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Tsierkezos, Nikos; Freiberger, Emma; Ritter, Uwe; Krischok, Stefan; Ullmann, Fabian; Köhler, Michael
Application of nitrogen-doped multi-walled carbon nanotubes decorated with gold nanoparticles in biosensing. - In: Journal of solid state electrochemistry, ISSN 1433-0768, Bd. 27 (2023), 10, S. 2645-2658

Novel films consisting of nitrogen-doped multi-walled carbon nanotubes (N-MWCNTs) were fabricated by means of chemical vapor deposition technique and decorated with gold nanoparticles (AuNPs) possessing diameter of 14.0 nm. Electron optical microscopy analysis reveals that decoration of N-MWCNTs with AuNPs does not have any influence on their bamboo-shaped configuration. The electrochemical response of fabricated composite films, further denoted as N-MWCNTs/AuNPs, towards oxidation of dopamine (DA) to dopamine-o-quinone (DAQ) in the presence of ascorbic acid (AA) and uric acid (UA) was probed in real pig serum by means of cyclic voltammetry (CV) and square wave voltammetry (SWV). The findings demonstrate that N-MWCNTs/AuNPs exhibit slightly greater electrochemical response and sensitivity towards DA/DAQ compared to unmodified N-MWCNTs. It is, consequently, obvious that AuNPs improve significantly the electrochemical response and detection ability of N-MWCNTs. The electrochemical response of N-MWCNTs/AuNPs towards DA/DAQ seems to be significantly greater compared to that of conventional electrodes, such as platinum and glassy carbon. The findings reveal that N-MWCNTs/AuNPs could serve as powerful analytical sensor enabling analysis of DA in real serum samples.
Stapf, Siegfried; Shikhov, Igor; Arns, Christoph; Gizatullin, Bulat; Mattea, Carlos
Dipolar NMR relaxation of adsorbates on surfaces of controlled wettability. - In: Magnetic resonance letters, ISSN 2772-5162, Bd. 3 (2023), 3, S. 220-231

In reservoir rocks, the term “ageing” refers to extended exposition to crude oil; a typically water-wet sandstone will then gradually become oil-wet as a consequence of the deposition of insoluble fractions of oil onto the surface grains. Rocks have been aged artificially by subjecting them to a bitumen solution at elevated temperature in order to achieve comparable surface properties for three different types of rock: Bentheimer, Berea Buff and Liège Chalk. Using saturated and aromatic model compounds as proxies for crude oil, the nuclear magnetic resonance (NMR) relaxation dispersion in native and aged rocks was compared and correlated to the properties of paramagnetic impurities in these rock types. Perfluorated liquids were found to follow the same trend as deuterated and naturally occurring oil components, suggesting they can be used as suitable tracers for wettability studies since the 19F nucleus is absent in natural sources. By combining electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) it becomes possible to identify and quantify the origin of the dominating relaxation processes between native and aged rocks, providing an alternative approach to assess wettability in natural rocks.
Gizatullin, Bulat; Mattea, Carlos; Stapf, Siegfried
Radicals on the silica surface: probes for studying dynamics by means of fast field cycling relaxometry and dynamic nuclear polarization. - In: Magnetic resonance letters, ISSN 2772-5162, Bd. 3 (2023), 3, S. 256-265

Determining the dynamics of adsorbed liquids on nanoporous materials is crucial for a detailed understanding of interactions and processes on the solid-liquid interface in many materials and porous systems. Knowledge of the influence of the presence of paramagnetic species on the surface or within the porous matrices is essential for fundamental studies and industrial processes such as catalysts. Magnetic resonance methods, such as electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR) and dynamic nuclear polarization (DNP), are powerful tools to address these questions and to quantify dynamics, electron-nuclear interaction features and their relation to the physical-chemical parameters of the system. This paper presents an NMR study of the dynamics of polar and nonpolar adsorbed liquids, represented by water, n-decane, deuterated water and nonane-d20, on the native silica surface as well as silica modified with vanadyl porphyrins. The analysis of the frequency dependence of the nuclear spin-lattice relaxation time is carried out by separating the intra- and intermolecular contributions, which were analyzed using reorientations mediated by translational displacements (RMTD) and force-free-hard-sphere (FFHS) models, respectively.
Dreßler, Christian; Hänseroth, Jonas; Sebastiani, Daniel
Coexistence of cationic and anionic phosphate moieties in solids: unusual but not impossible. - In: The journal of physical chemistry letters, ISSN 1948-7185, Bd. 14 (2023), 32, S. 7249-7255

