Permeability of Soft Particles at Electrical Fields.

The influence of the charge on the permeability of microgel particles is studied in the presence of electric fields. Electrophoresis experiments performed on pH-ionizable pNIPAM-AA microgels show that particles behave as permeable spheres when the network is ionized. However, they keep non-permeable in the absence of charge. The ionic nature of the network thus controls the permeability of the soft particles. A salt-dependent local viscosity explains these permeability changes. This is confirmed by NMR as alternative independent technique. Strongly hydrated counterions located around fixed charges on the network are considered responsible for the local viscosity variations.

Multifunctional hybrid nanogels for theranostic applications.

This paper reviews a wide set of theranostic applications based on the special properties associated with composite nanogels. The nanogels presented here are mostly hybridized with quantum dots, magnetic nanoparticles, and plasmonic metal noble nanoparticles. These inorganic components confer nanogels multifunctional properties that extend their applications from drug delivery systems to diagnosis and therapy. Nanogels can also be surface functionalized with specific ligands to achieve targeted therapy and reduce toxicity. This versatility makes hybrid nanogels very promising agents for imaging, diagnosis and treatment of cancer and other diseases.

Structural properties of thermoresponsive poly(N-isopropylacrylamide)-poly(ethyleneglycol) microgels.

We present investigations of the structural properties of thermoresponsive poly(N-isopropylacrylamide) (PNiPAM) microgels dispersed in an aqueous solvent. In this particular work poly(ethyleneglycol) (PEG) units flanked with acrylate groups are employed as cross-linkers, providing an architecture designed to resist protein fouling. Dynamic light scattering (DLS), static light scattering (SLS), and small angle neutron scattering (SANS) are employed to study the microgels as a function of temperature over the range 10 °C ≤ T ≤ 40 °C. DLS and SLS measurements are simultaneously performed and, respectively, allow determination of the particle hydrodynamic radius, R(h), and radius of gyration, R(g), at each temperature. The thermal variation of these magnitudes reveals the microgel deswelling at the PNiPAM lower critical solution temperature (LCST). However, the hydrodynamic radius displays a second transition to larger radii at temperatures T ≤ 20 °C. This feature is atypical in standard PNiPAM microgels and suggests a structural reconfiguration within the polymer network at those temperatures. To better understand this behavior we perform neutron scattering measurements at different temperatures. In striking contrast to the scattering profile of soft sphere microgels, the SANS profiles for T ≤ LCST of our PNiPAM-PEG suspensions indicate that the particles exhibit structural properties characteristic of star polymer configurations. The star polymer radius of gyration and correlation length gradually decrease with increasing temperature despite maintenance of the star polymer configuration. At temperatures above the LCST, the scattered SANS intensity is typical of soft sphere systems.

Coupled deswelling of multiresponse microgels.

In this article, we study the response of a thermosensitive and ionic microgel to various external stimuli where coupling between different contributions to the total osmotic pressure is needed to describe the observations. We introduce a new Flory solvency parameter chi ( T, Q, n) with strong dependence on the network charge, Q, and salt concentration, n. The scaling exponent for the salt-induced deswelling of the microgel is the signature of the coupling between the mixing and ionic osmotic pressures.

Thermal control over the electrophoresis of soft colloidal particles.

We study the electrophoresis of surface-charged thermosensitive microgel particles based on poly-N-isopropylacrylamide (PNIPAM); these deswell with increasing temperature T. Our results show that the electrophoretic mobility mu is affected by the temperature-induced volume phase transition. It increases with increasing temperature, as a result of the charge density increase induced by particle deswelling. Temperature thus allows control of mu, in contrast to the more conventional charged hard spheres for which mu is T independent. Salt also affects the mu behavior and gives rise to rich phenomenology, sharing common characteristics with charged hard spheres and polyelectrolyte-coated colloids depending on whether the microgels are swollen or deswollen. We interpret the effects of salt concentration n by considering that particle charges are located in an external shell, as confirmed by titrations, and that it is this shell-salt-induced compression that affects the resulting mu behavior.

Structure factor scaling in colloidal charge heteroaggregation.

The structure factor, S(q), of a system composed of a 1:1 mixture of oppositely charged colloids undergoing heteroaggregation is studied by Browninan dynamics simulations. A peak develops in S(q) at low wave vectors, which can be scaled for different times to overlap using the scaling of spinodal decomposition, as shown for DLCA. The same master function is obtained for different interaction ranges. The origin of the peak can be traced back to a depletion layer of clusters surrounding every aggregate. At those long distances, cluster-cluster interaction is negligible and the aggregation is diffusion limitted, as deduced from the evolution of peak position, and the S(q) scaling at different interaction ranges. The interaction is, nevertheless, strong enough to affect the internal cluster structure.

