Prof. Dr. Tobias Kraus

Dr. Helmut Cölfen, an exceptional interdisciplinary scientist, mentor, colleague, and dear friend, passed away in November 2023 at the age of 58. His untimely departure is a profound loss for the fields of analytical ultracentrifugation, colloid, crystallization, and polymer research. This obituary pays tribute to Helmut, honoring his remarkable academic career and contributions to the study of nanochemistry, biophysics, and life sciences. Helmut was renowned for his pioneering research contributions in several key research areas: (1) Development of advanced analytical techniques: Helmut made major contributions to techniques such as analytical ultracentrifugation and field flow fractionation, which are widely utilized to characterize the structure of biomolecules and the growth of nanostructured crystalline materials; (2) Study of nucleation and crystallization processes: Helmut explored the early stages of crystallization which led to the discovery of pre-nucleation clusters and mesocrystal intermediates, in the presence of additives and templates; and (3) Investigation of structure and morphogenesis of mesocrystals, examining their molecular properties.

The tailored synthesis of copolymer architectures provides insights into fundamental structure–property relationships for the formation of complex morphologies through microphase separation. In this way, classical areas within the phase diagram can be specifically influenced and also adapted for important applications. The exploration of copolymer architectures also offers the possibility to discover entirely new morphologies. In this study, we design a symmetric dendrimer-like second generation block copolymer by anionic polymerization. The structural design of the polymers influences the curvature of the interfaces to produce, in particular, bicontinuous morphologies and is investigated based on molecular chain architecture. After extensive molecular analysis of the new dendrimer-like block copolymers, the resulting morphology is analyzed using transmission electron microscopy, atomic force microscopy, and small-angle X-ray scattering measurements. We further combine the experimentally obtained morphologies with Monte Carlo simulations to better understand the relationship between tailored polymer architecture and the observed morphology. By changing the volume ratio of the copolymers used and also mixing this complex polymer architecture with a linear block copolymer, we gain insights into the polymer behavior at the phase boundaries. This knowledge has an impact on the optical and mechanical properties of thermoplastic elastomers and their corresponding blends.

Flexible and stretchable electronics require both sensing elements and stretching-insensitive electrical connections. Conductive polymer composites and liquid metals are highly deformable but change their conductivity upon elongation and/or contain rare metals. Solid conductive composites are limited in mechanoelectrical properties and are often combined with macroscopic Kirigami structures, but their use is limited by geometrical restraints. Here, we introduce “Electrofluids”, concentrated conductive particle suspensions with transient particle contacts that flow under shear that bridge the gap between classic solid composites and liquid metals. We show how Carbon Black (CB) forms large agglomerates when using incompatible solvents that reduce the electrical percolation threshold by 1 order of magnitude compared to more compatible solvents, where CB is well-dispersed. We analyze the correlation between stiffness and electrical conductivity to create a figure of merit of first electrofluids. Sealed elastomeric tubes containing different types of electrofluids were characterized under uniaxial tensile strain, and their electrical resistance was monitored. We found a dependency of the piezoresistivity with the solvent compatibility. Electrofluids enable the rational design of sustainable soft electronics components by simple solvent choice and can be used both as sensor and electrode materials, as we demonstrate.

Trotz erheblichen Interesses an heteroatomhaltigen konjugierten Polymeren sind Beispiele mit schwereren Elementen des p-Blocks im Konjugationspfad rar. Die kürzlich beschriebene Metathese schwererer acyclischer Diene (HADMET) ermöglichte die Synthese eines Ge=Ge-Doppelbindungen enthaltenden Polymers, wenn auch eines unlöslichen mit begrenztem Polymerisationsgrad. Durch Einführung langer Alkylketten erhielten wir nun lösliche Vertreter mit – nach diffusionsabhängiger NMR-Spektroskopie (DOSY) und dynamischer Lichtstreuung (DLS) – nahezu unendlichen Polymerisationsgraden. UV/Vis und NMR-Daten bestätigen das Vorliegen von σ,π-Konjugation entlang der Silylen-Phenylen-Verknüpfungen zwischen den Ge=Ge-Einheiten. Günstige intermolekulare Dispersionswechselwirkungen führen zu leiterartigen, zylindrischen Aggregaten, wie durch Röntgendiffraktometrie (XRD), Kleinwinkel-Röntgenstreuung (SAXS) und DLS bestätigt. AFM- und TEM-Bilder abgeschiedener dünner Schichten offenbaren eine lamellare Anordnung ausgedehnter Polymerbündel.

Despite considerable interest in heteroatom-containing conjugated polymers, there are only few examples with heavier p-block elements in the conjugation path. The recently reported heavier acyclic diene metathesis (HADMET) allowed for the synthesis of a polymer containing Ge=Ge double bonds—albeit insoluble and with limited degree of polymerization. By incorporation of long alkyl chains, we now obtained soluble representatives, which exhibit degrees of polymerization near infinity according to diffusion-ordered NMR spectroscopy (DOSY) and dynamic light scattering (DLS). UV/Vis and NMR data confirm the presence of σ,π-conjugation across the silylene-phenylene linkers between the Ge=Ge double bonds. Favorable intermolecular dispersion interactions lead to ladder-like cylindrical assemblies as confirmed by X-ray diffraction (XRD), small angle X-ray scattering (SAXS) and DLS. AFM and TEM images of deposited thin films reveal lamellar ordering of extended polymer bundles.

