NMR-Bildgebung und Materialforschung

Magnetic resonance imaging (MRI) is a powerful tool in medical diagnostics. But this technique of nuclear magnetic resonance (NMR) ist also receiving increasing attention in materials science because it is non-invasive and because of the abundance of different contrast parameters which reflect the molecular properties of the samples. For instance, distributions in microscopic molecular re-orientations as well as macroscopic coherent motion can be imaged by appropriate techniques. Applications of both aspects of MRI are reported by examples of images from elastomers, skin, and velocity profiles. In addition, a new method for investigations of surfacenear volume elements of arbitraril shaped samples is shown.     

Surface-to-Volume Ratio of Ganglia Trapped in Small-Pore Systems Determined by Pulsed-Field Gradient Nuclear Magnetic Resonance

Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) measurements of hydrocarbon diffusion are shown to provide a quantitative measure of the surface-to-volume (s/v) characteristics of slowly dissolving hydrocarbon ganglia, trapped in a water-saturated porous medium, for systems with pore sizes below the limit of spatial resolution of magnetic resonance imaging (MRI). The porous medium is in the form of a packed bed of glass ballotini. The PFG NMR approach is validated in two ways. First, both MRI and PFG analyses are performed on the same system containing ballotini with a diameter of 1 mm. The s/v ratio of the dissolving ganglia determined by the two methods is the same to within the accuracy of the experimental data. Second, below the spatial resolution limit of MRI, PFG NMR alone is used to characterize the s/v ratios of ganglia entrapped in two packings of ballotini with diameters 0.1 and 0.5 mm, respectively. The s/v data are then included into a one-dimensional advection-dispersion model of the ganglia dissolution process. The resultant mass transfer coefficients obtained are in agreement with those obtained, under the same conditions of aqueous superficial flow rate, following MRI analysis of hydrocarbon dissolution in larger pore structures. Copyright 2001 Academic Press.     

The role of water coordination in binary mixtures. A study of two model amphiphilic molecules in aqueous solutions by molecular dynamics and NMR

Two binary aqueous mixtures which contain the small amphiphilic molecules TMAO (trimethylamine-N-oxide) and TBA (tert-butyl alcohol) have been investigated by molecular dynamics simulations and NMR chemical shift and self-diffusion measurements. TMAO is an osmolyte, while TBA is a monohydrate alcohol. Both possess bulky hydrophobic groups and polar heads, namely, NO in TMAO and OH in TBA. The hydrophilic/hydrophobic content of these isosteric molecules strongly modulates the structure and dynamics of the hydration shell, which is thought to be responsible for the effects observed on proteins and phospholipids. Simulation results, especially on hydrogen-bond networking, spatial correlations, and self-diffusivity, are consistent with NMR data and agree well with previous numerical studies on similar solutions. The methods employed allow the elucidation of the microscopic features of the solutions. For TBA solutions, the hydration shell is found to have a low density and a large spatial spread, and thus, above the molar fraction of 0.03, reduction of hydrophobic hydration drives self-aggregation of the solute. This effect does not take place in TMAO solutions, where the hydration shell is more compact and stable, maintaining its structure over a wider range of solute concentrations.     

NMR microscopy - limitations and applications in material sciences -

Alongside the numerous applications of NMR spectroscopy in analytical chemistry, materials sciences and morphological studies by magnetic resonance imaging (MRI), NMR microscopy makes possible a whole new range of applications in materials sciences such as the development and non destructive testing of polymers and ceramic materials. This includes imaging of microscopic structures and structural changes in such materials. The contrast in the images is determined by the NMR specific parameters chemical shift δ, spin density ρ, spin lattice relaxation time T₁, spin spin relaxation time T₂ and spin lattice relaxation time in the rotating frame T_1ρ. The numerous well developed methods available make it possible to study dynamic processes by fast imaging, the measurement of diffusion constants of solvents or liquids, the mobility of fluids in polymers or ceramics or the three dimensional evaluation of pore sizes in porous materials.     

From Micro to Macro: Access to Long‐Range Li+ Diffusion Parameters in Solids via Microscopic 6, 7Li Spin‐Alignment Echo NMR Spectroscopy

The development of highly conductive solids is a rapidly growing research area in materials science. In particular, the study of Li-ion conductors is driven by the ambitious effort to design powerful lithium-ion batteries. A deeper understanding of Li dynamics in solids requires the availability of a large set of complementary techniques to probe Li self-diffusion on different length and time-scales. We report on (7)Li as well as (6)Li spin-alignment echo (SAE) nuclear magnetic resonance (NMR) spectroscopy, which is capable of probing long-range diffusion parameters from a microscopic, that is, atomic-scale, point of view. So far, variable-temperature SAE NMR spectroscopy has been applied to a number of polycrystalline and glassy Li-ion conductors. The materials investigated serve as model systems to unravel the interesting features of the technique in determining reliable Li jump rates and hopping activation energies. In particular, the latter are compared with those probed by macroscopic techniques such as dc-conductivity measurements that are sensitive to long-range translational motions.     

