Micro-Detection in Solid State NMR – MicrogramNMR

Solid state NMR is one of the most versatile characterization techniques used in materials science. The latest methodological developments (double and triple resonances, dipolar recoupling, J spectroscopy) and instrumental progress (ultra-high magnetic fields, ultra-fast MAS probes, gyrotrons for the generation of high power microwaves) allow us to obtain detailed information at the atomic level as well as connectivities in 3 dimensions. However, NMR suffers from an intrinsic sensitivity problem which makes its use unrealistic for the study of microscopic quantities of samples (typically < 100 micrograms). During the past years, remarkable efforts have been made in order to increase the signal-to-noise ratio of the NMR experiment (and consequently to decrease the corresponding experimental time): (i) hyperpolarized magnetic states (DNP, para-H2, hyperpolarized 129Xe), (ii) optimization of the NMR detector (micro-coils, cryogenic-NMR), (iii) novel signal treatments (non-usual schemes of sampling including Non Uniform Sampling), (iv) "exotic" detections of the NMR signal (mechanical and optical).

The MicrogramNMR program is fully connected to this innovative challenge, namely the net increase of sensitivity in solid state NMR.

The main goal of the project is perfectly defined: MicrogramNMR must popularize the study of solid samples for which the mass is roughly several tens of micrograms (using standard MAS probes with no hardware modification). The implemented methodology is based on the MACS approach (Magic Angle Coil Spinning), recently invented by D. Sakellariou and allowing gains in sensitivity of 10 (or 100 in time!). Durant the last years, C. Bonhomme and D. Sakellariou have obtained significant results on gold standards in solid state NMR by using MACS. Clearly, it is the right time to extend the MACS approach to much more complex materials for which the limitation of the mass is a critical issue in the fields of materials science and bio-nanocomposites (natural and synthetic samples). Another original goal of the project is to make the MACS approach "DNP-compatible", leading to unprecedented gains in sensitivity.

In order to succeed in our tasks, we propose a compact project in terms of duration (24 months), with a limited size of funding (< 199 keuros) and manpower (a specialist in micro-mechanics, a post-doctoral fellow and 2 Master students).

MicrogramNMR contains two main Tasks. The first Task is related to the optimization of micro-coils used in MACS in order to extend its application to NMR/EPR irradiation in DNP MAS. Two axes of research will be developed: (i) the generation of new coils with pure cylindrical geometry, (ii) the use of miniaturized dielectric resonators for DNP applications. The second Task of MicrogramNMR is related to the extension of MACS to complex topics in materials science for which the very small mass of the sample is a critical issue and a true challenge: (i) calcium phosphate germs at the surface of kidney stones called the Randall's plaques, in a very strong collaboration with physicians at the Tenon hospital, Paris, (ii) thin silica based mesoporous films labeled in 29Si and 17O. For the very first time, NMR experiments will be performed on a single thin film. The results will act as the first experimental proofs for the characterization of 2D nano-confinement effects on the oxide network, (iii) thin metallic oxide based mesoporous films. These films such as SrTiO3, BaxSr(1-x)TiO3, MgTa2O6 have many important applications in microelectronics. Here, the huge B1 fields delivered by the micro-coils will be of paramount importance for the study of "strong" quadrupolar nuclei such as 25Mg, 47/49Ti, 87Sr and 181Ta.