Pedro G. Toledo

How changes in the sequence of the peptide CLPFFD-NH2 can modify the conjugation and stability of gold nanoparticles and their affinity for β-amyloid fibrils

In a previous work, we studied the interaction of beta-amyloid fibrils (Abeta) with gold nanoparticles (AuNP) conjugated with the peptide CLPFFD-NH2. Here, we studied the effect of changing the residue sequence of the peptide CLPFFD-NH2 on the efficiency of conjugation to AuNP, the stability of the conjugates, and the affinity of the conjugates to the Abeta fibrils. We conjugated the AuNP with CLPFFD-NH 2 isomeric peptides (CDLPFF-NH2 and CLPDFF-NH2) and characterized the resulting conjugates with different techniques including UV-Vis, TEM, EELS, XPS, analysis of amino acids, agarose gel electrophoresis, and CD. In addition, we determined the proportion of AuNP bonded to the Abeta fibrils by ICP-MS. AuNP-CLPFFD-NH2 was the most stable of the conjugates and presented more affinity for Abeta fibrils with respect to the other conjugates and bare AuNP. These findings help to better understand the way peptide sequences affect conjugation and stability of AuNP and their interaction with Abeta fibrils. The peptide sequence, the steric effects, and the charge and disposition of hydrophilic and hydrophobic residues are crucial parameters when considering the design of AuNP peptide conjugates for biomedical applications.

Pore-level modeling of isothermal drying of pore networks accounting for evaporation, viscous flow, and shrinking

Abstract Simulation results of pore-level drying of non-hygroscopic, non-rigid, liquid-wet porous media are presented. Two- and three-dimensional pore networks represent pore spaces. Two kinds of mechanisms are considered: evaporation and hydraulic flow. The process is considered under isothermal conditions. Capillary forces thus dominate over viscous forces and the drying is considered as a modified form of invasion percolation. Liquid in pore corners allows for hydraulic connection throughout the network. During drying, liquid is replaced by vapor by two fundamental mechanisms: evaporation and pressure gradient–driven liquid flow. The development of capillary pressure as menisci turn concave induces shrinkage of the matrix, which contributes to the pressure gradient that drives liquid toward the surface of the network. Using Monte Carlo simulation, we find evaporation and drainage times; the shortest calculated indicates the controlling mechanism. Here we report distributions of liquid and vapor as drying time advances. For the calculation of transport properties, details of pore space and displacement are subsumed in pore conductances. Solving for the pressure field in each phase, vapor and liquid, we find a single effective conductance for each phase as a function of liquid saturation. Along with the effective conductance for the liquid-saturated network, the relative permeability of liquid and diffusivity of vapor are calculated.

Pore blocking and permeability reduction in cross-flow microfiltration

Particle capture at membrane surfaces in cross-flow microfiltration is studied both experimentally and computationally to better understand permeability reduction as function of membrane pore sizes and particle sizes and concentrations. Microfiltration experiments for various bentonite suspension concentrations are conducted in commercial polysulphone membranes of nominal pore sizes 2 and 0.2 μm. Permeability reduction is continuously evaluated. Scanning electron microscopy (SEM) and image analysis are used to determine membrane pore size and particle size distributions. For simulation purposes, the membrane pore space is represented by a bundle of nonintersecting tubes. Pore segments in the model membranes have circular cross-sections, random locations and sizes distributed according to distributions determined experimentally. Monte Carlo simulations of cross-flow microfiltration of a well-characterized particle suspension on well-defined model membranes are presented and permeability reduction calculated. Particle capture and size exclusion at pore segments are considered the dominant mechanisms of membrane fouling. In the simulations a matching size criterion between pore size and particle size is used to define pore blocking. Simulation results accord with those obtained experimentally. Permeability decreases more rapidly at higher particle concentrations. For all particle concentrations, permeability reduction of the thinner pore membrane is more intense. Membrane permeability scales linearly with porosity in both cases of membranes. This result is discussed in the light of simple theoretical arguments.

Nanoscale repulsive forces between mica and silica surfaces in aqueous solutions

Nanoscale repulsive forces between mineral surfaces in aqueous solutions were measured for the asymmetric mica-silica system. The force measured with an atomic force microscope (AFM) has universal character in the short range, less than ∼1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion (Na, Ca, Al), concentration (10(-6)-10(-2)M), and pH (3.9-8.2). Notably, the force is essentially the same as for the glass-silica system. Single force curves for a mica-silica system in a 10(-4)M aqueous NaCl solution at pH ∼ 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule, a consequence of a layer-by-layer dehydration of the surfaces when pushed together. This result provides additional support to the idea that nanoscale repulsive forces between mineral surfaces in aqueous solutions arise from a surface-induced water effect; the water between two mineral plates that are pushed together becomes structured and increasingly anchored to the surface of the plates by the creation of a hydrogen-bonding network that prevents dehydration of the surfaces.

Short-Range Forces between Glass Surfaces in Aqueous Solutions

We found that the force between glass surfaces measured with an atomic force microscope (AFM) has universal character in the short range, less than approximately 1 nm or about 3-4 water molecules, independent of solution conditions, that is, electrolyte ion size, charge and concentration and pH. Our results suggest that the excess DLVO force, obtained by subtracting the DLVO theory with a charge regulation model from the AFM force data, essentially does not change with the electrolytes Na, Ca, and Al, in the range of concentration from 10(-6) to 10(-2) M and the range of pH from 3.1 to 7.9. Single force curves for a glass-silica system in a 10-4 M aqueous NaCl solution at pH approximately 5.1 show oscillations with a period of about 0.25 nm, roughly the diameter of a water molecule. We postulate that the excess force between glass surfaces arises from a surface-induced solvent effect, from the creation of a hydrogen-bonding network at the surface level, rather than from a solvent-induced surface steric hindrance.

