Matthias Ballauff

Thermosensitive Au-PNIPA yolk-shell nanoparticles with tunable selectivity for catalysis.

Metallic nanoparticles have been the subject of intense research recently because of their catalytic properties, which may differ considerably from the bulk metal. As the free nanoparticles tend to aggregate and are difficult to handle in catalytic applications, colloidal carrier systems have been developed that encapsulate and stabilize the particles. 5] More recently, so-called smart carrier systems, such as thermosensitive microgels, have become the focus of research. These hybrids react on external stimuli and allow the catalytic properties to be altered accordingly. Thus, thermosensitive polystyrene (PS)-poly(N-isopropylacrylamide) (PNIPA) core–shell microgels were applied as the active nanoreactor for the immobilization of metal nanoparticles. The catalytic activity of immobilized metal nanoparticles can be tuned by the swelling and shrinking of the microgels. Liz-Marz n et al. have developed a AuPNIPA core–shell colloidal system. They found that the thermoresponsive PNIPA shell with limited cross-linking allows for particularly efficient control of the catalysis of encapsulated Au nanoparticles. Recently, yolk–shell structures that consist of a single metal nanoparticle within an inorganic or polymeric shell 19] have become the subject of intense research. These systems can be used to tune the catalytic activity of the enclosed nanoparticle by a suitable architecture of the shell. Yolk–shell structures have the clear advantages in that individual metal nanoparticles are enclosed in a compartment that prevents aggregation with other nanoparticles. Furthermore, the embedded gold nanoparticle has a free surface that is not blocked by any surface group or polymer compared to the Au-PNIPA core–shell system. Moreover, the permeability of the shell may be tuned to a certain extent. Therefore, yolk– shell systems may be regarded as true nanoreactors that allow the catalytic activity of single nanoparticles to be studied in a defined environment. Herein we present a thermosensitive yolk–shell system that uses temperature as a trigger for reaction. Figure 1a shows the underlying principle of these systems: A single Au nanoparticle is encapsulated in a hollow thermosensitive PNIPA shell. The porosity and the hydrophobicity of this shell can be tuned in a well-defined manner by temperature while the colloidal stability of the entire hybrid is fully maintained. We show this by monitoring the reduction of hydrophilic 4-

Self-Assembly of Janus Cylinders into Hierarchical Superstructures

We present in-depth studies of the size tunability and the self-assembly behavior of Janus cylinders possessing a phase segregation into two hemicylinders. The cylinders are prepared by cross-linking the lamella-cylinder morphology of a polystyrene-block-polybutadiene-block-poly(methyl methacrylate) block terpolymer. The length of the Janus cylinders can be adjusted by both the amplitude and the duration of a sonication treatment from the micro- to the nanometer length. The corona segregation into a biphasic particle is evidenced by selective staining of the PS domains with RuO(4) and subsequent imaging. The self-assembly behavior of these facial amphiphiles on different length scales is investigated combining dynamic light scattering (DLS), small-angle neutron scattering (SANS), and imaging procedures. Cryogenic transmission electron microscopy images of the Janus cylinders in THF, which is a good solvent for both blocks, exhibit unimolecularly dissolved Janus cylinders with a core-corona structure. These results are corroborated by SANS measurements. Supramolecular aggregation takes place in acetone, which is a nonsolvent for polystyrene, leading to the observation of fiber-like aggregates. The length of these fibers depends on the concentration of the solution. A critical aggregation concentration is found, under which unimolecularly dissolved Janus cylinders exist. The fibers are composed of 2-4 Janus cylinders, shielding the inner insoluble polystyrene hemicylinder against the solvent. Herein, the SANS data reveal a core-shell structure of the aggregates. Upon deposition of the Janus cylinders from more concentrated solution, a second type of superstructure is formed on a significantly larger length scale. The Janus cylinders form fibrillar networks, in which the pore size depends on the concentration and deposition time of the sample.

Gaussian effective interaction between flexible dendrimers of fourth generation: A theoretical and experimental study

We propose a theory for the effective interaction between soft dendritic molecules that is based on the shape of the monomer density profile of the macromolecules at infinite dilutions. By applying Flory-type arguments and making use of the experimentally measured density profiles, we derive a Gaussian effective interaction whose parameters are determined by the size and monomer number of the dendrimers that are derived from small-angle neutron scattering (SANS) measurements. By applying this theory to concentrated dendrimer solutions we calculate theoretical structure factors and compare them with experimental ones, derived from a detailed analysis of SANS-data. We find very good agreement between theory and experiment below the overlap concentration, where drastic shape deformations of the dendrimers are absent.