NMR studies of cooperative effects in adsorption

The conversion of gas adsorption isotherms into pore size distributions generally relies upon the assumption of thermodynamically independent pores. Hence, pore-pore cooperative adsorption effects, which might result in a significantly skewed pore size distribution, are neglected. In this work, cooperative adsorption effects in water adsorption on a real, amorphous, mesoporous silica material have been studied using magnetic resonance imaging (MRI) and pulsed-gradient stimulated-echo (PGSE) NMR techniques. Evidence for advanced adsorption can be seen directly using relaxation time weighted MRI. The number and spatial distributions of pixels containing pores of different sizes filled with condensate have been analyzed. The spatial distribution of filled pores has been found to be highly nonrandom. Pixels containing the largest pores present in the material have been observed to fill in conjunction with pixels containing much smaller pores. PGSE NMR has confirmed the spatially extensive nature of the adsorbed ganglia. Thus, long-range (≥40 μm) cooperative adsorption effects, between larger pores associated with smaller pores, occur within mesoporous materials. The NMR findings have also suggested particular types of pore filling mechanisms occur within the porous solid studied.     

NMR diffusion and relaxation correlation methods: New insights in heterogeneous materials

Heterogeneous materials, such as biological tissues, foodstuffs, and rocks, contain a range of microscopic environments where the molecular constituents often have different NMR relaxation time constants and self-diffusion coefficients. Multidimensional correlation methods have greatly improved the possibility for separating and assigning the NMR responses from distinct environments, thereby allowing for a more complete characterization of structure, dynamics, and molecular exchange in heterogeneous materials. Here, we review recent developments in experimental methodology and data analysis approaches.     

Noninvasive in situ visualization of supported catalyst preparations using multinuclear magnetic resonance imaging

Multinuclear magnetic resonance imaging (MRI) is employed as a new noninvasive tool for monitoring supported catalyst preparation by visualizing precursor transport within the porous support. In particular, liquid phase 31P MRI experiments were used to visualize the dynamics of H3PO4 penetration into an alumina pellet and have revealed a strong interaction of H3PO4 with the support. Solid state 31P MRI was applied to map the distribution of the adsorbed phosphate inside the support after its drying. Comparison of the liquid phase and solid phase MRI results confirms the correlation of the phosphate distribution in the liquid phase during impregnation and the phosphate adsorbed on the support. The possibility to monitor the transport of metal atoms within the support by a direct detection of their NMR signal is demonstrated for 195Pt nucleus during impregnation of an alumina pellet with an aqueous solution of H2PtCl6. Other possible strategies for the utilization of MRI to characterize in situ the preparation of supported catalysts and other supported materials are briefly discussed.     

Velocity distributions in a micromixer measured by NMR imaging

Velocity distributions (so-called propagators) with two-dimensional spatial resolution inside a chemical micromixer were measured by pulsed-field-gradient spin-echo (PGSE) nuclear magnetic resonance (NMR). A surface coil matching the volume of interest was built to enhance the signal-to-noise ratio. This enabled the acquisition of velocity maps with a very high spatial resolution of 29 μm × 39 μm. The measured propagators are compared with theoretical distributions and a good agreement is found. The results show that the propagator data provide much richer information about flow behaviour than conventional NMR velocity imaging and the information is essential for understanding the performance of a micromixer. It reveals, for example, deviations in the shape and size of the channel structures and multicomponent flow velocity distribution of overlapping channels. Propagator data efficiently compensate lost information caused by insufficient 3D resolution in conventional velocity imaging.     

Optically-actuated translational and rotational motion at the microscale for microfluidic manipulation and characterization

The single beam optical trap (optical tweezers), a highly focused beam, is on its way to revolutionizing not only the fields of colloidal physics and biology, but also materials science and engineering. Recently, spatially-extended three-dimensional light patterns have gained considerable usage for exerting force to alter, manipulate, organize and characterize materials. To advance the degree of manipulation, such as rotation of materials in microfluidic environments along with spatial structuring, other beam parameters such as phase and polarization have to be configured. These advances in optical tweezers' technology have enabled complex microfluidic actuation and sorting. In addition to remotely (in a non-contact way) applying force and torques in three-dimensions, which can be continuously varied unlike mechanical manipulators, optical tweezers-based methods can be used for sensing the force of interaction between microscopic objects in a microfluidic environment and for the characterization of micro-rheological properties. In this review, we place emphasis on applications of optical actuation based on novel beams in performing special functions such as rotation, transportation, sorting and characterization of the microscopic objects. Further, we have an extended discussion on optical actuation (transport and rotation) with fiber optic microbeams and spectroscopic characterization in the microfluidic environment. All these advancements in optical manipulation would further facilitate the growing use of optical tools for complex microfluidic manipulations.     

