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Does a strong pynocline impact organic-matter preservation and accumulation in an anoxic setting ? The case of the Orca Basin, Gulf of Mexico

Abstract : We show how the 29 Si NMR signals of dispersed inorganic nano-particles of laponite s can be enhanced by Dynamic Nuclear Polarization (DNP). The direct DNP enhances the signals of 29 Si nuclei near unpaired electrons, whereas the indirect DNP via 1 H enhances the signals of more remote sites. As a local and non-destructive technique, solid-state nuclear magnetic resonance (NMR) provides precious insight into the atomic-scale structure and dynamics of nanoparticles (NPs), i.e. materials with three external dimensions sized between 1 and 100 nm. 1,2 Nevertheless, the low sensitivity of NMR can preclude the observation of diluted species, such as the edge, corner, grafting or surface sites. The sensitivity limitation of NMR is even more acute for nuclei with long longitudinal relaxation times (T 1n), low natural abundance and/or low gyromagnetic ratio, such as 29 Si. 3,4 Herein, we show how the NMR signals of inorganic NPs dispersed in a frozen solution containing TOTAPOL 5 can be enhanced at high static magnetic field, B 0 , and under magic-angle spinning (MAS) using dynamic nuclear polarization (DNP). 3-12 Enhancements of 29 Si NMR signals in the order of 10 are reported in direct polarization (DP) and 1 H-29 Si cross-polarization (CP) experiments for laponite s (1), an industrial synthetic clay NP (see Fig. 1), with applications for hybrid materials and soft matter (cleanser, coating). 13-15 Hereafter, the DP and CP experiments with microwave irradiation are referred to as direct and indirect DNP, respectively. This protocol should become a standard for the DNP of NPs, since they are often dispersed in a liquid phase to prevent their aggregation. 16 The dispersion is supplementary to impregnation 4,17 and co-condensation, 18 which have been employed for high-field MAS DNP of porous solids or particle aggregates. 3,4,17-19 Hitherto, dispersion in frozen 1-(TEMPO-4-oxy)-3-(TEMPO-4-amino)-propan-2-ol (TOTAPOL) solutions has only been demonstrated for indirect 13 C and 15 N DNP of biological systems, such as bacteriophage or peptide mesocrystals. 10,11 We also show the complementarity of indirect and direct 29 Si DNP: in indirect DNP, 1 H spin diffusion distributes the polarization within the whole sample, whereas direct DNP enhances the signals of 29 Si sites nearer to TOTAPOL. The polarization buildups in direct and indirect DNP are compared and the polarization leakage is discussed. Fig. 2a shows how the indirect 29 Si DNP results in a 14-fold enhancement of 29 Si NMR signals of 1 dispersed in TOTAPOL solution. The concentration in unpaired electrons of 1 is lower than 100 nm (see the EPR spectrum in Fig. S1a, ESI †). Hence, in indirect DNP, the polarization is transferred from an exogenous TOTAPOL radical in the matrix to 29 Si nuclei of 1 via 1 H spin diffusion and 1 H-29 Si CP. The 29 Si NMR spectrum enhanced by indirect DNP displays two resolved 29 Si NMR signals. The intense peak at À94 ppm corresponds to the (SiO) 3 Si(OMg) (Q 3) sites, located inside the silicate framework, whereas the weak peak at À85 ppm is assigned to the (SiO) 2 Si(OMg)OH (Q 2) sites. 15 Fig. 1 (a) Schematic representation of 1 NPs, which are disk-shaped crystallites with a diameter of ca. 25-30 nm. 20 The specific surface area of 1 is 370 m 2 g À1. (b and c) Atomic-scale structures of the circular (b) and lateral (c) surfaces. The sheet of octahedrally coordinated magnesium atoms is sandwiched by two sheets of silicate.
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https://hal.archives-ouvertes.fr/hal-00371297
Contributor : Marie José Queyroy <>
Submitted on : Friday, March 27, 2009 - 11:55:39 AM
Last modification on : Friday, November 20, 2020 - 10:20:20 AM

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Nicolas Tribovillard, V. Bout-Roumazeilles, Thomas Sionneau, J.C Montero Serrano, Armelle Riboulleau, et al.. Does a strong pynocline impact organic-matter preservation and accumulation in an anoxic setting ? The case of the Orca Basin, Gulf of Mexico. Comptes Rendus Géoscience, Elsevier Masson, 2009, 341, pp.1-9. ⟨10.1016/j.crte.2008.10.002⟩. ⟨hal-00371297⟩

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