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Matthias Ballauff

Publications and source records attributed to Matthias Ballauff.

18 recordsLinked to original sources
PubMed2007

Nanoblossoms: light-induced conformational changes of cationic polyelectrolyte stars in the presence of multivalent counterions.

We analyze the structure of star-shaped polyelectrolytes in the presence of di- and trivalent counterions, and we use the gained knowledge to manipulate the polyelectrolyte's conformation by light. By applying dynamic light scattering and atomic force microscopy, we demonstrate that, at constant ionic strength, the arms of the cationic polyelectrolyte retract when adding multivalent counterions. Adding trivalent hexacyanocobaltate(III) ions leads to a collapse of the polyelectrolyte star even at low concentrations. This is shown by analysis of the star polyelectrolytes in solution as well as in the adsorbed state on mica surfaces. Considerably higher salt concentrations are necessary to obtain the same contraction of the polyelectrolyte star if the divalent tetracyanonickelate(II) ions are used. Sufficiently high multivalent counterion concentration leads finally to the precipitation of the polymer from the solution. We demonstrate that we can switch a polyelectrolyte star from the collapsed to the expanded state by transforming the trivalent hexacyanocobaltate(III) ions into a mixture of mono- and divalent ions by UV light. Thus, these collapsed stars react to light like "nanoblossoms". Moreover, polyelectrolyte stars precipitated through addition of the trivalent hexacyanocobaltate(III) ions can be redissolved by irradiation with light (photoinduced dissolution). Hence, the conformation and interaction of star polyelectrolytes can be switched by light. Possible applications of this novel way of manipulating polymers are discussed.

Cations↗
PubMed2006

The solution structure of stilbenoid dendrimers: a small-angle scattering study.

The spatial structure of a stilbenoid dendrimer is investigated by small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) in dilute solution. All measurements are performed in toluene. The dendrimer consists of a stilbenoid scaffold with appended hexyloxy chains. SAXS is mainly sensitive to the dendrimer scaffold whereas SANS intensity, measured in fully deuterated toluene, derives from the solute molecules. The resulting SAXS and SANS intensities are analyzed by comparison with various models. It is found that the model of a circular disk gives the best description of the data. SAXS data demonstrate that the stilbenoid scaffold is flat as expected for benzene rings conjugated through vinylene units. Thus, it can be described by a circular disk with a radius of 1.6 nm and a thickness of 0.7 nm. SANS, on the other hand, shows that the hexyloxy chains are not confined in the plane defined by the core. This is derived from modeling the SANS data with a much thicker circular disk (radius: 2.4 nm, thickness: 1.8 nm). The structure factor S(q), describing the interaction of the dendrimers at higher concentrations, is modeled quantitatively with the "polymer reference interaction site model" (PRISM) integral equation formalism for hard plates such as particles. Here the structural data obtained from the analysis of the SANS data are used so that no new adjustable parameter is necessary for this description.

Journal Article↗
PubMed2006

Collapse of spherical polyelectrolyte brushes in the presence of multivalent counterions.

We consider the interaction of multivalent counterions with spherical polyelectrolyte brushes (SPB). The SPB result if linear polyelectrolyte (PE) chains (contour length: 60 nm) are densely grafted to colloidal spheres of 116 nm in diameter. Dispersed in water, the surface layer consisting of chains of the strong PE poly(styrene sulfonic acid) (PSS) will swell. We demonstrate that successive addition of trivalent ions (La3+) leads to a collapse in which the surface layer is shrinking drastically. All findings are discussed on the base of a theoretical mean-field approach using the Donnan equilibrium. The ion exchange and a strong binding of trivalent ions by PE chains is followed up by a drop in the osmotic pressure inside the brush. This reduction is the driving force for the collapse. The strong ion-chain correlation is discussed with results obtained from molecular dynamics simulations.

Cations↗
PubMed2006

Adhesion of spherical polyelectrolyte brushes on mica: an in situ AFM investigation.