Ionic correlations in highly charge-asymmetric colloidal liquids.

We use electrophoretic mobility (mu) measurements of charged colloidal particles under the presence of multivalent counterions as a probe of the electrostatic correlations between them; they become important for sufficiently high surface charge densities of the colloid (sigma) and result in a decreasing mu upon increasing sigma. The physics of this decrease is the same as that giving rise to charge inversion. We account qualitatively for the observations by considering recent theoretical arguments that assume the counterions next to the colloid surface as a strongly correlated liquid of properties similar to that of a Wigner crystal.

Static light scattering from microgel particles: model of variable dielectric permittivity.

We perform static light scattering experiments on a dilute suspension of microgel particles and model the resultant form factors Pq by assuming an exponentially decaying dielectric permittivity. The result is that Pq is a Lorentzian function of the scattering wavevector q for length scales greater than the particle size; the width approximately corresponding to twice the particle radius. This simple model reasonably accounts for scattered light from both swollen and shrunken microgel phases.

Internal structure of clusters from charge heteroaggregation.

The internal structure of clusters formed by colloidal heteroaggregation of particles with opposite signs of charge is studied by means of computer simulations. Every particle is surrounded by a layer of particles of opposite sign, a second neighbors shell of particles mainly with the same sign, a third one of opposite sign, etc. As the distance from the particle increases, the system becomes more homogeneous and no difference between the numbers of particles with similar or opposite signs of charge can be noticed for distances larger than ten times the particle radius. For low ionic concentrations the local environment of particles is formed by quasi-straight branches, where the sign of charge alternates, and at high concentrations the structure of the cluster is typical of DLCA and the alternation is restricted to very short distances. However, this effect is not responsible for the low fractal dimensions observed in charge heteroaggregates.

Induced asymmetries in the heteroaggregation of oppositely charged colloidal particles.

Heterocoagulation of cationic and carboxylated polystyrene latexes is studied for a wide range of salt concentrations by static light scattering. The weak character of the surface groups providing the charges allows variation of the relative charge of the systems. Two situations are studied: both latexes with similar surface charges and with very different ones. In both cases at low ionic concentration pure heteroaggregation takes place, whereas diffusive aggregation is observed at high kappa, above the critical coagulation concentration (C.C.C.) of both latexes. The overall rate of aggregation describes a minimum at intermediate salt concentrations when both latexes bear similar charges. The heterocoagulation rate constant decreases continuously to reach the diffusive value at high salt. An interesting behavior is observed when the latexes have very different charge. The heterocoagulation kinetic constant becomes diffusive above the C.C.C. of the less charged latex.

Clustering under short-range finite interactions.

In this paper the aggregation of surface modified colloidal particles is presented, paying special attention to the cluster structure and growth. The surface was modified by adsorbing bovine serum albumin (BSA). The interaction potential develops a minimum of restricted depth, weakening the clusters which subsequently restructure and form more compact morphologies. This minimum is responsible for the reversibility of the aggregation processes (this is an important difference between diffusion-limited cluster aggregation and reaction-limited cluster aggregation). The energy minimum is associated with the presence of a steric term in the energy balance, which depends on the size of the adsorbed molecules. BSA molecules with different sizes were employed to test this point. In addition, the short-range interaction seems not to affect significantly the paths of approximating particles, since the aggregation of the clusters at long times is independent of the size of these particles. The long-time kinetics was interpreted in the frame of dynamic scaling concepts. A kinetics model, including surface-surface, protein-surface, and protein-protein aggregation, is used to determine the dominant mechanism controlling the aggregation.

Thermodynamics of ionic microgels.

We present a theory of dilute aqueous suspensions of microgel particles. It is found that as the number of charged monomers in the polymer network composing mesoscopic gel increases, the particles undergo a swelling transition. Depending on the hydrophobicity of the polymer, this transition can be either continuous or discontinuous. Furthermore, similar to charge stabilized colloidal particles, we find that the electrophoretic mobility of the microgel is controlled by an effective charge. Unlike the colloids, however, for which the effective charge grows asymptotically with the logarithm of the bare charge, the effective charge of an ionic microgel scales as Z(eff) approximately Z0.5. The findings are in good agreement with the experimental measurements.

Structural modifications in the swelling of inhomogeneous microgels by light and neutron scattering.

