1,721,075 research outputs found
NMR spectroscopy in the solid state
No full textNuclear magnetic resonance spectroscopy of solid materials has seen substantial advances in the past two decades and has now become a powerful tool for the materials scientist. Its main strengths lie in the possibility of obtaining and dynamic information with molecular specificity in highly disordered and even fully amorphous materials. In this article, the fundamental interactions that affect the dynamics of nuclear spins are introduced. These include the chemical shift, dipolar spin–spin coupling, and quadrupolar coupling. In contrast to the liquid state, where molecules tumble rapidly compared to the timescale of NMR, all these interactions are anistropic in the solid state, their magnitude depends on the orientation of the molecule with respect to the magnetic field. This anisotropy provides a wealth of structural and dynamic information, as discussed in the text.Solid-state NMR provides numerous opportunities to influence spin evolution and therefore render spectra selectively sensitive to structural and/or dynamic information of a desired type. Several such schemes, including magic angle spinning (MAS), cross-polarization, multiple-pulse, and Lee–Goldburg decoupling, as well as multiple-quantum techniques, are discussed. Finally, specific applications of NMR techniques to the study of organic and inorganic solids, including techniques such as rotor-synchronized magic angle spinning and DECODER (for the study of orientational order), spin–lattice relaxation time measurements and two-dimensional exchange spectroscopy (molecular dynamics on various timescales) and multiple-quantum magic angle spinning (chemical information from transition-metal nuclei) are introduced
Measurement of structural distribution functions in disordered systems: a general approach for sensitivity estimation
No full textA novel approach is proposed that allows the evaluation of distribution functions of structural parameters, such as dihedral angles and orientations of molecular segments, from suitable NMR spectra in disordered solids. The relationship between the distribution function and the observable spectrum is represented by a linear integral operator G. The distribution function corresponding to a measured spectrum is expanded into the eigenfunctions of G, the expansion coefficients being found by projecting the measured data onto the basis spectra associated with the eigenfunctions. The accuracy of the expansion coefficients, thus obtained, is related to the eigenvalues of G. Unlike other sets of orthogonal functions, the eigenfunctions of G have the interesting property that also their associated basis spectra (“eigenspectra”) are orthogonal. Therefore, we refer to this novel scheme as the “conjugate orthogonal functions” (COF) approach. Beyond the utility of the COF approach for data evaluation and sensitivity analysis, it also represents a means for comparing systematically the analytic power of spectroscopic techniques. The nature of the information delivered by a particular technique (e.g., its symmetry), is reflected in the eigenfunctions of G, whereas the eigenvalues are related to the “amount” of information that can be extracted from spectra with a given signal-to-noise ratio. By way of example, the approach is applied to the determination of the distribution of dihedral angles in doubly 13C-labeled bisphenol-A poly(carbonate) by solid-state NMR spectroscopy, and experimental data from heteronuclear separated-local-field and double quantum two-dimensional (2-D) correlation experiments are compared
Efficient reconstruction of multiphase morphologies from correlation functions
No full textA highly efficient algorithm for the reconstruction of microstructures of heterogeneous media from spatial correlation functions is presented. Since many experimental techniques yield two-point correlation functions, the restoration of heterogeneous structures, such as composites, porous materials, microemulsions, ceramics, or polymer blends, is an inverse problem of fundamental importance. Similar to previously proposed algorithms, the new method relies on Monte Carlo optimization, representing the microstructure on a discrete grid. An efficient way to update the correlation functions after local changes to the structure is introduced. In addition, the rate of convergence is substantially enhanced by selective Monte Carlo moves at interfaces. Speedups over prior methods of more than two orders of magnitude are thus achieved. Moreover, an improved minimization protocol leads to additional gains. The algorithm is ideally suited for implementation on parallel computers. The increase in efficiency brings new classes of problems within the realm of the tractable, notably those involving several different structural length scales and/or components
Microfluidic waveguides for frequency-based pumping
