A new NMR method for determining the particle thickness in nanocomposites, using T-2,T-H-selective X{H-1} recoupling

TitleA new NMR method for determining the particle thickness in nanocomposites, using T-2,T-H-selective X{H-1} recoupling
Publication TypeJournal Article
Year of Publication2007
AuthorsSchmidt-Rohr K, Rawal A, Fang XW
Journal TitleJournal of Chemical Physics
Date PublishedFeb
Type of ArticleArticle
ISBN Number0021-9606
Accession NumberISI:000244044600031
Keywordsbone, CLAY, COMPOSITE PULSES, ECHO DOUBLE-RESONANCE, nuclear-magnetic-resonance, OH, POLYMER, ROTATING SOLIDS, solid-state nmr, spectroscopy

A new nuclear magnetic resonance approach for characterizing the thickness of phosphate, silicate, carbonate, and other nanoparticles in organic-inorganic nanocomposites is presented. The particle thickness is probed using the strongly distant-dependent dipolar couplings between the abundant protons in the organic phase and X nuclei (P-31, Si-29, C-13, Al-27, Na-23, etc.) in the inorganic phase. This approach requires pulse sequences with heteronuclear dephasing only by the polymer or surface protons that experience strong homonuclear interactions, but not by dispersed OH or water protons in the inorganic phase, which have long transverse relaxation times T-2,T-H. This goal is achieved by heteronuclear recoupling with dephasing by strong homonuclear interactions of protons (HARDSHIP). The pulse sequence alternates heteronuclear recoupling for similar to 0.15 ms with periods of homonuclear dipolar dephasing that are flanked by canceling 90 degrees pulses. The heteronuclear evolution of the long-T-2,T-H protons is refocused within two recoupling periods, so that H-1 spin diffusion cannot significantly dephase these coherences. For the short-T-2,T-H protons of a relatively immobile organic matrix, the heteronuclear dephasing rate depends simply on the heteronuclear second moment. Homonuclear interactions do not affect the dephasing, even though no homonuclear decoupling is applied, because long-range H-1-X dipolar couplings approximately commute with short-range H-1-H-1 couplings, and heteronuclear recoupling periods are relatively short. This is shown in a detailed analysis based on interaction representations. The algorithm for simulating the dephasing data is described. The new method is demonstrated on a clay-polymer nanocomposite, diamond nanocrystals with protonated surfaces, and the bioapatite-collagen nanocomposite in bone, as well as pure clay and hydroxyapatite. The diameters of the nanoparticles in these materials range between 1 and 5 nm. Simulations show that spherical particles of up to 10 nm diameter can be characterized quite easily. (c) 2007 American Institute of Physics.

Alternate JournalJ. Chem. Phys.