of oppositely charged spider silk proteins is entirely dominated thereby. The presence of a fuzzy, ion-permeable particle interface with an extension of several tenths of a nanometer was confirmed by direct measurement of the resulting steric forces using the colloidal probe atomic force microscopy (AFM) technique.
Bioavailability of trace metals is a key parameter for assessment of toxicity on living organisms. Proper evaluation of metal bioavailability requires monitoring the various interfacial processes ...that control metal partitioning dynamics at the biointerface, which includes metal transport from solution to cell membrane, adsorption at the biosurface, internalization, and possible excretion. In this work, a methodology is proposed to quantitatively describe the dynamics of Cd(II) uptake by Pseudomonas putida. The analysis is based on the kinetic measurement of Cd(II) depletion from bulk solution at various initial cell concentrations using electroanalytical probes. On the basis of a recent formalism on the dynamics of metal uptake by complex biointerphases, the cell concentration-dependent depletion time scales and plateau values reached by metal concentrations at long exposure times (>3 h) are successfully rationalized in terms of limiting metal uptake flux, rate of excretion, and metal affinity to internalization sites. The analysis shows the limits of approximate depletion models valid in the extremes of high and weak metal affinities. The contribution of conductive diffusion transfer of metals from the solution to the cell membrane in governing the rate of Cd(II) uptake is further discussed on the basis of estimated resistances for metal membrane transfer and extracellular mass transport.
After size-selection, the phase behavior of aqueous suspensions of nontronite clay was analyzed by osmotic pressure measurements, rheological experiments, and small-angle X-ray scattering. All the ...measurements confirm that for ionic strength ≤10−3 M/L, the system is purely repulsive. By combining results from osmotic pressure measurements and X-ray scattering, it appears that the pressure of the system can be well-described using a simple Poisson−Boltzmann treatment based on the interaction between charged infinite parallel planes. In terms of rheological properties, even if the status of the sol/gel transition remains partially unclear as the number density of particles at the sol−gel transition exhibits a −2 power dependence with average particle size, the yield stress and elasticity of the gels can be easily renormalized for all particle sizes on the basis of the volume of the particles. Furthermore, rheological modeling of the flow curves shows that for all the particles, an approach based on excluded volume effects captures most features of nontronite suspensions. Still, the high shear flow properties of the suspensions that reveal a strong orientation of particles in the flow are affected by electrostatic interactions. This study then shows that the rich phase behavior of clay minerals, notably the fact that some clay minerals display an isotropic/nematic transition while others exhibit a sol−gel transition, requires a full understanding of all the interactions in the system that can only be achieved by working on well-characterized size-selected samples.
Electrokinetic phenomena, such as electrophoresis, are valuable tools for determining the interfacial (double layer) properties of colloidal particles. The theoretical formalisms employed to ...interpret electrokinetic data (electrophoretic mobility) were initially derived for the restrictive case of hard (non-permeable) particles with the electrokinetic potential as unavoidable primary variable. In this paper, we underline the inadequacy of such models for addressing the electrostatic and hydrodynamic characteristics of microbes like bacteria, viruses or yeast cells. These bioparticles are characterized by heterogeneous, soft, permeable interphases formed with the outer electrolytic medium, which requires advanced electrokinetic analyses where the concept of zeta-potential must be abandoned. We review the progresses made in the measurement and analysis of interphasial properties of bioparticles under electrokinetic conditions. In particular, emphasis is given on the necessity to couple appropriately interpreted electrokinetics with other physico-chemical measurements (
e.g. issued from AFM imaging/force spectroscopy) and microbiological techniques (genetic manipulation of microbes). Using such a combination, a clear connection between complex interphase properties of microbes and
e.g. their propensity to adhere onto charged surfaces should be achieved.
Nanoparticles (NPs) are generally believed to derive their high reactivity from the inherently large specific surface area. Here we show that this is just the trivial part of a more involved picture. ...Nanoparticles that carry electric charge are able to generate chemical reaction rates that are even substantially larger than those for similar molecular reactants. This is achieved by Boltzmann accumulation of ionic reactants and the Debye acceleration of their transport. The ensuing unique reactivity features are general for all types of nanoparticles but most prominent for soft ones that exploit the accelerating mechanisms on a 3D level. These features have great potential for exploitation in the catalysis of ionic reactions: the reactivity of sites can be enhanced by increasing the indifferent charge density in the NP body.
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