No full textFrequency-based control of fluid transport using fluidic networks with deformable features holds the potential to greatly simplify flow control in bioanalytical microchips, by enabling channel switching with a single active element. This paper describes a fluidic waveguide (analogous to cable in electronics) consisting of a millimeter-sized channel enclosed on one side with a deformable membrane. Dynamic coupling between the membrane deformation and pressure waves in channel is shown to lower the wave speed to 10-50 m/, thus combining frequencies in the range of ~0.5-3 kHz with wavelengths of 1-50 millimeters, with acceptably low attenuation. As a result, it is feasible to construct fluidic filters that transmit pressures to remote locations (centimeters from the source), by combining the waveguide with passive pressure-activated diodes (check valves). Results are shown to illustrate effective device characteristics that will lead to useful performance for flow control
Uniqueness of reconstruction of multiphase morphologies from two-point correlation functions
No full textThe restoration of the spatial structure of heterogeneous media, such as composites, porous materials, microemulsions, ceramics, or polymer blends from two-point correlation functions, is a problem of relevance to several areas of science. In this contribution we revisit the question of the uniqueness of the restoration problem. We present numerical evidence that periodic, piecewise uniform structures with smooth boundaries are completely specified by their two-point correlation functions, up to a translation and, in some cases, inversion. We discuss the physical relevance of the results
Characterisation of oxygen permeation into a microfluidic device for cell culture by in-situ NMR spectroscopy
No full textA compact microfludic device for perfusion culture of mammalian cells under in-situ metabolomic observation by NMR spectroscopy is presented. The chip is made from poly(methylmethacrylate) (PMMA), and uses a poly(dimethyl siloxane) (PDMS) membrane to allow gas exchange. It is integrated with a generic micro-NMR detector developed recently in our group [J. Magn. Reson. 262, 73-80 2016]. While PMMA is an excellent material in the context of NMR, PDMS is known to produce strong background signals. To mitigate this, the device keeps the PDMS away from the detection area. The oxygen permeation into the device is quantified using a flow chemistry approach. A solution of glucose is mixed on the chip with one of glucose oxidase, before flowing through the gas exchanger. The resulting concentration of gluconate is measured by 1H NMR spectroscopy as a function of flow rate. An oxygen equilibration rate constant of 2.4 s -1 is found for the device, easily sufficient to maintain normoxic conditions in a cell culture at modest perfusion flow rates
Electromechanical characterization of polyelectrolyte gels by indentation
No full textWe report an indentation method to quantify the electromechanical coupling in polyelectrolyte gels (PGs). PGs produce electric fields in response to mechanical stress and are therefore promising for mechanical sensor applications. The method exposes thin gel samples to well-defined pressure distributions through a spherical indentor, while the electrical response is measured with an array of platinum electrodes embedded in the support. A series of copolymer gels of acrylamide and acrylic acid were synthesized and equilibrated at a fixed pH, leading to samples with systematically varying spatial densities of both charged groups and cross-links. They were characterized by measuring the potential difference between the gel and the equilibrating solution (Donnan potential) as well as their electromechanical coupling through the indentation method. The electromechanical coupling was found to be proportional to the Donnan potential, while the latter is a universal function of the spatial density of ionizable groups in the gel, irrespective of the cross-link density
Electromechanical equilibrium properties of poly(acrylic acid/acrylamide) hydrogels
No full textThermomechanical properties of poly(acrylic acid-co-acrylamide)hydrogels have been measured for a range of gels while systematically varying the acrylamide/acrylic acid ratio. The gels have been equilibrated with a buffer solution at constant pH and salinity. The gels were characterized in terms of their equilibrium swelling ratio, elastic modulus, and electrochemical potential. The results are in quantitative agreement with the predictions from a recently published thermodynamic field theory
Nuclear magnetic resonance in microfluidic environments using inductively coupled radiofrequency resonators
No full textInductively coupled radiofrequency resonators can provide NMR signals from small samples wirelessly and with high sensitivity. We explore the achievable sensitivity depending on the resonator’s Q-factor and its cross-inductance to the NMR probe. Even for small resonators with modest Q, the sensitivity can be close to that of directly (impedance) coupled microcoils. Sensitivity and excitation power inside inductively coupled solenoids were monitored experimentally by microimaging. The flow velocity profile inside a capillary of 200 µm diameter was measured with a resolution and sensitivity that rivals recent work based on directly coupled microcoils
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