Pore-Level Modeling of Gas and Condensate Flow in Two- and Three-Dimensional Pore Networks: Pore Size Distribution Effects on the Relative Permeability of Gas and Condensate

We present a mechanistic model of retrograde condensation processes in two- and three-dimensional capillary tube networks under gravitational forces. Condensate filling-emptying cycles in pore segments and gas connection–isolation cycles are included. With the pore-level distribution of gas and condensate in hand, we determine their corresponding relative permeabilities. Details of pore space and displacement are subsumed in pore conductances. Solving for the pressure field in each phase, we find a single effective conductance for each phase as a function of condensate saturation. Along with the effective conductance for the saturated network, the relative permeability for each phase is calculated. Our model porous media are two- and three-dimensional regular networks of pore segments with distributed size and square cross-section. With a Monte Carlo sampling we find the optimum network size to avoid size effects and then we investigate the effect of network dimensionality and pore size distribution on the relative permeabilities of gas and condensate.

Nanoscale adhesive forces between silica surfaces in aqueous solutions

Nanoscale adhesive forces between a colloidal silica probe and a flat silica substrate were measured with an atomic force microscope (AFM) in a range of aqueous NaCl, CaCl2, and AlCl3 solutions, with concentrations ranging from 10(-)(6) to 10(-)(2) M at pH ∼5.1. Notably, the measured force curves reveal large pull-off forces in water which increase in electrolyte solutions, with jump-off-contact occurring as a gradual detachment of the probe from the flat substrate rather than as a sharp discontinuous jump. The measured force curves also show that the number and size of the steps increase with concentration and notably with electrolyte valence. For the higher concentration and valence the steps become jumps. We propose that these nanoscale adhesive forces between mineral surfaces in aqueous solutions may arise from newly born cavities or persistent subnanometer bubbles. Formation of cavities or nanobubbles cannot be observed directly in our experiments; however, we cannot disregard them as responsible for the discontinuities in the measured force data. A simple model based on several cavities bridging the two surfaces we show that is able to capture all the features in the measured force curves. The silica surfaces used are clean but not intentionally hydroxylated, as contact angle measurements show, and as such may be responsible for the cavities.

Population balance modelling of particle flocculation with attention to aggregate restructuring and permeability

A population balance model based on a detailed literature review is used to describe coagulation and flocculation kinetics as well as the time evolution of aggregate size distribution in a turbulent shear flow simultaneously with the breakage and restructuring of aggregates. The fractal nature and permeability of the aggregates and their evolution with time are also part of the model. Restructuring is absent in coagulation with soluble salts, but is present in flocculation caused by large polyelectrolyte molecules; in the latter, aggregates never reach a steady-state size, but a size that decreases gradually through particle and polymer rearrangement. The model is tested against available experimental data for monodisperse polystyrene particles coagulated with hydrated aluminium sulphate at different shear rates, and precipitated calcium carbonate flocculated with a cationic polyelectrolyte of very high molecular weight at different flocculant dosages. The numerical solution of the model requires adjusting three parameters, i.e, maximum collision efficiency (αmax), critical force needed for the breakage of the aggregates (B) and rate of aggregate restructuring (γ), which are obtained from minimising the difference between experimental data and model predictions. The model studied for the two very different systems shows excellent agreement with experimental flocculation kinetics and a reasonably good fit for aggregate size distributions. The model is most sensitive to the fragmentation rate through parameter B, somewhat less to the collision efficiency through parameter αmax and little to γ. When the aggregates undergo restructuring, properties such as permeability, breakage rate and collision rate change considerably over time. When the aggregates are permeable, the collision frequency is significantly smaller than when they are impervious.

Discrete sedimentation model for ideal suspensions

The sedimentation process of ideal suspensions is simulated using a discrete model in which gravitational, hydrodynamic, particle interaction and dispersive motions are considered as competitive processes. These mechanisms define motion rules that are implemented in regular two-dimensional lattices. Results show that the model is capable of producing the whole spectrum of particle-settling of ideal suspensions. Computer simulation results for the batch settling of rigid spheres in water are obtained for monodisperse systems. Results compared fairly well with experimental data.

A molecular dynamics study of the force between planar substrates due to capillary bridges

Molecular dynamics simulations are used to study capillary liquid bridges between two planar substrates and the origin, strength and range of the resulting force between them. Pairwise interactions are described by the Lennard-Jones potential. Surface wettability is tuned by varying the fluid-substrate well depth interaction parameter. The force between the substrates due to a bridge of liquid is estimated by different methods including non-equilibrium simulations of moving substrates connected by liquid bridges and macroscopic balance of forces. The latter involves knowledge of liquid-vapor interfacial free energy, curvature radii, radius of wetted area and contact angle at the triple-phase contact line. All these physical quantities are estimated from equilibrium simulations. The force is attractive when the substrates are solvophilic or moderately solvophobic; and thus for cavities surrounded by the same liquid the force is attractive even when the substrates are moderately solvophilic. Two threshold values for the fluid-substrate potential interaction parameter can be identified; one for which the effective interaction between substrates due to liquid bridges changes from repulsive to attractive and another for which the capillary bridge becomes mechanically unstable and breaks into droplets.