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Item type:Publication, Access status: with files , Equilibrium shape and surface termination of supported magnetite nanoparticles(Nature Publishing Group, 2026-12-01); ; ; ; The equilibrium shape and surface termination of magnetite nanoparticles (NPs) are fundamental properties that determine the physical and chemical properties of the supercrystal structure of oleic acid-coated magnetite NPs. Here, we studied the equilibrium shape of magnetite (Fe3O4) NPs supported by Al2O3(0001) single-crystalline surfaces. We report the growth of epitaxial (111)-oriented NPs exhibiting a triangular shape with a height-to-diameter aspect ratio of 0.42 over a wide growth temperature range. We probed the surface termination of the NP facets by adsorbing formic acid as a prototypical molecule representing the adsorption behavior of oleic acid. We identified infrared absorption bands characteristic of dissociative adsorption on (111) facets with the iron tetrahedral (Fetet1) termination, as well as on mixed-terminated (100) side facets. Our experimental findings are supported by predictions of the NP shape using surface-free energy calculations from ab initio thermodynamics. The experimentally observed nanoparticle shape can only be rationalized by the presence of bulk-terminated {100} type facets. Such a fundamental understanding of the shape and surface terminations of oxide nanoparticles is crucial for tailoring the properties of hybrid hierarchical materials and drug carriers and for their development.Publicationtype: Journal Article60 13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Integrated preservation of water activity as key to intensified chemoenzymatic synthesis of bio-based styrene derivatives(Macmillan Publishers Limited, part of Springer Nature, 2024-12-01); ; ; The valorization of lignin-derived feedstocks by catalytic means enables their defunctionalization and upgrading to valuable products. However, the development of productive, safe, and low-waste processes remains challenging. This paper explores the industrial potential of a chemoenzymatic reaction performing the decarboxylation of bio-based phenolic acids in wet cyclopentyl methyl ether (CPME) by immobilized phenolic acid decarboxylase from Bacillus subtilis, followed by a base-catalyzed acylation. Key-to-success is the continuous control of water activity, which fluctuates along the reaction progress, particularly at high substrate loadings (triggered by different hydrophilicities of substrate and product). A combination of experimentation, thermodynamic equilibrium calculations, and MD simulations revealed the change in water activity which guided the integration of water reservoirs and allowed process intensification of the previously limiting enzymatic step. With this, the highly concentrated sequential two-step cascade (400 g·L–1) achieves full conversions and affords products in less than 3 h. The chemical step is versatile, accepting different acyl donors, leading to a range of industrially sound products. Importantly, the finding that water activity changes in intensified processes is an academic insight that might explain other deactivations of enzymes when used in non-conventional media.Publicationtype: Journal Article30 46 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Reconstructing the infrared spectrum of a peptide from representative conformers of the full canonical ensemble(Macmillan Publishers Limited, part of Springer Nature, 2023-12); ; ; ; Leucine enkephalin (LeuEnk), a biologically active endogenous opioid pentapeptide, has been under intense investigation because it is small enough to allow efficient use of sophisticated computational methods and large enough to provide insights into low-lying minima of its conformational space. Here, we reproduce and interpret experimental infrared (IR) spectra of this model peptide in gas phase using a combination of replica-exchange molecular dynamics simulations, machine learning, and ab initio calculations. In particular, we evaluate the possibility of averaging representative structural contributions to obtain an accurate computed spectrum that accounts for the corresponding canonical ensemble of the real experimental situation. Representative conformers are identified by partitioning the conformational phase space into subensembles of similar conformers. The IR contribution of each representative conformer is calculated from ab initio and weighted according to the population of each cluster. Convergence of the averaged IR signal is rationalized by merging contributions in a hierarchical clustering and the comparison to IR multiple photon dissociation experiments. The improvements achieved by decomposing clusters containing similar conformations into even smaller subensembles is strong evidence that a thorough assessment of the conformational landscape and the associated hydrogen bonding is a prerequisite for deciphering important fingerprints in experimental spectroscopic data.Publicationtype: Journal ArticleScopus© Citations 4 121 112 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Adsorption of oleic acid on magnetite facets(Macmillan Publishers Limited, part of Springer Nature, 2022-10-22); ; ; ; The microscopic understanding of the atomic structure and interaction at carboxylic acid/oxide interfaces is an important step towards tailoring the mechanical properties of nanocomposite materials assembled from metal oxide nanoparticles functionalized by organic molecules. We have studied the adsorption of oleic acid (C17H33COOH) on the most prominent magnetite (001) and (111) crystal facets at room temperature using low energy electron diffraction, surface X-ray diffraction and infrared vibrational spectroscopy complemented with molecular dynamics simulations used to infer specific hydrogen bonding motifs between oleic acid and oleate. Our experimental and theoretical results give evidence that oleic acid adsorbs dissociatively on both facets at lower coverages. At higher coverages, the more pronounced molecular adsorption causes hydrogen bond formation between the carboxylic groups, leading to a more upright orientation of the molecules on the (111) facet in conjunction with the formation of a denser layer, as compared to the (001) facet. This is evidenced by the C=O double bond infrared line shape, in depth molecular dynamics bond angle orientation and hydrogen bond analysis, as well as X-ray reflectivity layer electron density profile determination. Such a higher density can explain the higher mechanical strength of nanocomposite materials based on magnetite nanoparticles with larger (111) facets.Publicationtype: Journal ArticleScopus© Citations 2 67 208 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Carboxylic acid induced near-surface restructuring of a magnetite surface(Macmillan Publishers Limited, part of Springer Nature, 2019-08-09); ; ; ; A fundamental knowledge of the interaction of carboxylic acids, such as formic acid, with magnetite surfaces is of prime importance for heterogeneous catalysis and the synthesis of novel materials. Despite this, little is known about the atomic scale adsorption mechanisms. Here we show by in-situ surface X-ray diffraction that the oxygen rich subsurface cation vacancy reconstruction of the clean magnetite (001) surface is lifted by dissociative formic acid adsorption, reestablishing a surface with bulk stoichiometry. Using density functional theory, the bulk terminated, fully formic acid covered surface is calculated to be more stable than the corresponding clean, reconstructed surface. A comparison of calculated and experimental infrared bands supports the bidentate adsorption geometry and a specific adsorption site. Our results pave the way for a fundamental understanding of the bonding mechanism at carboxylic acid/oxide interfaces.Publicationtype: Journal ArticleScopus© Citations 16 401 347