Quantifying the Reversible Association of Thermosensitive Nanoparticles

Under many conditions, biomolecules and nanoparticles associate by means of attractive bonds, due to hydrophobic attraction. Extracting the microscopic association or dissociation rates from experimental data is complicated by the dissociation events and by the sensitivity of the binding force to temperature (T). Here we introduce a theoretical model that combined with light-scattering experiments allows us to quantify these rates and the reversible binding energy as a function of T. We apply this method to the reversible aggregation of thermoresponsive polystyrene/poly(N-isopropylacrylamide) core-shell nanoparticles, as a model system for biomolecules. We find that the binding energy changes sharply with T, and relate this remarkable switchable behavior to the hydrophobic-hydrophilic transition of the thermosensitive nanoparticles

Anisotropic dynamics and kinetic arrest of dense colloidal ellipsoids in the presence of an external field studied by differential dynamic microscopy

Anisotropic dynamics on the colloidal length scale is ubiquitous in nature. Of particular interest is the dynamics of systems approaching a kinetically arrested state. The failure of classical techniques for investigating the dynamics of highly turbid suspensions has contributed toward the limited experimental information available up until now. Exploiting the recent developments in the technique of differential dynamic microscopy (DDM), we report the first experimental study of the anisotropic collective dynamics of colloidal ellipsoids with a magnetic hematite core over a wide concentration range approaching kinetic arrest. In addition, we have investigated the effect of an external magnetic field on the resulting anisotropic collective diffusion. We combine DDM with small-angle x-ray scattering and rheological measurements to locate the glass transition and to relate the collective short- and long-time diffusion coefficients to the structural correlations and the evolution of the zero shear viscosity as the system approaches an arrested state

Giant hollow fiber formation through self-assembly of oppositely charged polyelectrolyte brushes and gold nanoparticles

We report on the use of binary mixtures of oppositely charged gold nanoparticles (AuNPs) and spherical polyelectrolyte brushes (SPBs), consisting of a polystyrene core onto which long polystyrene sulfonate chains are grafted, as a simple model system to investigate the influence of directional interactions on self-assembly. We demonstrate that the mixing ratio, i.e., the number of AuNPs per SPB, has a profound influence on self-assembly. In particular we report on the formation of giant hollow fibers, and present a thorough characterization of these nanostructures. We speculate that the adsorption of a few AuNPs on the SPBs appears to direct the tubular self-assembly, and discuss the analogy to the case of modified proteins such as tubulin under the action of nucleotides

Asymmetric self-assembly of oppositely charged composite microgels and gold nanoparticles

The electrostatically driven self-assembly of oppositely charged gold nanoparticles (Au NPs) and polystyrene/poly(N-isopropylacrylamide) (PS/PNIPAm) core-shell microgels (CSMs) has been investigated. The co-assembly was accomplished by addition of smaller Au NPs to CSMs in dilute conditions up to a number ratio of about 1 : 1, when the suspension is destabilized. A combination of different techniques (i.e. turbidimetric titration, electrophoretic mobility, UV-visible spectroscopy, dynamic light scattering and microscopy techniques) were used to investigate the association between the two particles and the stability of the different mixtures. Hereby we demonstrate that the size ratio between the two particles (about 4 to 1) and the asymmetric character of the association result in the formation of electrostatic hybrid complexes, analogous to dipolar colloidal molecules, which further rearrange into finite sized clusters for number ratios N(AuNPs)/N(CSMs) < 1