可充电池的磁共振研究

快速增长的对安全能源的需求,促使科研工作者不断探索高能量密度的可充锂离子电池(LIBs)。发展原位表征技术能更好地研究电池工作中的锂离子镶嵌机制和电池失效因素。固体核磁共振(NMR)能有效的测试短程化学环境:通过对~1H、~(6,7)Li、~(11)B、~(13)C、~(17)O、~(19)F、~(23)Na和~(31)P等同位素来探测电池材料的微观结构。除了魔角旋转(MAS)高分辨NMR谱图研究电池材料的精细结构之外,核磁共振还能无损地捕获、研究电池材料在充放电循环中的演化。因此,原位核磁共振NMR及成像(MRI)可拓展到电池充放电循环中的锂离子的动态演化以及锂离子浓度的时空分布信息。互为补充地,电子顺磁共振(EPR)及成像(EPRI)能有效地跟踪和捕获电极过渡金属、阴氧离子(O_2~(n-))的氧化还原过程。这些实时捕获的动态信息能更好地指导电极材料的构效、微观设计和电池组装的改进,最终获得优异的电化学性能。     

Optical Doppler Tomography: Imaging in vivo Blood Flow Dynamics Following Pharmacological Intervention and Photodynamic Therapy

A noninvasive optical technique has been developed for imaging in vivo blood flow dynamics and vessel structure with high spatial resolution. The technique is based on optical Doppler tomography, which combines Doppler velocimetry with optical coherence tomography to measure blood flow velocity at discrete spatial locations in turbid biological tissue. Applications of this technique for monitoring changes in blood flow dynamics and vessel structure following pharmacological intervention and photodynamic therapy are demonstrated.     

Numerical analysis of electroosmotic flow in dense regular and random arrays of impermeable, nonconducting spheres

We present a numerical scheme for analyzing steady-state isothermal electroosmotic flow (EOF) in three-dimensional random porous media, involving solution of the coupled Poisson, Nernst-Planck, and Navier-Stokes equations. While traditional finite-difference methods were used to resolve the Poisson-Nernst-Planck problem, the (electro)hydrodynamics has been addressed with high efficiency using the lattice-Boltzmann method. The developed model allows simulation of electrokinetic transport under most general conditions, including arbitrary value and distribution of electrokinetic potential at the solid-liquid interface, electrolyte composition, and pore space morphology. The approach provides quantitative information on a spatial distribution of simulated velocities. This feature was utilized to characterize EOF fields in regular and random, confined and bulk packings of hard (i.e., impermeable, nonconducting) spheres. Important aspects of pore space morphology (sphere size distribution), surface heterogeneity (mismatch in electrokinetic potentials at confining wall and sphere surface), and fluid phase properties (electrical double layer thickness) were investigated with respect to their influence on the EOF dynamics over microscopic and macroscopic spatial domains. Most important is the observation of a generally nonuniform pore-level EOF velocity profile in the sphere packings (even in the thin double layer limit) which is caused by pore space morphology and which is in contrast to the pluglike velocity distribution in a single, straight capillary under the same conditions.     

Shear banding in semidilute entangled polymer solutions

Shear banding in semidilute polymer solutions and other soft materials is one of the most intensely debated topics in current rheology. By means of rheo-optical experiments, physical modeling, and numerical simulations, researchers have started to develop a more thorough understanding of this flow instability within the past few years. Nevertheless, much effort is still required to identify the exact microscopic mechanisms leading to shear band formation and to clarify whether the phenomenon is universal for polymers. For this purpose, basic rheological characteristics, such as the appearance of a stress overshoot during start-up of a simple shear flow, have to be revisited and better related to the dynamics of the polymeric network.     

Flow structure of water in carbon nanotubes: poiseuille type or plug-like?