Silver nanowires (AgNW) find use in transparent conductive electrodes with applications in solar cells, touch screens, and wearables. Unprotected AgNW are prone to atmospheric corrosion and lose conductivity over time. Known passivation techniques either require submersion of pre-deposited AgNW in liquid compounds or the modification of AgNW inks prior to deposition, which alters viscosity and complicates deposition. Here, new possibilities for stabilization of pre-deposited AgNW networks without need for submersion are explored. It is demonstrated that AgNW networks can be stabilized either by argon or hydrogen plasma treatment or by solvent vapor annealing with ethanol, methanol, or ethyl acetate. These treatments yielded stable electrical resistance over at least nine weeks, whereas untreated or thermally annealed AgNW layers quickly lost conductivity. The potential of solvent vapor annealing is further explored by demonstrating a new processing technique for stable polymer matrix composites containing AgNW. Co-deposited layers of AgNW with polystyrene microbeads are annealed in ethyl acetate vapor to stabilize the AgNW while at the same time merging polymer beads into a closed film around the AgNW. The resulting composites maintained stable resistance and transmittance for at least seven weeks.

We study the effect of additives on the colloidal stability of alkanethiol-coated gold nanoparticles. Cyclic amines and sulfides of different sizes were added to dispersions in decane at additive concentrations below 128 mM. Small-angle X-ray scattering (SAXS) indicated that tetrahydrothiophene reduced the agglomeration temperature, Tagglo, by up to 29 °C, a considerable increase in colloidal stability. Amines had a much weaker stabilizing effect of up to 2.5 °C. We found an unexpected maximum of stabilization for low additive concentrations, where Tagglo increased at concentrations above 64 mM. Molecular dynamics simulations were used to correlate these observations with the ligand shell structure. They excluded the physisorption of additives as a stabilization mechanism and suggested that sulfides replace hexadecanethiol on the AuNP surfaces by chemisorption. This hinders ligand ordering, thereby reducing Tagglo, which explains the stabilizing effect. Clustering of chemisorbed additive molecules at high concentration restabilized the ligand ordered state, explaining the detrimental effect of higher additive concentrations. The predictions of the simulations were confirmed by using thermogravimetric analyses and SAXS measurements of washed samples that indicated that the structure of the ligand shell itself, not the presence of physisorbed additives, changes Tagglo. Finally, we calculated potentials of mean force, which show that larger sulfide-based additives have a weaker affinity for the gold surface than smaller ones due to stronger steric hindrance. This explains why smaller cyclic sulfides were the most efficient stabilizers.

The expansion of T cells ex vivo is crucial for effective immunotherapy but currently limited by a lack of expansion approaches that closely mimic in vivo T cell activation. Taking inspiration from bottom-up synthetic biology, a new synthetic cell technology is introduced based on dispersed liquid-liquid phase-separated droplet-supported lipid bilayers (dsLBs) with tunable biochemical and biophysical characteristics, as artificial antigen presenting cells (aAPCs) for ex vivo T cell expansion. These findings obtained with the dsLB technology reveal three key insights: first, introducing laterally mobile stimulatory ligands on soft aAPCs promotes expansion of IL-4/IL-10 secreting regulatory CD8+ T cells, with a PD-1 negative phenotype, less prone to immune suppression. Second, it is demonstrated that lateral ligand mobility can mask differential T cell activation observed on substrates of varying stiffness. Third, dsLBs are applied to reveal a mechanosensitive component in bispecific Her2/CD3 T cell engager-mediated T cell activation. Based on these three insights, lateral ligand mobility, alongside receptor- and mechanosignaling, is proposed to be considered as a third crucial dimension for the design of ex vivo T cell expansion technologies.

Mechanical metamaterials are known for their prominent mechanical characteristics such as programmable deformation that are due to periodic microstructures. Recent research trends have shifted to utilizing mechanical metamaterials as structural substrates to integrate with functional materials for advanced functionalities beyond mechanical, such as active sensing. This study reports on the ultra-stretchable kirigami piezo-metamaterials (KPM) for sensing coupled large deformations caused by in- and out-of-plane displacements using the lead zirconate titanate (PZT) and barium titanate (BaTiO3) composite films. The KPM are fabricated by uniformly compounding and polarizing piezoelectric particles (i.e., PZT and BaTiO3) in silicon rubber and structured by cutting the piezoelectric rubbery films into ligaments. Characterizes the electrical properties of the KPM and investigates the bistable mechanical response under the coupled large deformations with the stretching ratio up to 200% strains. Finally, the PZT KPM sensors are integrated into wireless sensing systems for the detection of vehicle tire bulge, and the non-toxic BaTiO3 KPM are applied for human posture monitoring. The reported kirigami piezo-metamaterials open an exciting venue for the control and manipulation of mechanically functional metamaterials for active sensing under complex deformation scenarios in many applications.

Engineered living materials (ELMs) encapsulate microorganisms within polymeric matrices for biosensing, drug delivery, capturing viruses, and bioremediation. It is often desirable to control their function remotely and in real time and so the microorganisms are often genetically engineered to respond to external stimuli. Here, we combine thermogenetically engineered microorganisms with inorganic nanostructures to sensitize an ELM to near infrared light. For this, we use plasmonic gold nanorods (AuNR) that have a strong absorption maximum at 808 nm, a wavelength where human tissue is relatively transparent. These are combined with Pluronic-based hydrogel to generate a nanocomposite gel that can convert incident near infrared light into heat locally. We perform transient temperature measurements and find a photothermal conversion efficiency of 47 %. Steady-state temperature profiles from local photothermal heating are quantified using infrared photothermal imaging and correlated with measurements inside the gel to reconstruct spatial temperature profiles. Bilayer geometries are used to combine AuNR and bacteria-containing gel layers to mimic core-shell ELMs. The thermoplasmonic heating of an AuNR-containing hydrogel layer that is exposed to infrared light diffuses to the separate but connected hydrogel layer with bacteria and stimulates them to produce a fluorescent protein. By tuning the intensity of the incident light, it is possible to activate either the entire bacterial population or only a localized region.