Category Archives: Material Science

Small Molecules, Non-Covalent Interactions, and Confinement #DNPNMR

Buntkowsky, Gerd, and Michael Vogel. “Small Molecules, Non-Covalent Interactions, and Confinement.” Molecules 25, no. 14 (July 21, 2020): 3311.

https://doi.org/10.3390/molecules25143311

This review gives an overview of current trends in the investigation of small guest molecules, confined in neat and functionalized mesoporous silica materials by a combination of solid-state NMR and relaxometry with other physico-chemical techniques. The reported guest molecules are water, small alcohols, and carbonic acids, small aromatic and heteroaromatic molecules, ionic liquids, and surfactants. They are taken as characteristic role-models, which are representatives for the typical classes of organic molecules. It is shown that this combination delivers unique insights into the structure, arrangement, dynamics, guest-host interactions, and the binding sites in these confined systems, and is probably the most powerful analytical technique to probe these systems.

Shedding light on the atomic-scale structure of amorphous silica–alumina and its Brønsted acid sites #DNPNMR

Perras, Frédéric A., Zichun Wang, Takeshi Kobayashi, Alfons Baiker, Jun Huang, and Marek Pruski. “Shedding Light on the Atomic-Scale Structure of Amorphous Silica–Alumina and Its Brønsted Acid Sites.” Physical Chemistry Chemical Physics 21, no. 35 (2019): 19529–37.

https://doi.org/10.1039/C9CP04099D

In spite of the widespread applications of amorphous silica–aluminas (ASAs) in many important industrial chemical processes, their high-resolution structures have remained largely elusive. Specifically, the lack of long-range ordering in ASA precludes the use of diffraction methods while NMR spectroscopy has been limited by low sensitivity. Here, we use conventional as well as DNP-enhanced 29Si–29Si, 27Al–27Al, and 29Si–27Al solid-state NMR experiments to shed light on the ordering of atoms in ASAs prepared by flame-spray-pyrolysis. These experiments, in conjunction with a novel Monte Carlo-based approach to simulating RESPDOR dephasing curves, revealed that ASA materials obey Loewenstein’s rule of aluminum avoidance. 3D 17O{1H} and 2D
17O{1H, 27Al} experiments were developed to measure site-specific O–H and HO–Al distances, and show that the Brønsted acid sites originate predominantly from the pseudo-bridging silanol groups.

Dynamic nuclear polarization and ESR hole burning in As doped silicon #DNPNMR

Järvinen, J., D. Zvezdov, J. Ahokas, S. Sheludiakov, L. Lehtonen, S. Vasiliev, L. Vlasenko, Y. Ishikawa, and Y. Fujii. “Dynamic Nuclear Polarization and ESR Hole Burning in As Doped Silicon.” Physical Chemistry Chemical Physics 22, no. 18 (2020): 10227–37.

https://doi.org/10.1039/C9CP06859G

We present an experimental study of the Dynamic Nuclear Polarization (DNP) of 29Si nuclei in silicon crystals of natural abundance doped with As in the temperature range 0.1-1 K and in strong magnetic field of 4.6 T. This ensures very high degree of electron spin polarization, extremely slow nuclear relaxation and optimal conditions for realization of Overhauser and resolved solid effects. We found that the solid effect DNP leads to an appearance of a pattern of holes and peaks in the ESR line, separated by the super-hyperfine interaction between the donor electron and 29Si nuclei closest to the donor. On the contrary, the Overhauser effect DNP mainly affects the remote 29Si nuclei having the weakest interaction with the donor electron. This leads to an appearance of a very narrow ( 3 mG wide) hole in the ESR line. We studied relaxation of the holes after burning, which is caused by the nuclear spin diffusion. Analyzing the dynamics of the hole in the spectrum with a simple one-dimensional diffusion model leads to a value of the diffusion coefficient D = 8(3)10􀀀9 G2/s. Our data indicate that the spin diffusion is not completely prevented even in the frozen core near the donors. The emergence of the narrow hole after the Overhauser DNP may be explained by a partial “softening” of the frozen core caused by decoupling of the donor electron and remote 29Si nuclei.

Enabling Natural Abundance 17O Solid-State NMR by Direct Polarization from Paramagnetic Metal Ions #DNPNMR

Jardón-Álvarez, Daniel, Guy Reuveni, Adi Harchol, and Michal Leskes. “Enabling Natural Abundance 17O Solid-State NMR by Direct Polarization from Paramagnetic Metal Ions.” The Journal of Physical Chemistry Letters 11, no. 14 (July 16, 2020): 5439–45.

https://doi.org/10.1021/acs.jpclett.0c01527.