We demonstrate that the adsorption of cationic spherical polyelectrolyte brushes (SPB) on negatively charged mica substrates can be controlled in situ by the ionic strength of the suspension. The SPB used in our experiments consist of colloidal core particles made of polystyrene. Long cationic polyelectrolyte chains are grafted onto these cores that have diameters in the range of 100 nm. These particles are suspended in aqueous solution with a fixed ionic strength. Atomic force microscopy (AFM) in suspension as well as in air was used for surface characterization. In pure water the polymer particles exhibit a strong adhesion to the mica surface. AFM investigations of the dry samples show that the particles occupy the identical positions as they did in liquid. They were not removed by the capillary forces within the receding water front during the drying process. The strong interaction between the particles and the mica surface is corroborated by testing the adhesion of individual particles on the dried surface by means of the AFM tip: after a stepwise increase of the force applied to the surface by the AFM tip, the polymer particles still were not removed from the surface, but they were cut through and remained on the substrate. Moreover, in situ AFM measurements showed that particles which adsorb under liquid in a stable manner are easily desorbed from the surface after electrolyte is added to the suspension. This finding is explained by a decreasing attractive particle-substrate interaction, and the removal of the particles from the surface is due to the significant reduction of the activation barrier of the particle desorption. All findings can be explained in terms of the counterion release force.

Aluminum Silicates↗
PubMed2006

Micellar spheres in a high frequency oscillatory field.

The viscoelasticity of aqueous micellar solutions of two oxyethylene/oxybutylene block copolymers (E(92)B(18) and B(20)E(510)) has been investigated using a torsional resonator operated at 26 kHz. For both systems considered, values of the dynamic viscosity (eta'(infinity)) point to partial draining of the micellar corona induced by the high-frequency oscillatory field. At low effective volume fractions, values of the elastic modulus (G'(infinity)) indicate that the repulsive interactions between micelles can be modeled by a power law function u(r) proportional to 1/r(nu) with exponents close to 13 and 6 for copolymers E(92)B(18) and B(20)E(510) respectively. At a critical copolymer concentration (c*) plots of log(G'(infinity)) against log(c) deviate from the straight lines established at low concentrations, implying that the systems undergo ergodic/nonergodic transitions.

Journal Article↗
PubMed2006

Imaging the volume transition in thermosensitive core-shell particles by cryo-transmission electron microscopy.

We report a study of colloidal thermosensitive core-shell particles by cryo-transmission electron microscopy (cryo-TEM). The particles consist of a solid core of poly(styrene), onto which a network of cross-linked poly(N-isopropylacrylamide) (PNIPAM) is affixed. In water, the shell of these particles swells when the temperature is low. Raising the temperature above 32 degrees C leads to a marked shrinking of the shell. In this letter, we present the first study of these core-shell particles by cryo-TEM in situ, that is, in aqueous solution. We demonstrate that the core-shell particles are well-defined and exhibit a narrow size distribution. In particular, the PNIPAM shell is compact and has a defined outer surface of a slightly irregular shape. The micrographs show that there are density fluctuations within the network. Cryo-TEM of the system above and below the transition temperature furnishes information about the thermosensitive particles that had not been available through other methods employed in previous investigations.

Journal Article↗
PubMed2006

Thermosensitive core-shell particles as carrier systems for metallic nanoparticles.

We present a new system that allows us to modulate the catalytic activity of metal nanoparticles (Ag) by a thermodynamic transition that takes place within the carrier system. Thermosensitive core-shell particles have been used as the carrier system in which the core consists of poly(styrene) (PS), whereas the shell consists of a poly(N-isopropylacrylamide) (PNIPA) network cross-linked by N,N'-methylenebisacrylamide (BIS). Immersed in water, the shell of these particles is swollen. Heating the suspension above 32 degrees C leads to a volume transition within the shell that is followed by a marked shrinking of the network of the shell. The maximum degree of swelling can be adjusted by the degree of cross-linking. Silver nanoparticles with diameters ranging from 6.5 to 8.5 nm have been embedded into thermosensitive PNIPA networks with different cross-linking densities. The Ag nanoparticles do not influence the swelling and the shrinking of the network in the shell. The surface plasmon absorption band of the nanoparticles is shifted to higher wavelengths with temperature. This is traced back to the varying distance of the nanoparticles caused by the swelling and the shrinking of the shell. The catalytic activity is investigated by monitoring photometrically the reduction of 4-nitrophenol by an excess of NaBH4 in the presence of the silver nanocomposite particles. The rate constant kapp was found to be strictly proportional to the total surface of the nanoparticles in the system. Moreover, kapp is first decreasing with increasing temperature when approaching the volume transition. This is due to the strong shrinking of the network. Only at temperatures above the volume transition is the normal Arrhenius-type dependence of kapp found again. In this way, catalytic activity of the metal nanoparticles enclosed in a "nanoreactor" can be modulated by volume transition over a wide range.