Phosphoric acid is commonly known either as a neutral molecule or as an anion (phosphate). We theoretically confirm by ab initio molecular dynamics simulations (AIMD) that a cationic form H4PO4+ coexists with the anionic form H2PO4- in the same salt. This paradoxical situation is achieved by partial substitution of Cs+ by H4PO4+ in CsH2PO4. Thus, HnPO4 acts simultaneously as both the positive and the negative ion of the salt. We analyze the dynamical protonation pattern within the unusual hydrogen bond network that is established between the ions. Our AIMD simulations show that a conventional assignment of protonation states of the phosphate groups is not meaningful. Instead, a better description of the protonation situation is achieved by an efficiently fractional assignment of the strongly hydrogen-bonded protons to both its nearest and next-nearest oxygen neighbors.
Flecken, Franziska; Knapp, Anna; Grell, Toni; Dreßler, Christian; Hanf, Schirin
Acute bite angle POP- and PSP-type ligands and their trinuclear copper(I) complexes: synthesis and photo-luminescence properties. - In: Inorganic chemistry, ISSN 1520-510X, Bd. 62 (2023), 32, S. 13038-13049

In the current work, the rational synthesis of trinuclear copper complexes, incorporating acute bite angle POP- and PSP-type ligands, is reported. The in situ formation of POP (Ph2P–O–PPh2) or PSP (Ph2P–S–PPh2) ligands in the presence of a copper(I) precursor gave access to various trinuclear copper complexes of the form [Cu3(μ3-Hal)2(μ-PXP)3]PF6 [X = O; Hal = Cl (1), Br (2), I (3) and X = S; Hal = Cl (5), Br (6), I (7)]. Related iodide-containing complexes and clusters, such as [Cu4(μ3-I)4(Ph2PI)4] (4) and [Cu3(μ3-I)2(μ-I)(μ-PSP)2] (8), could also be obtained via the variation of the reaction stoichiometry. The investigation of the photo-optical properties by photo-luminescence spectroscopy has demonstrated that the phosphorescence in the visible region can be switched off through the mere change of the heteroatom in the ligand backbone (POP vs PSP ligand scaffold). Theoretical studies have been conducted to complement the experimental photo-optical data with detailed insights into the occurring electronic transitions. Consequently, this systematic study paves the way for tuning the photo-optical properties of transition metal complexes in a more rational way.
Stapf, Siegfried; Siebert, Niklas; Spalek, Timo; Hartmann, Vincent; Gizatullin, Bulat; Mattea, Carlos
Binary fluids in mesoporous materials: phase separation studied by NMR relaxation and diffusion. - In: Magnetic resonance letters, ISSN 2772-5162, Bd. 3 (2023), 2, S. 108-117

Relaxation and diffusion measurements were carried out on single and binary liquids filling the pore space of controlled porous glass Vycor with an average pore size of about 4 nm. The dispersion of the longitudinal relaxation time T1 is discussed as a means to identify liquid-surface interaction based on existing models developed for metal-free glass surfaces. In addition, the change of T1 and T2 with respect to their bulk values is discussed, in particular T2 serves as a probe for the strength of molecular interactions. As the native glass surface is polar and contains a large amount of hydroxyl groups, a pronounced interaction of polar and protic adsorbate liquids is expected; however, the T1 dispersion, and the corresponding reduction of T2, are also observed for non-polar liquids such as alkanes and cyclohexane. Deuterated liquids are employed for simplifying data analysis in binary systems, but also for separating the respective contributions of intra- and intermolecular interactions to the overall relaxation rate. Despite the lack of paramagnetic impurities in the glass material, 1H and 2H relaxation dispersions of equivalent molecules are frequently found to differ from each other, suggesting intermolecular relaxation mechanisms for the 1H nuclei. The variation of the T1 dispersion when comparing single and binary systems gives clear evidence for the preferential adsorption of one of the two liquids, suggesting complete phase separation in several cases. Measurement of the apparent tortuosity by self-diffusion experiments supports the concept of a local variation of sample composition within the porespace.
Selzer, Silas A.; Bauer, Fabian; Bohm, Sebastian; Runge, Erich; Bretschneider, Peter
Physics-guided machine learning techniques for improving temperature calculations of high-voltage transmission lines. - In: Die Energiewende beschleunigen, (2023), S. 353-360

