The relationship between polymer/substrate charge density and charge overcompensation by adsorbed polyelectrolyte layers

This work examines polyelectrolyte adsorption (exclusively driven by electrostatic attractions) for a model system (DMAEMA, polydimethylaminoethyl methacrylate, adsorbing onto silica) where the adsorbing polycation is more densely charged than the substrate. Variations in the relative charge densities of the polymer and substrate are accomplished by pH, and the polycation is of sufficiently low molecular weight that the adsorbed conformation is generally flat under all conditions examined. We demonstrate, quantitatively, that the charge overcompensation observed on the isotherm plateau can be attributed to the denser positive charge on the adsorbing polycation and that the ultimate coverage obtained corresponds to the adsorption of one oligomer onto each original negative silica charge, when the silica charge is most sparse, at pH 6. This limiting behavior breaks down at higher pHs where the greater silica charge density accommodates single chains adsorbing onto multiple negative sites. As a result of the greater substrate charge density and reduced polycation charge at higher pHs, the extent of charge overcompensation diminishes while the coverage increases on the plateau of the isotherm. Ultimately at the highest pHs, a regime is approached where the coil's excluded surface area, not surface charge, limits the ultimate coverage. In addition to quantifying the crossover from the charge-limiting to the area-limiting behaviors, this paper quantitatively reports adsorption-induced changes in bound counterion density and ionization at the interface, which were generally found to be independent of coverage for this model system.     

Nuclear spin-lattice and nuclear spin-spin relaxation time measurements in EuB6 at low temperatures using the spin-echo technique

Experimental results on the nuclear spin-lattice and nuclear spin-spin relaxation times in the ferromagnetic EuB₆ at temperatures below 4·2 K are presented using the external magnetic field,H _ext, in the range of 0 ⩽H _ext ⩽ 10 kG. Nuclear spin-spin relaxation time computed on the basis of the Suhl-Nakamura process turns out to be 3·2μs, which compares well with the experimental value 11·1μs obtained with the 10 kG magnetic field at 1·7 K. It is found that in the ferromagnetic EuB₆,T ₁ is approximately 5 × 10³ times larger thanT ₂ at 1·7 K with the 10 kG magnetic field. Thus the effect ofT ₁ onT ₂ can be neglected. From the experimental value ofT ₂, the value of the homogeneous line broadening is found to be 14 kHz. The corresponding value obtained from the cw method is 175 kHz. This evidently shows the presence of the inhomogeneous line broadening in the cw NMR.     

Hydrodynamic crossover in dynamic microparticle adhesion on surfaces of controlled nanoscale heterogeneity

This note documents the crossover from a regime where shear flow hinders microparticle adhesion on collecting surfaces to that where increased flow aids particle capture. Flow generally works against adhesion and successfully hinders particle capture when the net physicochemical attractions between the particles and collector are weak compared with hydrodynamic forces on the particle. Conversely, with strong attractions between particles and collector, flow aids particle capture by increasing the mass transport of particles to the interfacial region. Here, local hydrodynamics still generally oppose adhesion but are insufficient to pull particles off of the surface. Thus, flow actually increases the particle capture rate through the increased transport to the surface. These behaviors are demonstrated using 1 mum silica spheres flowing over electrostatically heterogeneous (length scales near 10 nm) collecting surfaces at shear rates from 22 to 795 s(-1). The net surface charge on the collector is varied systematically from strongly negative (pure silica) to strongly positive (a saturated polycationic overlayer), demonstrating the interplay between physicochemical and hydrodynamic contributions. These results clearly apply to situations where heterogeneous particle-surface interactions are electrostatic in nature; however, qualitatively similar behavior was previously reported for the effect receptor density on bacterial adhesion.     

