Patrick van Rijn

Ferritin: a versatile building block for bionanotechnology.

Günther Jutz,†,§ Patrick van Rijn,†,‡,§ Barbara Santos Miranda,‡ and Alexander Böker*,† †DWI Leibniz-Institut für Interaktive Materialien e.V., Lehrstuhl für Makromolekulare Materialien und Oberflac̈hen, RWTH Aachen University, Forckenbeckstrasse 50, D-52056 Aachen, Germany ‡Department of Biomedical Engineering-FB40, W.J. Kolff Institute for Biomedical Engineering and Materials Science, University of Groningen, University Medical Center Groningen, A. Deusinglaan 1, 9713 AV Groningen, The Netherlands

Formation of catalytically active gold–polymer microgel hybrids via a controlled in situ reductive process

A newly developed N-vinylcaprolactam/acetoacetoxyethyl methacrylate/acrylic acid based microgel displays in situ reductive reactivity towards HAuCl4, forming hybrid polymer–gold nanostructures at ambient temperature without additional reducing agents. The colloidal gold nanostructure is selectively formed in the core of the microgel and the composite structure is used as a noble metal catalyst, the activity of which can be tuned depending on the size of the formed core. The hybrid particles can easily be isolated after catalysis via centrifugation and re-used with retention of the catalytic activity.

Challenges and advances in the field of self-assembled membranes

Self-assembled membranes are of vital importance in biological systems e.g. cellular and organelle membranes, however, more focus is being put on synthetic self-assembled membranes not only as an alternative for lipid membranes but also as an alternative for lithographic methods. More investigations move towards self-assembly processes because of the low-cost preparations, structural self-regulation and the ease of creating composite materials and tunable properties. The fabrication of new smart membrane materials via self-assembly is of interest for delivery vessels, size selective separation and purification, controlled-release materials, sensors and catalysts, scaffolds for tissue engineering, low dielectric constant materials for microelectronic devices, antireflective coatings and proton exchange membranes for polymer electrolyte membrane fuel cells. Polymers and nanoparticles offer the most straightforward approaches to create membrane structures. However, alternative approaches using small molecules or composite materials offer novel ultra-thin membranes or multi-functional membranes, respectively. Especially, the composite material membranes are regarded as highly promising since they offer the possibility to combine properties of different systems. The advantages of polymers which provide elastic and flexible yet stable matrices can be combined with nanoparticles being either inorganic, organic or even protein-based which offers pore-size control, catalytic activity or permeation regulation. It is therefore believed that at the interface of different disciplines with each offering different materials or approaches, the most novel and interesting membrane structures are going to be produced. The combinations and approaches presented in this review offer non-conventional self-assembled membrane materials which exhibit a high potential to advance membrane science and find more practical applications.

Aggregation-Driven Reversible Formation of Conjugated Polymers in Water

Come together: Self-assembly can drive the formation of conjugated imine polymers in water, and stabilization of otherwise unstable imine bonds is used to obtain fully π-conjugated, responsive dynamic covalent polyimines in aqueous environments. Both the optical properties and the aggregate morphology can be tuned by varying the aromatic monomers.

Bionanoparticles and hybrid materials: tailored structural properties, self-assembly, materials and developments in the field

We discuss new developments with respect to bionanoparticles as well as bionanoparticle hybrid systems and their use in composite materials. The recognition of the particle character and behavior of proteins and viral particles has a major impact on the development of novel nanoparticle systems and adds new functions and possibilities. The types of particles discussed in this review will give access to new functional materials, not only in medical- and biotechnological applications but also in electronic devices and possibly membrane-technology. Though, many systems have been developed, still more is to be discovered and significant advances are expected if more (sub-) disciplines are combined.

Programmed Morphological Transitions of Multisegment Assemblies by Molecular Chaperone Analogues

Supramolecular morphological transitions are of great importance in many processes in molecular biology. The Golgi apparatus bilayer, for instance, constantly transforms into cargo vesicles mediated by coaggregation with protein coats, but also proteins fold and unfold by coaggregation with chaperone proteins. In the latter case, chaperones selectively switch off the self-assembly of parts of the protein, thereby controlling their structure and function. Such a strategy can be of use for the development of smart materials, in which an external trigger induces a morphological transition, altering the macroscopic properties of the material. The use of coaggregation to induce morphological transitions has indeed been applied successfully to artificial systems, hence leading to the development of smart materials. The main strategy so far is to alter the packing parameter of a surfactant leading to morphological changes following the structure–shape concept. By this strategy, the transition of vesicles to hexagonal phases by the addition of trimethylbenzene, the conversions of spheres into rods into tubes by addition of ions as well as other additives have been induced. Other strategies to induce morphological changes have also been applied, for example, the use of twocomponent gel systems or photoisomerization of the building block. Although in all cases the mechanisms of these morphological transitions are well understood, the outcome can be hard to predict. Therefore, it remains a challenge to program the outcome of such transitions within the initial building block, which is necessary to use this approach for smart materials. Herein we demonstrate that morphological transitions can easily be programmed by separately addressing the complex aggregation behavior of a segmented self-assembling molecule, using small molecule chaperone analogues, that is, small molecules that selectively switch off self-assembly of another molecule through selective association. As a model system, we use a previously described multisegment amphiphile that consists of two covalently linked but orthogonally self-assembling building blocks, and which forms architectures characterized by each individual segment. Through the addition of a chaperone analogue we can selectively switch off the self-assembly of each of these segments individually, resulting in architectures of the other segment. Interestingly, not only the morphologies of the other segment are retained, but also its dynamics of self-assembly. By such an approach it is possible to design the morphological transitions within the multisegment amphiphile. The multisegment amphiphile (MA 3) is constructed of a gelator segment reminiscent of gelator 1 and a surfactant segment reminiscent of EO4C8 (surfactant 2), which assemble orthogonally (Figure 1a). We have previously shown that MA 3 in water assembles into architectures that display properties of both parental segments. The gelator segment