Journal Article↗
PubMed2005

High catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes.

We present a study on the catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes that act as carriers. The spherical polyelectrolyte brushes consist of a solid core of poly(styrene) onto which long chains of poly(2-methylpropenoyloxyethyl) trimethylammonium chloride are grafted. These positively charged chains form a dense layer of polyelectrolytes on the surface of the core particles ("spherical polyelectrolyte brush") that tightly binds divalent PtCl6-(2) ions. The reduction of these ions within the brush layer leads to nearly monodisperse nanoparticles of metallic platinum. The average size of the particles is approximately 2 nm. The composite particles exhibit excellent colloidal stability. The catalytic activity is investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4 in the presence of the nanoparticles. The kinetic data could be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. In all cases, a delay time t0 has been observed, after which the reactions start. This time is shorter when the catalyst has already been used. All data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles.

Journal Article↗
PubMed2005

High elongation of polyelectrolyte chains in the osmotic limit of spherical polyelectrolyte brushes: a study by cryogenic transmission electron microscopy.

We investigate the conformation of long polyelectrolyte chains attached to colloidal latex particles by cryogenic transmission electron microscopy (cryo-TEM). The dense grafting of the polyelectrolyte chains ("polyelectrolyte brush") leads to a confinement of the counterions and a concomitantly high osmotic pressure within the polyelectrolyte layer attached to the core particles. Cryo-TEM has provided first model-independent direct proof for the strong stretching of the polyelectrolyte chains by direct visualization. If salt is added, cryo-TEM clearly shows how chains collapse because of the strong screening of the electrostatic interaction. Moreover, the analysis of interacting particles by cryo-TEM shows that the polyelectrolyte chains retract at close contact. Hence, we demonstrate how cryo-TEM can be used to analyze directly the spatial structure of polyelectrolyte brushes in situ.

Journal Article↗
PubMed2005

Polyelectrolyte-mediated protein adsorption: fluorescent protein binding to individual polyelectrolyte nanospheres.

We have used confocal fluorescence microscopy with single molecule sensitivity to characterize uptake and release of fluorescent protein (mEosFP) molecules by individual spherical polyelectrolyte brush (SPB) nanoparticles that were immobilized on a glass surface. The SPB particles consisted of a solid core particle of 100 nm diameter onto which long polyelectrolyte chains were affixed. They could be loaded with up to 30 000 mEosFP molecules in a solvent of low ionic strength. The concentration dependence of protein loading can be described with a simple bimolecular binding model, characterized by an equilibrium dissociation coefficient of 0.5 microM. Essentially complete release of the bound proteins was observed after increasing the ionic strength by adding 250 mM NaCl to the solvent. Fluorescence emission spectra and time-resolved fluorescence intensity decays were measured on individual, mEosFP-loaded SPB nanoparticles, and also on the dissolved mEosFP before and after adsorption. These results indicate that the mEosFP molecules remained structurally intact in this procedure. Hence, the present investigation demonstrates unambiguously that polyelectrolyte-mediated protein adsorption onto SPB particles presents a viable process for protein immobilization.

Adsorption↗
PubMed2005

Flow curves of dense colloidal dispersions: schematic model analysis of the shear-dependent viscosity near the colloidal glass transition.

A recently proposed schematic model for the nonlinear rheology of dense colloidal dispersions is compared to flow curves measured in suspensions that consist of thermosensitive particles. The volume fraction of this purely repulsive model system can be adjusted by changing temperature. Hence, high volume fractions (phi</=0.63) can be achieved in a reproducible manner. The quantitative analysis of the flow curves suggests that the theoretical approach captures the increase of the low shear viscosity with increasing density, the shear thinning for increasing shear rate, and the yielding of a soft glassy solid. Variations of the high shear viscosity can be traced back to hydrodynamic interactions which are not contained in the present approach but can be incorporated into the data analysis by an appropriate rescaling.

Journal Article↗
PubMed2005

Temperature-induced unfolding of ribonuclease A embedded in spherical polyelectrolyte brushes.

