Browsing by Author "Kirchhof, Manuel"

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    Charged triazole cross-linkers for hyaluronan-based hybrid hydrogels
    (2016) Martini, Maike; Hegger, Patricia S.; Schädel, Nicole; Minsky, Burcu B.; Kirchhof, Manuel; Scholl, Sebastian; Southan, Alexander; Tovar, Günter E. M.; Boehm, Heike; Laschat, Sabine
    Polyelectrolyte hydrogels play an important role in tissue engineering and can be produced from natural polymers, such as the glycosaminoglycan hyaluronan. In order to control charge density and mechanical properties of hyaluronan-based hydrogels, we developed cross-linkers with a neutral or positively charged triazole core with different lengths of spacer arms and two terminal maleimide groups. These cross-linkers react with thiolated hyaluronan in a fast, stoichiometric thio-Michael addition. Introducing a positive charge on the core of the cross-linker enabled us to compare hydrogels with the same interconnectivity, but a different charge density. Positively charged cross-linkers form stiffer hydrogels relatively independent of the size of the cross-linker, whereas neutral cross-linkers only form stable hydrogels at small spacer lengths. These novel cross-linkers provide a platform to tune the hydrogel network charge and thus the mechanical properties of the network. In addition, they might offer a wide range of applications especially in bioprinting for precise design of hydrogels.
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    A complementary experimental and theoretical approach for probing the surface functionalization of ZnO with molecular catalyst linkers
    (2023) Kousik, Shravan R.; Solodenko, Helena; YazdanYar, Azade; Kirchhof, Manuel; Schützendübe, Peter; Richter, Gunther; Laschat, Sabine; Fyta, Maria; Schmitz, Guido; Bill, Joachim; Atanasova, Petia
    The application of ZnO materials as solid-state supports for molecular heterogeneous catalysis is contingent on the functionalization of the ZnO surface with stable self-assembled monolayers (SAMs) of catalyst linker molecules. Herein, experimental and theoretical methods are used to study SAMs of azide-terminated molecular catalyst linkers with two different anchor groups (silane and thiol) on poly and monocrystalline (0001, ) ZnO surfaces. Angle-resolved and temperature-dependent X-ray photoelectron spectroscopy (XPS) is used to study SAM binding modes, thermal stabilities, and coverages. The binding strengths and atomistic ordering of the SAMs are determined via atom-probe tomography (APT). Density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations provide insights on the influence of the ZnO surface polarity on the interaction affinity and conformational behavior of the SAMs. The investigations show that SAMs based on 3-azidopropyltriethoxysilane possess a higher binding strength and thermal stability than the corresponding thiol. SAM surface coverage is strongly influenced by the surface polarity of ZnO, and the highest coverage is observed on the polycrystalline surface. To demonstrate the applicability of linker-modified polycrystalline ZnO as a catalyst support, a chiral Rh diene complex is immobilized on the azide-terminal of the SAM and its coverage is evaluated via XPS.
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    Efficient and spatially controlled functionalization of SBA‐15 and initial results in asymmetric Rh‐catalyzed 1,2‐additions under confinement
    (2021) Beurer, Ann‐Katrin; Kirchhof, Manuel; Bruckner, Johanna R.; Frey, Wolfgang; Baro, Angelika; Dyballa, Michael; Giesselmann, Frank; Laschat, Sabine; Traa, Yvonne
    Selectively functionalized mesoporous silica may considerably advance heterogeneous catalysis through the controlled immobilization of highly selective complex catalysts inside the mesopores. However, spatially controlled functionalization and the precise analytical verification are still a challenge. In this publication, we report a method, which ensures a selective functionalization of the mesopore surface with a clickable linker and thus makes it possible to study confinement effects during catalyzed reactions. First, we passivate the silanol groups on the particle surface and in the pore entrances of the mesoporous silica material SBA‐15 with 1,1,1‐trimethyl‐N‐(trimethylsilyl)silanamine. Then we remove the template by solvent extraction and functionalize the pore walls with 3‐azidopropyltriethoxysilane before we click the catalyst. In initial experiments of asymmetric Rh‐catalyzed 1,2‐addition, we investigate the performance of a catalyst clicked selectively in the mesopores and compare it to the dissolved catalyst as well as to the catalyst immobilized exclusively on the external surface of SBA‐15.
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    Triazole-based cross-linkers in radical polymerization processes: tuning mechanical properties of poly(acrylamide) and poly(N,N-dimethylacrylamide) hydrogels
    (2018) Götz, Tobias; Schädel, Nicole; Petri, Nadja; Kirchhof, Manuel; Bilitewski, Ursula; Tovar, Günter E. M.; Laschat, Sabine; Southan, Alexander
    Triazole-based cross-linkers with different spacer lengths and different functional end groups (acrylamides, methacrylamides, maleimides and vinylsulfonamides) were synthesized, investigated for cytotoxic and antibacterial activity, and incorporated into poly(acrylamide) (PAAm) and poly(N,N-dimethylacrylamide) (PDMAAm) hydrogels by free radical polymerization. Hydrogels prepared with different cross-linkers and cross-linker contents between 0.2% and 1.0% were compared by gel yields, equilibrium degrees of swelling (S) and storage moduli (G’). Generally with increasing cross-linker content, G’ values of the hydrogels increased, while S values decreased. The different polymerizable cross-linker end groups resulted in a decrease of G’ in the following order for cross-linkers with C4 spacers: acrylamide > maleimide > methacrylamide > vinylsulfonamide. Longer cross-linker alkyl spacer lengths caused an increase in G’ and a decrease in S. Independent of the cross-linker used, a universal correlation between G’ and equilibrium polymer volume fraction phi was found. For PAAm hydrogels, G’ ranged between 4 kPa and 23 kPa and and phi between 0.07 and 0.14. For PDMAAm hydrogels, G’ ranged between 0.1 kPa and 4.9 kPa and and phi between 0.02 and 0.06. The collected data were used to establish an empirical model to predict G’ depending on phi. G’ of PAAm and PDMAAm hydrogels is given by G' = 4034 kPa * phi^2.66 and G' = 4297 kPa * phi^2.46, respectively.