Self-Assembly Process of Soft Ferritin-PNIPAAm Conjugate Bionanoparticles at Polar–Apolar Interfaces

We describe an in-depth investigation on the dynamics and assembly behavior at polar-apolar interfaces of ferritin-PNIPAAm conjugates (poly-N-isopropylacrylamide). The stabilization of oil-water interfaces by the modified ferritin was investigated by dynamic surface tension measurements and compared to the individual components of the bionanoparticle conjugate, namely, unmodified ferritin and pure PNIMAAm of similar molecular weight. It was found that the modified ferritin, even at a low particle concentration, rapidly reduces the interfacial tension. The difference in interfacial stabilization was also shown by cryo-scanning electron microscopy and scanning force microscopy, which displayed very different morphologies at the polar-apolar interface for the unmodified ferritin, pure PNIPAAm, and the ferritin-PNIPAAm conjugate, respectively. The self-assembly of the ferritin-PNIPAAm was further analyzed by cryo-transmission electron microscopy and fluorescence microscopy, for which a fluorescently labeled polymer was used. Both techniques revealed details on the assembly of the protein-polymer conjugate at the oil-water interface.

Synthetic inorganic materials by mimicking biomineralization processes using native and non-native protein functions

Nature is able to produce various inorganic structures with very specific fine structures in the micro- and nano-regime, which are facilitated and controlled by protein-based systems. Enzymes like silicateines catalyse biomineralization and provide organisms with exoskeletons with specific material properties. While these structures are interesting materials in biology, they also offer ample opportunities for material scientists to create man-made materials with the same biological species in a non-natural setting. While natural organisms rely on specific proteins for certain processes, other more accessible proteins show similar capabilities even though it is not their native function. Mimicking biomineralization provides a route for the formation of new materials of various shapes and compositions. In this article, synthetic processes and the resulting materials will be discussed, describing the tools and bio-inspired systems used and comparing the original biological function of the protein to its role in the non-natural process.

Biomaterial-stem cell interactions and their impact on stem cell response

In this review, current research in the field of biomaterial properties for directing stem cells are discussed and placed in a critical perspective. Regenerative medicine, in which stem cells play a crucial role, has become an interdisciplinary field between cell biology and materials science. New insights are generated, different approaches to determine material features and stem cell properties are implemented, but also many misconceptions exist. According to the current state-of-the-art and combined with basic principles from two different disciplines the topic is critically addressed. We take into account what seem to be the most important material properties and their influence towards stem cells but also the various stem cells available with respect to their origin, tissue source and culturing conditions.

Cross-linking density and temperature effects on the self-assembly of SiO2-PNIPAAm core-shell particles at interfaces.

SiO2-PNIPAAm core-shell microgels (PNIPAAm=poly(N-isopropylacrylamide)) with various internal cross-linking densities and different degrees of polymerization were prepared in order to investigate the effects of stability, packing, and temperature responsiveness at polar-apolar interfaces. The effects were investigated using interfacial tensiometry, and the particles were visualized by cryo-scanning electron microscopy (SEM) and scanning force microscopy (SFM). The core-shell particles display different interfacial behaviors depending on the polymer shell thickness and degree of internal cross-linking. A thicker polymer shell and reduced internal cross-linking density are more favorable for the stabilization and packing of the particles at oil-water (o/w) interfaces. This was shown qualitatively by SFM of deposited, stabilized emulsion droplets and quantitatively by SFM of particles adsorbed onto a hydrophobic planar silicon dioxide surface, which acted as a model interface system. The temperature responsiveness, which also influences particle-interface interactions, was investigated by dynamic temperature protocols with varied heating rates. These measurements not only showed that the particles had an unusual but very regular and reversible interface stabilization behavior, but also made it possible to assess the nonlinear response of PNIPAAm microgels to external thermal stimuli.