We use Fourier Transform infrared spectroscopy (FT-IR) spectroscopy to study the thermal unfolding and refolding behavior of ribonuclease (RNase A) adsorbed to spherical polyelectrolyte brushes (SPB). The SPB consist of a solid poly(styrene) core of ca. 100 nm diameter onto which long chains of poly(styrene sulfonic acid), PSS have been densely attached. The particles bearing the adsorbed protein are dispersed in aqueous buffer solution at a pH close to the isoelectric point (9.6) of the protein. The secondary structure of the protein was analyzed by FT-IR spectroscopy and compared to the structure of the native protein before adsorption. The unfolding of the free RNase A in solution was found to be fully reversible with an unfolding temperature of 65 degrees C, in accordance to previous studies. However, after adsorption to the SPB, the unfolding temperature of the protein molecule is lowered by 10 degrees C and the Van't Hoff enthalpy of the unfolding process is significantly reduced. Moreover the unfolding of the adsorbed protein is irreversible. The phenomenon may be explained by an increase in binding sites due to unfolding of the globular structure. Protein adsorption to a spherical polyelectrolyte brush.

Adsorption↗
PubMed2004

Dendrimers in solution: insight from theory and simulation.

A variety of experimental and theoretical approaches show that, akin to linear polymers, dendrimers in good solvent conditions are best described as flexible macromolecular aggregates with a dense core and fluctuating monomer groups. We present theoretical and simulational evidence of how the shape and inner structure of dendrimers depends on the generation number as well as the effective interactions that exist between dendrimers in solution. These approaches based on simplified dendritic structures show there is a tunable and ultrasoft interaction between the centers of the solublized dendrimers. Results from small-angle neutron scattering data confirm the theory and indicate that dendrimers are model systems of ultrasoft colloids that bridge the gap between polymers and hard spheres. Dendrimers can form a class of materials analogous to the related systems of star polymers and block copolymer micelles which exhibit special properties.

Journal Article↗
PubMed2004

Secondary structure analysis of proteins embedded in spherical polyelectrolyte brushes by FT-IR spectroscopy.

The adsorption of bovine serum albumin (BSA), bovine beta-lactoglobulin, and bovine pancreatic ribonuclease A onto spherical polyelectrolyte brushes (SPB) is reported. The SPB consist of narrowly distributed poly(styrene) core particles (diameter approximately 100 nm) onto which linear chains of anionic polyelectrolytes are grafted. The polyelectrolyte shell consists of either the weak polyelectrolyte poly(acrylic acid) or the strong polyacid poly(styrenesulfonate). The SPB particles are dispersed in H(2)O at room temperature. The secondary structure of the proteins was investigated by Fourier transform infrared spectroscopy in transmission mode before and during adsorption to these colloidal brushes. The alpha-helix and beta-sheet content of the proteins was nearly fully retained in the adsorbed state for all systems. Only in the case of BSA interacting with poly(styrenesulfonic) brushes could a slight loss of alpha-helix structure be observed. As the interaction of SPB and proteins can be controlled by the ionic strength in the buffer, additional experiments were performed to release the adsorbed protein. The amount of released protein was quantified and was found to be strongly dependent on the kind of protein and brush used. The secondary structure of the released proteins could be analyzed as well. An almost full preservation of secondary structure was found. This demonstrates that SPB are well-suited to immobilize proteins. The SPB can be charged and decharged under retention of the secondary structure of the biomolecules.

Animals↗
PubMed2004

High activity of enzymes immobilized in colloidal nanoreactors.

We present a novel type of nanoreactor suitable for the immobilization of enzymes. The particles used consist of a polystyrene core onto which long chains of poly(acrylic acid) are grafted ("spherical polyelectrolyte brush"). Proteins adsorbed spontaneously onto these particles from aqueous solutions if the ionic strength is low. We immobilized glucoamylase on these particles and showed that this enzyme keeps nearly its full activity. This is shown by analyzing the enzymatic activity in terms of the Michaelis-Menten kinetics. No leaching out of the enzyme takes place during the reaction and the colloidal stability is not impeded by the adsorbed biomolecules. The data presented here suggest that the principle of immobilizing enzymes on these particles may be of general use.The Figure shows a schematic representation of the colloidal nanoreactors.

Acrylic Resins↗