The calculation of the temperature of high-voltage transmission lines is usually done by the commercially used standard models, the CIGRE Standard No. 601 and the IEEE Standard No. 738. These turn out to be prone to errors in application. Based on data analysis, new models based on machine learning techniques and their combination with physics-based models, called physics-guided machine learning techniques, were developed and compared with the results of the established physical models and measurement results. The improved models achieve a reduction of the mean absolute estimation error as well as a significant reduction of the values that deviate more than 5 K from the measured conductor temperature. Also, the mean underestimation of the conductor temperature was changed into an applicationtechnically unproblematic overestimation by the transition from the best standard to the best data-scientific model. The optimization of the models could be achieved by eliminating the incorrect determination of the physical parameters, a compensation of the conservative estimation of the physical effects as well as the consideration of the neglected thermal components of the heat balance. The investigations are based on measured data of the conductor temperature and electrical quantities from the grid area of 50Hertz Transmission GmbH.

Köster, Felix; Patel, Dhruvit; Wikner, Alexander; Jaurigue, Lina; Lüdge, Kathy
Data-informed reservoir computing for efficient time-series prediction. - In: Chaos, ISSN 1089-7682, Bd. 33 (2023), 7, S. 073109-1-073109-11

We propose a new approach to dynamical system forecasting called data-informed-reservoir computing (DI-RC) that, while solely being based on data, yields increased accuracy, reduced computational cost, and mitigates tedious hyper-parameter optimization of the reservoir computer (RC). Our DI-RC approach is based on the recently proposed hybrid setup where a knowledge-based model is combined with a machine learning prediction system, but it replaces the knowledge-based component by a data-driven model discovery technique. As a result, our approach can be chosen when a suitable knowledge-based model is not available. We demonstrate our approach using a delay-based RC as the machine learning component in conjunction with sparse identification of nonlinear dynamical systems for the data-driven model component. We test the performance on two example systems: the Lorenz system and the Kuramoto-Sivashinsky system. Our results indicate that our proposed technique can yield an improvement in the time-series forecasting capabilities compared with both approaches applied individually, while remaining computationally cheap. The benefit of our proposed approach, compared with pure RC, is most pronounced when the reservoir parameters are not optimized, thereby reducing the need for hyperparameter optimization.
Phi, Hai Binh; Bohm, Sebastian; Runge, Erich; Dittrich, Lars; Strehle, Steffen
3D passive microfluidic valves in silicon and glass using grayscale lithography and reactive ion etching transfer. - In: Microfluidics and nanofluidics, ISSN 1613-4990, Bd. 27 (2023), 8, 55, S. 1-12

A fabrication strategy for high-efficiency passive three-dimensional microfluidic valves with no mechanical parts fabricated in silicon and glass substrates is presented. 3D diffuser-nozzle valve structures were produced and characterized in their added value in comparison to conventional diffuser-nozzle valve designs with rectangular cross sections. A grayscale lithography approach for 3D photoresist structuring combined with a proportional transfer by reactive ion etching allowed to transfer 3D resist valve designs with high precision into the targeted substrate material. The efficiency with respect to the rectification characteristics or so-called diodicity of the studied valve designs is defined as the ratio of the pressure drops in backward and forward flow directions. The studied valve designs were characterized experimentally as well as numerically based on finite element simulations with overall matching results that demonstrate a significantly improved flow rectification of the 3D valves compared to the corresponding conventional structure. Our novel 3D valve structures show, for instance, even without systematic optimization a measured diodicity of up to 1.5 at low flow rates of only about 10 μl/s.
Mathew, Sobin; Abedin, Saadman; Kurtash, Vladislav; Lebedev, Sergei P.; Lebedev, Alexander A.; Hähnlein, Bernd; Stauffenberg, Jaqueline; Jacobs, Heiko O.; Pezoldt, Jörg
Evaluation of hysteresis response in achiral edges of graphene nanoribbons on semi-insulating SiC. - In: Materials science forum, ISSN 1662-9752, Bd. 1089 (2023), S. 15-22

Hysteresis response of epitaxially grown graphene nanoribbons devices on semi-insulating 4H-SiC in the armchair and zigzag directions is evaluated and studied. The influence of the orientation of fabrication and dimensions of graphene nanoribbons on the hysteresis effect reveals the metallic and semiconducting nature graphene nanoribbons. The hysteresis response of armchair based graphene nanoribbon side gate and top gated devices implies the influence of gate field electric strength and the contribution of surface traps, adsorbents, and initial defects on graphene as the primary sources of hysteresis. Additionally, passivation with AlOx and top gate modulation decreased the hysteresis and improved the current-voltage characteristics.