Using flow to switch the valency of bacterial capture on engineered surfaces containing immobilized nanoparticles

Toward an understanding of nanoparticle-bacterial interactions and the development of sensors and other substrates for controlled bacterial adhesion, this article describes the influence of flow on the initial stages of bacterial capture (Staphylococcus aureus) on surfaces containing cationic nanoparticles. A PEG (poly(ethylene glycol)) brush on the surface around the nanoparticles sterically repels the bacteria. Variations in ionic strength tune the Debye length from 1 to 4 nm, increasing the strength and range of the nanoparticle attractions toward the bacteria. At relatively high ionic strengths (physiological conditions), bacterial capture requires several nanoparticle-bacterial contacts, termed "multivalent capture". At low ionic strength and gentle wall shear rates (on the order of 10 s(-1)), individual bacteria can be captured and held by single surface-immobilized nanoparticles. Increasing the flow rate to 50 s(-1) causes a shift from monovalent to divalent capture. A comparison of experimental capture efficiencies with statistically determined capture probabilities reveals the initial area of bacteria-surface interaction, here about 50 nm in diameter for a Debye length κ(-1) of 4 nm. Additionally, for κ(-1) = 4 nm, the net per nanoparticle binding energies are strong but highly shear-sensitive, as is the case for biological ligand-receptor interactions. Although these results have been obtained for a specific system, they represent a regime of behavior that could be achieved with different bacteria and different materials, presenting an opportunity for further tuning of selective interactions. These finding suggest the use of surface elements to manipulate individual bacteria and nonfouling designs with precise but finite bacterial interactions.     

Non-specific adhesion on biomaterial surfaces driven by small amounts of protein adsorption

This work explores how long-range non-specific interactions, resulting from small amounts of adsorbed fibrinogen, potentially influence bioadhesion. Such non-specific interactions between protein adsorbed on a biomaterial and approaching cells or bacteria may complement or even dominate ligand–receptor mating. This work considers situations where the biomaterial surface and the approaching model cells (micron-scale silica particles) exhibit strong electrostatic repulsion, as may be the case in diagnostics and lab-on-chip applications. We report that adsorbed fibrinogen levels near 0.5 mg/m² produce non-specific fouling. For underlying surfaces that are less fundamentally repulsive, smaller amounts of adsorbed fibrinogen would have a similar effect. Additionally, it was observed that particle adhesion engages sharply and only above a threshold loading of fibrinogen on the collector. Also, in the range of ionic strength, I, below about 0.05 M, increases in I reduce the fibrinogen needed for microparticle capture, due to screening of electrostatic repulsions. Surprisingly, however, ionic strengths of 0.15 M reduce fibrinogen adsorption altogether. This observation opposes expectations based on DLVO arguments, pointing to localized electrostatic attractions and hydration effects to drive silica–fibrinogen adhesion. These behaviors are benchmarked against microparticle binding on silica surfaces carrying small amounts of a polycation, to provide insight into the role of electrostatics in fibrinogen-driven non-specific adhesion.     

Adsorption of copolymers aggregates: from kinetics to adsorbed layer structure

We examined the adsorption, on hydrophobic and hydrophilic surfaces, of 4 rake-type poly(dimethyl siloxane) (PDMS) copolymers varying the amount of poly(ethylene glycol) (PEG) graft arms from 41 to 72%. The copolymers formed large aggregates in solution, complicating their adsorption kinetics and layer structures. We found the adsorption process always to be dominated by the adsorption of large aggregates, with strongly bound layers resistant to rinsing in adsorbing buffer. Adsorbed amounts were nearly independent of the substrate. However, subtleties in the adsorption kinetics suggested different layer structures for the different systems. On hydrophilic silica, aggregates adsorbed at the transport limited rate until surface saturation, and associated interfacial structures were likely retained. On the hydrophobic surface, a subset of the copolymers exhibited retarded late stage adsorption kinetics suggestive of brush formation. This work demonstrates how subtle differences in adsorption kinetics provide insight into potential interfacial layer structures.     

Sur quelques récents résultats dans la théorie des surfaces algébriques

Sans résumé     

Depletion versus deflection: how membrane bending can influence adhesion

During depletion-driven vesicle adhesion, a stiff membrane's resistance to bending at fixed tension prevents contact angle equilibrium and vesicle spreading over an opposing vesicle, while more flexible vesicles partially engulf opposing vesicles. Estimates of the bending cost associated with the spreading contact line, relative to the adhesion energy, were consistent with the observed spreading or lack of spreading for the flexible and stiff membranes, respectively, and predicted a lag time sometimes preceding spreading.