We have conducted molecular dynamics simulations of water flow in carbon nanotubes (CNTs) for (6,6) to (20,20) CNTs at a streaming velocity of 100 ms. The fluidized piston model (FPM) and the ice piston model (IPM) are employed to drive flow through the CNTs. The results show that the single-file water flow inside (6,6) CNT has a convex upward streaming velocity profile, whereas the velocity profiles in (10,10) to (20,20) CNTs are flat except near the tube wall. The flow structure of cylindrical water in the (8,8) CNT is intermediate between that for the (6,6) CNT and the larger CNTs. The flow parameters are found not to exhibit any dependence on streaming velocity at up to 300 ms in the (12,12) CNT. The hydrogen bond lifetimes of water flowing in CNTs tend to be longer than for the corresponding equilibrium states, and nonzero flow does not reduce the microscopic structure or structural robustness (hydrogen bond lifetime). Although the atomic density profile varies with tube diameter, reflecting the change in static microscopic structure of flow from single file to cylindrical, tube diameter does not induce a clear transition in streaming velocity, temperature, or hydrogen bond lifetime over this diameter range. The results suggest that water flow in CNTs of this size is more pluglike than Poiseuille type, although the flow structure does not strictly accord with either definition.     

Probing Spatial Distribution of Alignment by Deuterium NMR Imaging

Deuterium NMR imaging was used to evaluate the spatial distribution of the degree of alignment in different types of alignment media by monitoring the deuterium quadrupolar splitting using spatially resolved NMR techniques in conventional liquid state NMR instruments. These images allow the unambiguous distinction of magnetic field and alignment inhomogeneities present in partially aligned samples, revealing the underlying reasons for linebroadening within an alignment medium that cannot be explained by the sole analysis of 1D ²H NMR spectra. For example, alignment inhomogeneities due to broken gels or the presence of concentration gradients in liquid crystalline solutions are clearly detected by the imaging methods proposed in this work.     

The effects of molecular diffusion in ultrafast two-dimensional nuclear magnetic resonance

The so-called "ultrafast" nuclear magnetic resonance (NMR) methods enable the collection of multidimensional spectra within a single scan. These experiments operate by replacing traditional t(1) time increments, with a series of combined radiofrequency-irradiation/magnetic-field-gradient manipulations that spatially encode the effects of the indirect-domain spin interactions. Barring the presence of sizable displacements, the spatial patterns thus imparted can be read out following a mixing period with the aid of oscillating acquisition gradients, leading to a train of t(2)-modulated echoes carrying in their positions and phases the indirect- and the direct-domain spin interactions. Both the initial spatial encoding as well as the subsequent spatial decoding procedures underlying ultrafast NMR were designed under the assumption that spins remain static within the sample during their execution. Most often this is not the case, and motion-related effects can be expected to affect the outcome of these experiments. The present paper focuses on analyzing the effects of diffusion in ultrafast two-dimensional (2D) NMR. Toward this end both analytical and numerical formalisms are derived, capable of dealing with the nonuniform spin manipulations, macroscopic sample sizes, and microscopic displacements involved in this kind of sequences. After experimentally validating the correctness of these formalisms these were used to analyze the effects of diffusion for a variety of cases, including ultrafast experiments on both rapidly and slowly diffusing molecules. A series of prototypical schemes were considered including discrete and continuous encoding modes, constant- and real-time manipulations, homo- and heteronuclear acquisitions, and single versus multiple quantum modalities. The effects of molecular diffusion were also compared against typical relaxation-driven losses as they happen in these various prototypical situations; from all these situations, general guidelines for choosing the optimal ultrafast 2D NMR scheme for a particular sample and condition could be deduced.     

Folding dynamics of tethered giant DNA under strong flow

Using a microfluidic device, we investigate the folding dynamics of individual linear long DNA, whose one end is tethered under a strong flow in the presence of a condensing agent. Direct observations of the folding process of DNA molecules reveal a characteristic dynamics with pronounced non-monotonic velocity of the folded part at the free end against the flow. We discuss this unique dynamics in relation to the inhomogeneous spatial fluctuation and the structure change at the multiple order levels along the stretched DNA, which is induced by the increasing tension due to the build-up of the hydrodynamic drag force.     

二维双量子魔角旋转核磁共振技术在功能材料研究中的应用

本文简要介绍了二维双量子魔角旋转核磁共振（DQ-MAS NMR）新技术的基本原理,详细综述了1H、19F、29Si、31P、19F和27Al DQ-MAS NMR技术在各种固体功能材料中的应用,并展望了该技术的应用前景.     

Mechanical manipulation of molecular lattice parameters in smectic elastomers

Smectic liquid crystalline elastomers (SLCE) represent unique materials that combine a 1-D molecular lattice arrangement and orientational order with rubber-elasticity mediated by a polymer network. Such materials may exhibit large thermo-mechanical, opto-mechanical and electro-mechanical effects, due to the coupling of macroscopic sample geometry and microscopic structural features. It is shown that the molecular layer dimensions in the smectic phases can be influenced reversibly by macroscopic strain of the material. We present a microscopic model on the basis of experimental results obtained by mechanical dilatation measurements, optical interferometry, X-ray scattering, (13)C NMR, FTIR and polarizing microscopy data. The model gives an explanation of the controversial results obtained in different types of smectic elastomers.