Dynamic nuclear polarization (DNP) significantly enhances the sensitivity of nuclear magnetic resonance (NMR), increasing applications and quality of NMR as a characterization tool for materials. Efficient spin diffusion among the nuclear spins is considered to be essential for spreading the hyperpolarization throughout the sample enabling large DNP enhancements. This scenario mostly limits the polarization enhancement of low sensitivity nuclei in inorganic materials to the surface sites when the polarization source is an exogenous radical. In metal ions based DNP, the polarization agents are distributed in the bulk sample and act as both source of relaxation and of polarization enhancement. We have found that as long as the polarization agent is the main source of relaxation, the enhancement does not depend on the distance between the nucleus and dopant. As a consequence, the requirement of efficient spin diffusion is lifted and the entire sample can be directly polarized. We exploit this finding to measure high quality NMR spectra of 17O in the electrode material Li4Ti5O12 doped with Fe(III) despite its low abundance and long relaxation time.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS–PIETA NMR Spectroscopy #DNPNMR

Piveteau, Laura, Dmitry N. Dirin, Christopher P. Gordon, Brennan J. Walder, Ta-Chung Ong, Lyndon Emsley, Christophe Copéret, and Maksym V. Kovalenko. “Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS–PIETA NMR Spectroscopy.” Nano Letters 20, no. 5 (May 13, 2020): 3003–18.

https://doi.org/10.1021/acs.nanolett.9b04870

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands−phase incremented echo-train acquisition (PASS−PIETA). The improved sensitivity and resolution of DNP enhanced PASS−PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the 111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin−orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS–PIETA NMR Spectroscopy #DNPNMR

Piveteau, Laura, Dmitry N. Dirin, Christopher P. Gordon, Brennan J. Walder, Ta-Chung Ong, Lyndon Emsley, Christophe Copéret, and Maksym V. Kovalenko. “Colloidal-ALD-Grown Core/Shell CdSe/CdS Nanoplatelets as Seen by DNP Enhanced PASS–PIETA NMR Spectroscopy.” Nano Letters 20, no. 5 (May 13, 2020): 3003–18

https://doi.org/10.1021/acs.nanolett.9b04870

Ligand exchange and CdS shell growth onto colloidal CdSe nanoplatelets (NPLs) using colloidal atomic layer deposition (c-ALD) were investigated by solid-state nuclear magnetic resonance (NMR) experiments, in particular, dynamic nuclear polarization (DNP) enhanced phase adjusted spinning sidebands−phase incremented echo-train acquisition (PASS−PIETA). The improved sensitivity and resolution of DNP enhanced PASS−PIETA permits the identification and study of the core, shell, and surface species of CdSe and CdSe/CdS core/shell NPLs heterostructures at all stages of c-ALD. The cadmium chemical shielding was found to be proportionally dependent on the number and nature of coordinating chalcogen-based ligands. DFT calculations permitted the separation of the the 111/113Cd chemical shielding into its different components, revealing that the varying strength of paramagnetic and spin−orbit shielding contributions are responsible for the chemical shielding trend of cadmium chalcogenides. Overall, this study points to the roughening and increased chemical disorder at the surface during the shell growth process, which is not readily captured by the conventional characterization tools such as electron microscopy.

Aqueous aging of a silica coated TiO2UV filter used in sunscreens: investigations at the molecular scale with dynamic nuclear polarization NMR #DNPNMR

Slomberg, Danielle L., Riccardo Catalano, Fabio Ziarelli, Stéphane Viel, Vincent Bartolomei, Jérôme Labille, and Armand Masion. “Aqueous Aging of a Silica Coated TiO 2 UV Filter Used in Sunscreens: Investigations at the Molecular Scale with Dynamic Nuclear Polarization NMR.” RSC Advances 10, no. 14 (2020): 8266–74

https://doi.org/10.1039/D0RA00595A. , 

Short-term, aqueous aging of a commercial nanocomposite TiO2 UV filter with a protective SiO2 shell was examined in abiotic simulated fresh- and seawater. Under these conditions, the SiO2 layer was quantitatively removed (∼88–98%) within 96 hours, as determined using inductively coupled plasma-atomic emission spectroscopy (ICP-AES). While these bulk ICP-AES analyses suggested almost identical SiO2 shell degradation after aging in fresh- and seawater, surface sensitive 29Si dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (SSNMR), with signal enhancements of 5–10× compared to standard SSNMR, was able to distinguish differences in the aged nanocomposites at the molecular level. DNP-SSNMR revealed that the attachment of the silica layer to the underlying TiO2 core rested on substantial Si–O–Ti bond formation, bonds which were preserved after freshwater aging, yet barely present after aging in seawater. The removal of the protective SiO2 layer is due to ionic strength accelerated dissolution, which could present significant consequences to aqueous environments when the photoactive TiO core becomes exposed. This work demonstrates the importance of characterizing aged nanocomposites not only on the bulk scale, but also on the molecular level by employing surface sensitive techniques, such as DNP-NMR. Molecular level details on surface transformation and elemental speciation will be crucial for improving the environmental safety of nanocomposites.

