Volume 229, Issue 7-8 - Special Issue: Self-Assembled Soft Matter Nano-Structures at Interfaces Dedicated to Gerhard Findenegg on the occasion of his 75th birthday / Regine v. Klitzing, Michael Gradzielski

This paper deals with the synthesis of Pt and Pt/Pb nanoparticles via oil-in-water microemulsions. Studying systematically the phase behavior of microemulsions which contain metal precursors, it was possible to determine the influence of the metal precursors on the phase behavior of the base microemulsion. It was found that the addition of Pb(ethylhexanoate) 2 to the microemulsion shifts the phase boundaries to slightly lower temperatures, while adding dimethyl-(cyclooctadiene) platinum has no effect on the phase boundaries. SAXS measurements confirmed that the addition of the metal precursors does not influence the size of the microemulsion droplets along the emulsification failure boundary. Using transmission electron microscopy (TEM) we found that by increasing the oil content w B in the microemulsion from 0.04 to 0.08 the size of the resulting mono- and bimetallic Pt and Pt/Pb nanoparticles changes from 3 to 4.4 nm.

The self-assembly of colloidal particles into well-defined structures, such as anisometric ones, has shown to be both a challenging and promising field in today's research. We report a detailed analysis of the processes of drying and the associated colloidal assembly-induced deformation of aqueous droplets containing fumed silica (FS) on a superhydrophobic surface. The use of FS as colloidal building block can lead to the formation of anisometric supraparticles by droplet deformation due to the formation of a dense silica shell at the water-air interface. We demonstrate how this can be done in a controlled way by adjusting the initial suspension's ionic strength using NaCl. An interpretation of the critical deformation point is given and we derive an empirical formula to predict the extent of anisometry depending on the initially applied salt and FS concentration. By addition of polystyrene microspheres we prove the general applicability of this method to droplets containing dispersions of various types of colloidal material, thereby forming hybrid supraparticles. This allows for introducing additional functionality to anisometric particles prepared with FS.

External fields (magnetic and electric) present a simple, robust and efficient route to manipulate and assemble colloidal particles. We report how biparticle dispersions can be assembled into well-defined arrays of tunable morphology using external AC electric field. Binary dispersions of strongly and weakly charged colloidal particles were arranged into linear composite chains via dipole-dipole attraction. The frequency of the applied electric field was the first control parameter for reversibly tuning the biparticle attraction from longitudinal assembly (in the direction of field) to the traverse one (perpendicular to the field). We show that in addition to frequency, spatial limitations play a key role in the assembly process and may assist in the formation of short bidirectional chain-like clusters or characteristic highly structured strings of colloidal triplets. Thus, we control the long-range organization through a combination of particle size ratio, concentration ratio and field frequency. The new strategy to reconfigure the microstructures can find application in better control of the field driven colloidal assembly processes and may be extended to the formation of more complex and precisely arranged particle networks.

This review article summarizes the progress of research in the field of polymer brush/metal nanoparticle hybrid materials. We will discuss the mutual influence of polymer brush matrix and particles. Self-assembly of particles within polymer brushes, and ways to control the loading and location of nanoparticles inside polymer brushes will be described, as well as the possibility to use the brush templates as nanoreactors to generate metal nanoparticles. The combination of stimuli-responsive polymer brushes and nanoparticles exhibiting surface plasmon resonance, such as gold or silver, enables the design of optical sensors based on reversible variations of the brush conformation. Sensing devices are capable of detecting a variety of extrinsic variations in their surrounding enviroments. The progress in the development of such optical sensors using brush/particle hybrids will be discussed in more detail.

Planar polyelectrolyte brushes are prepared by Langmuir–Schaefer based grafting of perdeuterated (styrene) 49 -b-(acrylic acid) 222 block copolymer (dPS-b-PAA) to dPS pre-coated silicon supports with grafting density σ PAA from 0.07 to 0.11 nm – 2 . The structure of the solvent-swollen brushes, i. e. the volume fraction profile of polymer segments, ϕ PAA , as a function of altitude z from the grafting plane into the liquid phase is extracted from neutron reflectivity measurements. We find that for all cases investigated ϕ PAA ( z ) resembles a Gaussian profile. Although very condensed, the PAA brushes can be loaded with bovine serum albumin (BSA). The integral amount of adsorbed BSA scales linearly with grafting density. We compare our z -resolved volume fraction profile ϕ BSA ( z ) of BSA on PAA brushes with existing literature on that system. It is found that a cross-over takes place in the adsorption scheme from ternary compressive, where proteins can approach the grafting surface only by compressing the brush, to ternary insertive, where proteins enter the brush with only local perturbation of the concentration profile, as a function of R P / H max , where R P is the Stokes-Radius of the protein, and H max is the experimentally determined maximum height of the brush.

