Markus Küppers

In situ observation of diffusion and reaction dynamics in gel microreactors by chemically resolved NMR microscopy

The enzymatically catalyzed esterification reaction of propionic acid+1-butanol ⇆ propionic-acid-butyl-ester+water inside an immobilizing hydrogel environment has been investigated by means of spectroscopically resolved nuclear magnetic resonance (NMR) imaging. The alginate hydrogel was in the shape of a 3–4 mm diameter bead, with both the gel-forming water and the solvent (cyclohexane) being fully deuterated to simplify the identification of small amounts of reactants. In the absence of enzymes, the self-diffusion process of the separate reactants (propionic acid and butanol) proved to be severely slowed down compared to pure Fickian self-diffusion, and the concentration buildup inside the gel bead was shown to depend strongly on the properties of the reactants. Two-dimensional, non-chemically resolved images revealed that the diffusion process is not radially symmetric as expected, thus complicating the modelling of the diffusion and reaction kinetics. The reaction itself has been observed with chemical resolution in a time series of up to 40 h, clearly demonstrating the reduction of 1-butanol and production of water inside the gel bead.

New approaches to the visualization, quantification and explanation of acid-induced water loss from Ca-alginate hydrogel beads

The water loss of Ca-alginate hydrogels at pHs below 4.0 was visualized with 1HNMR-imaging by covering a single alginate bead with cyclohexane-d12 in a specially equipped NMR-tube and adding propionic acid at defined concentrations. The exact amount of water expelled from the beads was calculated from their weight loss and correlated with the acid concentrations and pHs within the hydrogel matrix. The maximum water loss of 52% (w/w) occurred at pH 1.0, while only 5% (w/w) of the initial water content were lost at pH 3.6. The analysis of the water collected from several alginate beads for Ca2+-ions and free polysaccharides led to the assumption that, due to the acid-induced protonation of the carboxyl functions, the ionotropic network is gradually converted to an alginic acid gel structured by H-bonds. This contradicts existing theories explaining the pH-induced water loss by a lower solubility of the alginate chains and decreased repulsion between protonated carboxyl functions, but explains previously reported pH-dependent alterations of mass transport and drug retention of Ca-alginate gels. Thus, the presented experiments enable a more precise and complete view of the acid-induced process within Ca-alginate hydrogels. The transfer to the characterization of other hydrogels is possible and should be advantageous, especially if a calibration of the NMR-measurement could be achieved.

NMR and MRI of Blood-Dissolved Hyperpolarized Xe-129 in Different Hollow-Fiber Membranes

Magnetic resonance of hyperpolarized (129)Xe has found a wide field of applications in the analysis of biologically relevant fluids. Recently, it has been shown that the dissolution of hyperpolarized gas into the fluid via hollow-fiber membranes leads to bubble-free (129)Xe augmentation, and thus to an enhanced signal. In addition, hollow-fiber membranes permit a continuous operation mode. Herein, a quantitative magnetic resonance imaging and spectroscopy analysis of a customized hollow-fiber membrane module is presented. Different commercial hollow-fiber membrane types are compared with regard to their (129)Xe dissolution efficiency into porcine blood, its constituents, and other fluids. The presented study gives new insight into the suitability of these hollow-fiber membrane types for hyperpolarized gas dissolution setups.

Sodium NMR Relaxation: A Versatile Non-invasive Tool for the Monitoring of Phase Transitions and the Estimation of Effective Pore Sizes of Supramolecular Hydrogels

Nuclear magnetic resonance (NMR) relaxometry of liquids is a widely used tool to characterize porous media. In particular, 23Na NMR is an especially suitable method when applied to gels and biological tissues. In this work we investigated the thermoreversible melting and gelation processes of supramolecular hydrogels formed by succinamic acid-based amphiphiles (SAn) in a saturated aqueous NaHCO3 solution (sat. aq. NaHCO3 sol.). We could show that it is not only possible to determine the melting points and to monitor the gelation process with 23Na relaxometry, but also to estimate the effective pore size based on the expanded Brownstein-Tarr model. Our findings are in good agreement with data from differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) experiments.

Imaging of copper oxygenation reactions in a bubble flow

Reactions of gases with liquids play a crucial role in the production of many bulk chemicals. Often, the gas is bubbled into the chosen reactor. Most of the processes at the gas–liquid interface of the bubbles and in their tails are not fully understood and warrant further investigation. For this purpose, NMR imaging or Magnetic Resonance Imaging has been applied to visualize some of the processes in the bubble tail. To generate sufficient contrast, a magnetogenic gas–liquid reaction associated with a change of magnetic state, from diamagnetic to paramagnetic, was employed. In this work, a copper(I)‐based compound was oxidized to copper(II) to exploit relaxation contrast. To match the speed of the rising bubbles to the acquisition time of the spin‐echo imaging sequence, polyethylene glycol was added to increase the viscosity of the reacting solution. Images of the oxygen ingress into a static solution as well as of oxygen bubbles rising in the solution are presented. In both cases, changes in magnetism were observed, which reported the hydrodynamic processes.

Visualization of Hydrogel Shrinkage Due to Ion Replacement by 27Al and 23Na Magnetic Resonance Imaging

Ionic hydrogels are polymer networks that have the ability to store large amounts of water. Due to this behavior hydrogels based on sodium acrylate (SA) are mostly used in diapers and soil conditioners. In this work diffusion and exchange processes in polyelectrolyte hydrogels were followed by 23Na and 27Al magnetic resonance imaging (MRI) techniques. We present 23Na and 27Al MRI experiments of the replacement reaction in hydrogels (classical superabsorbers, hydrogels with an inherent charge gradient and with a higher charge) as a feasibility study of monitoring the combined diffusion/exchange process as well as the shrinking and local defect growth in macroscopic model hydrogels. It is shown that the contraction of the gel induced by increasing the number of physical crosslinks through aluminum ions is of non-linear nature and takes place in several steps in contrast to high charge-density hydrogels based on acrylic acid and maleic acid.