Small-angle neutron scattering and dynamic light scattering have been used to study the thermodynamics of swelling and the associated structure modifications of highly cross-linked temperature-sensitive poly (N-isopropylacrylamide) [poly(NIPAM)] microgels in D2O. A particle core-shell model is proposed, with the core containing most of the cross-linker molecules. The Flory-Rehner theory, with the inclusion of a concentration dependent Flory solvency parameter, successfully describes the experimental swelling, despite the inhomogeneous character of the particles. Interestingly, the shell evolution with temperature controls the whole particle swelling, exerting an external pressure over the core, which in turn influences its size during the swelling process. Scaling laws for the correlation lengths were found with respect to temperature and polymer concentration. Finally, it has been encountered that for the collapsed microgel states, the particle surface seems to have a fractal character.

Structure formation from mesoscopic soft particles.

In this work, the aggregation of mesoscopic gel particles (soft colloids) has been experimentally investigated. The interaction between particles was controlled through the addition of salt, above the critical coagulation concentration, resulting in aggregation with finite bond energies. Attention has been paid to the structure of the clusters formed in the process as well as to the aggregation kinetics. The results indicate that the clusters are fractal and the kinetics of aggregation can be described through the dynamic scaling solution of the Smoluchowski equation. As the energy minimum increases in depth the resultant clusters pass from a very compact structure to typical diffusion-limited cluster aggregation (DLCA) fractal dimension values. In addition, the kinetics of growth change from those observed in reaction controlled aggregation to DLCA. These results can be explained within the framework of a reversible growth model, arising from the fact that aggregation takes place in an energy minimum of restricted depth. Moreover, they show that structure and kinetics decouple for such a soft sphere system, in contrast to what is encountered for DLCA and reaction-limited processes. Finally, an unexpected return to a reaction controlled aggregation kinetics was observed for sufficiently deep energy minima, which could be due to the polymerlike particularities of the soft particles considered in this work.

Nonlinear effects in the stability of highly charged colloidal suspensions.

We investigate the nonlinear effects related to the formation of particle-counterion clusters in highly charged asymmetric colloidal suspensions. The ocurrence of such clustering is experimentally probed by studying the stability of the colloidal system. The results demonstrate that a renormalized charge is needed in order to explain the observed critical coagulation concentrations. This renormalization is predicted by an extension of the Debye-Hückel-Bjerrum liquid state theory [A. Diehl, M. C. Barbosa, and Y. Levin, Europhys. Lett. 53, 86 (2001)]. Therefore, counterion condensation seems to become apparent in particle aggregation processes through control of the repulsive barrier that keeps the system stable. As a consequence of the agreement, new insights into the microscopic state of highly charged complex fluids follow.

Charge heteroaggregation between hard and soft particles.

In this paper, the heteroaggregation of opposite sign hard and soft colloidal particles has been studied by static and dynamic light scattering. The structure of the aggregates, as well as the aggregation kinetics, have been investigated. At low electrolyte concentration, where both long-range electrostatic repulsive and attractive forces are present, the aggregates were found to be more open than expected for diffusion-limited cluster aggregation (DLCA). However, the aggregate size time evolution is characteristic of diffusion-controlled processes. At high electrolyte concentration, where DLCA would be expected, very compacted clusters were found, as well as very rapid aggregation, leading to high polydispersity. These latter findings are interpreted in terms of the differences in the homoaggregation speeds for the hard and soft particles.

Particle-counterion clustering in highly charge-asymmetric complex fluids.

The formation of particle-counterion clusters through electrostatic interaction is studied in this work for a highly charge asymmetric colloidal suspension. The occurrence of such clustering is probed by the particle electrophoretic mobility, i.e., with the aid of a transport property. The results show that the effective charge manifesting under the presence of an external electric field is the renormalized charge predicted by an extension of the Debye-Hückel-Bjerrum theory to the fluid state of highly charged colloids.

Measurement of Absolute Coagulation Rate Constants for Colloidal Particles: Comparison of Single and Multiparticle Light Scattering Techniques

The absolute coagulation rate constants of monodisperse spherical colloids in aqueous suspension were measured at the early stage of the coagulation processes by using two different techniques: single particle light scattering (SPLS) and simultaneous static and dynamic light scattering (SLS + DLS). Single particle light scattering determines the cluster-size distribution directly by counting the number of different clusters during the coagulation process and therefore can be used to test the kinetic growth model used for obtaining the coagulation rate constants. The simultaneous static and dynamic light-scattering method is an in-situ experiment, where the average cluster size is estimated by simultaneous static and dynamic light-scattering measurements at different angles on a multiangle fiber-optics-based setup. A combined evaluation of the static and dynamic light-scattering data permits the determination of the absolute rate constants without the explicit use of light scattering form factors and hydrodynamic properties for dimers. In this paper, we compare results obtained with both techniques. Good agreement was found for the absolute coagulation rate constants of two different latex particle suspensions. Furthermore, we show that the two techniques complement one another and the limitations of the first are overcome by the second and vice versa. Copyright 1997Academic Press