The surface chemistry of a nanocellulose drug carrier unravelled by MAS-DNP #DNPNMR

Kumar, Akshay, Hippolyte Durand, Elisa Zeno, Cyril Balsollier, Bastien Watbled, Cecile Sillard, Sébastien Fort, et al. “The Surface Chemistry of a Nanocellulose Drug Carrier Unravelled by MAS-DNP.” Chemical Science, 2020, 10.1039.C9SC06312A.

https://doi.org/10.1039/C9SC06312A.

Cellulose nanofibrils (CNF) are renewable bio-based materials with high specific area, which makes them ideal candidates for multiple emerging applications including for instance on-demand drug release. However, in-depth chemical and structural characterization of the CNF surface chemistry is still an open challenge, especially for low weight percentage of functionalization. This currently prevents the development of efficient, cost-effective and reproducible green synthetic routes and thus the widespread development of targeted and responsive drug-delivery CNF carriers. We show in this work how we use dynamic nuclear polarization (DNP) to overcome the sensitivity limitation of conventional solid-state NMR and gain insight into the surface chemistry of drug-functionalized TEMPO-oxidized cellulose nanofibrils. The DNP enhanced-NMR data can report unambiguously on the presence of trace amounts of TEMPO moieties and depolymerized cellulosic units in the starting material, as well as coupling agents on the CNFs surface (used in the heterogeneous reaction). This enables a precise estimation of the drug loading while differentiating adsorption from covalent bonding (∼1 wt% in our case) as opposed to other analytical techniques such as elemental analysis and conductometric titration that can neither detect the presence of coupling agents, nor differentiate unambiguously between adsorption and grafting. The approach, which does not rely on the use of 13C/15N enriched compounds, will be key to further develop efficient surface chemistry routes and has direct implication for the development of drug delivery applications both in terms of safety and dosage.

Structural analysis of cross-linked poly(vinyl alcohol) using high-field DNP-NMR #DNPNMR

Kanda, Taiji, Mayuka Kitawaki, Toshiaki Arata, Yoh Matsuki, and Toshimichi Fujiwara. “Structural Analysis of Cross-Linked Poly(Vinyl Alcohol) Using High-Field DNP-NMR.” RSC Advances 10, no. 14 (2020): 8039–43.

https://doi.org/10.1039/D0RA00399A.

Poly(vinyl alcohol) (PVOH) is a water-soluble synthetic polymer, widely used in materials for functional films and moldings, fiber fabric sizing agents, paper coating resins, and adhesives. PVOH is mainly applied in the form of an aqueous solution, yet after its application, insolubility (water resistance) is required. To achieve this, additives are introduced. These additives used with PVOH are cross-linking agents which react with the hydroxyl groups and modified functional groups in PVOH. Because of the poor reactivity of unmodified PVOH, it does not react with cross-linking agents that have functional reactive groups. Therefore, modified PVOH that reacts with a cross-linking agent more successfully is required. These chemical bonding sites are so low in abundance that it is difficult to characterize the cross-linking structure. Solid-state 13C NMR is a powerful technique that can be used for the structural analysis of a polymer material. However, its sensitivity is low, hence it is difficult to determine crosslinking in a polymer, as it makes up only a small proportion of the product. Therefore, solid-state 13C NMR sensitivity can be enhanced by high-field dynamic nuclear polarization (DNP) using strong electron polarization. In this study, the reaction of acetoacetylated PVOH with a cross-linking agent, adipic dihydrazide, was analyzed. This crosslinked PVOH is the most popular vinyl alcohol polymer on the commercial market. The sensitivity enhanced 13C NMR spectra reveal that the carbonyl of the acetoacetyl group of PVOH crosslinks with adipic hydrazide by forming an imine bond (>CN–) this study also shows that the product has only seven crosslinking sites per molecular chain with a polymerization degree of 1000 and is water resistant.

DNP in Materials Science: Touching the Surface #DNPNMR

Berruyer, Pierrick, Lyndon Emsley, and Anne Lesage. “DNP in Materials Science: Touching the Surface,” 7:12, 2018.

https://doi.org/10.1002/9780470034590.emrstm1554.

Dynamic nuclear polarization (DNP)-enhanced solid-state NMR spectroscopy under magic-angle spinning has recently emerged as a unique analytical method to probe surfaces at atomic resolution. In this article, we first describe the basic principles of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS). The article continues with a large review of recent literature that illustrates the versatility of this technique and its incredible potential to reveal new structural features at surfaces with details at an unprecedented level. The most recent developments, such as the application of DNP SENS to highly reactive surface sites, are finally covered.

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