Experimental study of electrostatically stabilized colloidal particles: Colloidal stability and charge reversal

We consider the interaction of colloidal spheres in the presence of mono-, di-, and trivalent ions. The colloids are stabilized by electrostatic repulsion due to surface charges. The repulsive part of the interaction potential Ψ(d) is deduced from precise measurements of the rate of slow coagulation. These microsurface potential measurements allow us to determine a weak repulsion in which Ψ(d) is of the order of a few k(B)T. These data are compared to ζ potential measured under similar conditions. At higher concentrations both di- and trivalent counterions accumulate at the very proximity of the particle surface leading to charge reversal. The salt concentration c(cr) at which charge reversal occurs is found to be always above the critical coagulation concentration c(ccc). The analysis of Ψ(d) and of the ζ potential demonstrates, however, that adsorption of multivalent counterions starts far below c(cr). Hence, colloid stability in the presence of di- and trivalent ions cannot be described in terms of a DLVO ansatz assuming a surface charge that is constant with regard to the ionic strength.

Sphere-to-Rod Transition of Micelles formed by the Semicrystalline Polybutadiene-block-Poly(ethylene oxide) Block Copolymer in a Selective Solvent

We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)-block-poly(ethylene oxide), in the selective solvent n-heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5u2009kgu2009·u2009mol(-1) , respectively. In this solvent, micellization into a liquid PEO-core and a corona of PB-chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at -30u2009°C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (-196u2009°C; liquid nitrogen) leads to the transition from spherical to rod-like micelles. With time these rod-like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO-cores of the rod-like micelles increases. The transition from a spherical to a rod-like morphology can be explained by a decrease of solvent power of the solvent n-heptane for the PB-corona chains: n-Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology.

Core–shell microgels as “smart” carriers for enzymes

We present a thermodynamic study of the adsorption of lysozyme on a negatively charged core–shell microgel at pH 7.2. The carrier particles consist of a polystyrene core onto which a charged poly(N-isopropylacrylamide-co-acrylic acid) network is attached. Isothermal titration calorimetry (ITC) is used to investigate the temperature and salt dependence of lysozyme binding. Our ITC analysis unequivocally shows that the adsorption of lysozyme onto the charged gel is driven by entropy. The addition of salt strongly decreases the binding affinity, indicating significant electrostatic contributions to the adsorption process. However, at high salt concentrations, substantial protein binding with unaltered entropies is still observed pointing to large contributions from hydrophobic interactions. Furthermore, the calorimetric analysis suggests that protonation of lysozyme takes place upon binding. This is directly shown by analysis of the enzymatic activity of adsorbed lysozyme. It was found that the activity is enhanced about ∼3.5 times, indicating that lysozyme has taken up approximately one proton when entering the gel. The entire set of data demonstrates that core–shell microgels present “smart” colloidal carriers for lysozyme that enhance its activity.

Analysis of thermosensitive core–shell colloids by small-angle neutron scattering including contrast variation

We present an investigation of thermosensitive core–shell particles by small-angle neutron scattering (SANS). The particles consist of a solid poly(styrene) core and a shell of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. These latex particles are dispersed in water and have a diameter of ca. 150 nm. At ambient temperature the PNIPA-network in the shell is swollen but at higher temperature water is expelled and the shell undergoes a continuous volume transition. The radial extension of the shell is investigated as a function of temperature by use of SANS. The analysis by SANS is performed at different contrasts using appropriate mixtures of H2O and D2O. It demonstrates that the shell has a well-defined compact structure above the n volume transition. The swelling of the shell upon cooling can be described in terms of an affine expansion of the n network. This is followed by a slight decrease of the volume fraction with increasing distance to the surface of the cores. The analysis by SANS demonstrates that the phase behavior of the network in the shell may be undertaken in terms n of average volume fractions. It thus supplements the previous analysis by SAXS in a decisive manner.

Can dendrimers be viewed as compact colloids? A simulation study of the fluctuations in a dendrimer of fourth generation

By employing monomer-resolved Monte Carlo simulations, we analyze the conformations, density distributions, correlation functions, and the form factor of model dendrimers of fourth generation. We find that these objects are hybrids between polymer chains and compact colloidal particles, with the fluctuations of the monomers being correlated at length scales of the order of the bond length but practically uncorrelated for lengths exceeding this scale. We discuss the implications of this finding on the possibility of regarding dendrimers as “soft colloids,” on the detection of these fluctuations in scattering experiments and on the inversion of intensity profiles obtained in small-angle neutron scattering measurements.

Self-diffusion and cooperative diffusion in semidilute polymer solutions as measured by fluorescence correlation spectroscopy

We present a comprehensive investigation of polymer diffusion in the semidilute regime by fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS). Using single-labeled polystyrene chains, FCS leads to the self-diffusion coefficient while DLS gives the cooperative diffusion coefficient for exactly the same molecular weights and concentrations. Using FCS we observe a new fast mode in the semidilute entangled concentration regime beyond the slower mode which is due to self-diffusion. Comparison of FCS data with data obtained by DLS on the same polymers shows that the second mode observed in FCS is identical to the cooperative diffusion coefficient measured with DLS. An in-depth analysis and a comparison with current theoretical models demonstrates that the new cooperative mode observed in FCS is due to the effective long-range interaction of the chains through the transient entanglement network.

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.