The interactions of two fragments of the human antimicrobial LL-37 (LL-32 and LL-20) with lipid monolayers at the soft liquid/air interface have been characterized. To model the interaction with mammalian cell membranes, lipid monolayers composed of the zwitterionic DPPC and DOPC were used. To investigate the interaction with bacterial cell membranes, lipid monolayers of anionic DPPG and POPG were used. DPPC and DPPG exhibit a first-order phase transition from the disordered liquid to the ordered condensed state, whereas POPG and DOPC monolayers are in the fluid disordered state at all surface pressures studied. Therefore, the influence of the monolayer phase state on peptide-lipid interactions can be studied. To obtain insight into the peptide structure and their influence on phospholipid membranes, film balance measurements were coupled with surface sensitive Infrared Reflection-Absorption Spectroscopy (IRRAS). The results were compared to CD measurements in bulk. LL-32 is more surface active and can better intercalate into lipid monolayers than LL-20. Even though LL-32 has no cell-selectivity, our results show how the peptide interacts differently with zwitterionic compared to anionic membrane models. The interaction with DPPC monolayers is based on simple intercalation of the peptides between the lipid molecules. However, the peptides bind in a two-step process to DPPG monolayers, which results in a fluidization of the lipid film. This can be related to a membrane thinning.

Depletion potentials induced by the rod-like fd-virus between colloidal probe spheres and a flat glass wall were measured using total internal reflection microscopy. The objective of this investigation was to determine the limits of the approximations used for the theoretical description of the depletion pair potential. Data were obtained at rod concentrations as high as 15 times their overlap concentration and the size ratio between the rod length and the sphere radius increased up to L / R  = 1.76. With this, the basic assumptions for low density approximation and Derjaguin approximation were clearly violated. Nevertheless, we observed good agreement between experimental data and predictions up to the highest size ratio and in rod concentrations of seven times their overlap concentration. Only at higher concentrations and at the lowest size ratio investigated the experimental data deviate significantly from the simple model.

The role of condensed counterions in transport properties, such as electrical conductivity and viscosity, has been investigated with solutions of a flexible polyelectrolyte. Comparisons with existing theories are proposed. Viscosity is strongly affected by confinement in thin films, depending whether polyelectrolyte chains are adsorbed or not at the film surfaces. The role of counterion mobility is however difficult to assess because the measurements are not accurate enough. It is proposed that this role could be tested by electrical conductivity measurements.

Experimental results on sorption-induced deformation during n-pentane desorption were obtained by in-situ dilatometry and in-situ small angle X-ray scattering (SAXS). The sample investigated was a silica-based monolith with hierarchical pore structure comprising a macroporous network of struts, each strut containing well-defined cylindrical mesopores ordered on a 2D hexagonal lattice. In-situ dilatometry and in-situ SAXS measurements revealed strain isotherms of similar shape, which are qualitatively in good agreement with recent theoretical predictions. From the relative pressure range of the liquid filled mesopores a pore load modulus of 1.5 GPa is determined. The relative pressure region of mono- and multilayer formation, however, reveals differences between the two independent methods. In particular, the net strain at saturation pressure is considerably larger for in-situ dilatometry. We attribute this observation to the different sensitivity of the two methods to anisotropic deformation in the hierarchical solid framework. While in-situ SAXS measures the mesopore lattice strain and is therefore exclusively sensitive to radial deformation of the struts, dilatometry measures the linear deformation of the isotropic macroscopic sample, being sensitive to the volumetric strain of the whole network.

Interactions of ammonia and water with the oxygen-containing functional groups of graphite oxide have been studied by ab initio density functional theory method. The results show that ammonia can dissociate on the carboxyl, epoxy and hydroxyl groups. The dissociation on the epoxy group is an exothermic reaction with a small activation energy barrier. Water is found to form a hydrogen bond with the carboxyl, epoxy and hydroxyl groups, and thus will block ammonia from interacting with those functional groups. The results in this work provide a fundamental understand of previous experiments about ammonia adsorption on graphene oxide materials.

Stimuli responsive surface structures attract increasing attention due to a large variety of envisioned applications. The controlled organization of poly( N -isopropyl acrylamid), PNIPAM microgel particles at solid surfaces inspired numerous research activities. In this review article, we briefly discuss the swelling/deswelling properties of adsorbed microgel particles in comparison to the behavior in the bulk phase. The presence of the solid interface highly influences and changes their behavior with respect to the properties in solution. Furthermore, the confinement on a solid substrate allows the direct and in-situ investigation of the mechanical properties of the microgel particles. Additionally, we briefly review the research on microgel particles at liquid interfaces. At these interfaces new interesting effects occur. Moreover, we discuss some interesting work on potential applications. In this context, microgel particles are often used as an active component for responsive coatings of various functionality envisioning applications, e.g. in medicine, biotechnology, and nanooptics.

Organic photovoltaic (OPVs) devices are promising due to their low cost, light weight, and compatibility with high throughput processing on flexible substrates. This paper demonstrates a simple process utilizing thin-film instabilities to enhance light absorption in OPVs in a way that is compatible with planar processing and the customary thermal annealing steps. Placing a thin, transparent polystyrene (PS) film between the glass substrate and the transparent conductive indium tin oxide (ITO) electrode results in the formation of periodic surface buckles in the PS layer due to induced strain caused by thermal expansion mismatch between the ITO and PS films. OPVs comprising bilayer laminates of copper phthalocyanine (CuPc) and fullerene (C 60 ) deposited onto buckled the ITO/PS substrate show enhanced light absorption due to the longer path-length and improved power conversion efficiency (20%) relative to a similar planar device. This approach is appealing because it takes advantage of naturally-occurring surface topography (i. e., buckling) without the need for any sophisticated patterning. This work is distinguished from other buckling strategies for OPVs by the use of ITO as a transparent, conductive electrode and the absence of additional processing steps.