Effective Charge of Adsorbed Poly(amido amine) Dendrimers: Transition from Heterogeneous to Homogeneous Charge Distribution

Nanoparticle and bioparticle deposition kinetics

Mechanisms and kinetic of particle deposition at solid surfaces leading to the formation of self-assembled layers of controlled structure and density were reviewed. In the first part theoretical aspects were briefly discussed, comprising limiting analytical solutions for the linear transport under flow and diffusion. Methods of the deposition kinetics analysis for non-linear regimes affected by surface blocking were also considered. Characteristic monolayer formation times under diffusion and flow for the nanoparticle size range were calculated. In the second part illustrative experimental results obtained for micro- and nanoparticles were discussed. Deposition at planar substrates was analyzed with emphasis focused on the stability of layers and the release kinetics of silver particles. Applicability of the quartz microbalance measurements (QCM) for quantitative studies of nanoparticle deposition kinetic was also discussed. Except for noble metal and polymer particles, representative results for virus deposition at abiotic surfaces were analyzed. Final part of the review was devoted to nanoparticle corona formation at polymer carrier particles investigated by combination of the concentration depletion, AFM, SEM and the in situ electrokinetic method. It is argued that the results obtained for colloid particles can be used as reliable reference systems for interpretation of protein and other bioparticle deposition, confirming the thesis that simple is universal.

The Next Generation of Colloidal Probes: A Universal Approach for Soft and Ultra-Small Particles

The colloidal probe technique, which is based on the atomic force microscope, revolutionizes direct force measurements in many fields, such as interface science or biomechanics. It allows for the first time to determine interaction forces on the single particle or cell level. However, for many applications, important "blind spots" remain, namely, the possibility to probe interaction potentials for nanoparticles or complex colloids with a soft outer shell. Definitely, these are colloidal systems that are currently of major industrial importance and interest from theory. The here-presented novel approach allows for overcome the aforementioned limitations. Its applicability has been demonstrated for 300 nm sized carboxylate-modified latex particles as well as sub-micron core-shell particles with a soft poly-N-isopropylacrylamide hydrogel shell and a rigid silica core. For the latter, which until now cannot be studied by the colloidal probe technique, determined is the temperature dependency of electrosteric and adhesion forces has been determined on the single particle level.

Formation of hematite nanoparticle monolayers of controlled coverage and structure at polymeric microparticles

The deposition of hematite nanoparticles (22nm and 29nm in diameter) on negatively charged polystyrene microspheres (820nm in diameter) was studied by micro-electrophoretic measurements and AFM. The influence of ionic strength, varied between 10^-4 and 10^-2M, was determined. Initially, the electrophoretic mobility change of microspheres upon the addition of controlled amount of hematite nanoparticles were measured. These dependencies were quantitatively interpreted in terms of the general electrokinetic model. This allowed to determine the coverage of nanoparticles on microspheres under in situ conditions, which increased with ionic strength attaining 0.35 for the ionic strength of 10^-2M and 29 in diameter hematite particles. This effect, attributed to the decreasing range of lateral electrostatic repulsion among deposited particles, was accounted for by the random sequential adsorption model. However, the coverages attained for lower ionic strength exceeded the theoretical predictions. This effect was interpreted in terms of an additional electrostatic screening due to polymeric chains present at the microparticle surface. The acid base properties of the hematite monolayers were also acquired by applying thorough micro-electrophoretic measurements. The obtained results confirmed a feasibility of preparing hematite nanoparticle monolayers on polymeric carrier microspheres having well-defined coverage and structure.

Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation

This review summarizes the current understanding of adsorption of polyelectrolytes to oppositely charged solid substrates, the resulting interaction forces between such substrates, and consequences for colloidal particle aggregation. The following conclusions can be reached based on experimental findings. Polyelectrolytes adsorb to oppositely charged solid substrates irreversibly up to saturation, whereby loose and thin monolayers are formed. The adsorbed polyelectrolytes normally carry a substantial amount of charge, which leads to a charge reversal. Frequently, the adsorbed films are laterally heterogeneous. With increasing salt levels, the adsorbed mass increases leading to thicker and more homogeneous films. Interaction forces between surfaces coated with saturated polyelectrolyte layers are governed at low salt levels by repulsive electric double layer interactions, and particle suspensions are stable under these conditions. At appropriately high salt levels, the forces become attractive, principally due to van der Waals interactions, but eventually also through other forces, and suspensions become unstable. This situation can be rationalized with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Due to the irreversible nature of the adsorption process, stable unsaturated layers form in colloidal particle suspensions at lower polyelectrolyte doses. An unsaturated polyelectrolyte layer can neutralize the overall particle surface charge. Away from the charge reversal point, electric double layer forces are dominant and particle suspensions are stable. As the charge reversal point is approached, attractive van der Waals forces become important, and particle suspensions become unstable. This behaviour is again in line with the DLVO theory, which may even apply quantitatively, provided the polyelectrolyte films are sufficiently laterally homogeneous. For heterogeneous films, additional attractive patch-charge interactions may become important. Depletion interactions may also lead to attractive forces and suspension destabilization, but such interactions become important only at high polyelectrolyte concentrations.

Dendrimer induced interaction forces between colloidal particles revealed by direct force and aggregation measurements

Interaction forces and aggregation rates were determined in order to characterize colloid stability of negative carboxyl latex particles in the presence of oppositely charged poly(amido amine) (PAMAM) dendrimers of three different generations G4, G7 and G10. The force profiles were measured by the atomic force microscopy (AFM) based on multi-particle colloidal probe technique. Close to the isoelectric point, the measured force profiles were more attractive than the pure van der Waals interactions. This behavior was rationalized in term of an additional electrostatic patch-charge contribution whose magnitude increases by increasing the dendrimer generation. The aggregation rates were calculated from these results using the classical theory developed by Derjaguin, Landau, Verwey and Overbeek (DLVO) as well as including the additional attractive term and a radially symmetric force field. The calculated aggregation rates were compared to the ones obtained directly from time-resolved dynamic light scattering (DLS) measurements using exactly the same latex particles as in the AFM study. The results from these two methods were in good agreement in the case of dendrimers of lower generation, while at higher generation, significant differences were found. In the latter case, the stability ratio in the slow aggregation regime extracted from direct force measurements was much higher than the one measured experimentally by DLS. Despite the fact that the additional attractive term was included in the calculation, the discrepancy between the two different stability ratios suggests that the assumption of radial symmetric interaction is weak.

Mechanism of nanoparticle deposition on polystyrene latex particles

The deposition of positive amidine latex particles (98 nm in diameter) on negative polystyrene latex particles (820 nm in diameter) was studied by SEM imaging, microelectrophoretic and concentration depletion methods involving AFM. The role of ionic strength varied between 10(-4) and 10(-2) M and was systematically studied. The number of deposited positive latex particles (surface coverage) was evaluated by a direct counting procedure exploiting the SEM images. This allowed one to calibrate the results obtained from measurements of the electrophoretic mobility of larger latex particles covered by a controlled amount of the positive latex. These dependencies were quantitatively interpreted in terms of the 3D electrokinetic model previously used for planar interfaces. This allowed us to determine the coverage of nanoparticles on latex carriers under in situ conditions. Additionally, the maximum coverage of the positive latex was determined via AFM where the kinetics of the residual amidine latex deposition on mica was measured. The maximum coverage monotonically increased with ionic strength, attaining 0.52 for 10(-2) M NaCl. This effect was interpreted in terms of reduced electrostatic repulsion among positive latex particles and theoretically accounted for by the random sequential adsorption model. The obtained results have significance for basic science, indicating that the results obtained for curved interfaces (polymeric carrier particles) by the microelectrophoretic method can be exploited to interpret the deposition of nanoparticles and proteins on planar interfaces and vice versa.

Conformational transitions of weak polyacids grafted to nanoparticles

The charge distribution on polyelectrolytes is a key factor, which controls their conformation and interactions. In weak polyelectrolytes, this distribution is determined by a number of factors, including the solvent conditions and local environment. In this work, we investigate charge distributions of chains end-grafted on a spherical nanoparticle in a salt solution, using grand canonical titration Monte Carlo simulations of a coarse-grained polymer model. In this approach, the ionization state of each polymer bead fluctuates based on the dissociation constant, pH of the solution, and interactions with other particles in the system. We determine charge and polymer conformations as functions of the pH and solvent quality. We compare the results to a fixed charge model and also investigate the role of grafting density and the effect of curvature on the film morphologies.