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High-resolution DNP-NMR accesses ultra-low temperatures

IRIG researchers have developed innovative instrumentation for DNP and high-resolution solid-state NMR at very low temperatures. A major milestone has just been reached: a cryostat and a probe can be used continuously to cool the helium gas flows, enabling the sample holder to be sustained and driven at more than 10,000 revolutions per second and at ultra-low temperatures. Thermal and fluidic performance puts this pre-industrial-level instrument at the forefront of worldwide prototyping.

Published on 24 November 2020
Having an instrument to work at very low temperatures (< 100 K) is the wish of many researchers in the field of Nuclear Magnetic Resonance (NMR). The interest becomes obvious for Dynamic Nuclear Polarization (DNP), a hyperpolarization technique that allows increasing the sensitivity of an NMR experiment by several orders of magnitude. This NMR approach makes it possible to study with unprecedented precision the structure and defects of amorphous or crystalline systems, to explore the surface or interface of complex systems, either biological / organic / inorganic or even hybrid. The fields of application are very varied, from the development of innovative functional materials (catalysis, storage, biomaterials, etc.) to the understanding of complex biomolecular systems (fibrillar protein, membrane protein, etc.) in vitro today and in cellulo in the future.

By combining IRIG's skills in cryogenics and DNP, the researchers of our institute have just reached a decisive milestone in this quest for low temperatures for high-resolution and high-sensitivity NMR. The teams developed an innovative experimental system consisting of two cryostats in tandem. The first one, SACRYPAN, generates the cryogenic feed streams of the second one, PAVLOT, which allows NMR/DNP experiments at low temperatures. This "cryogen-free" device imagined at the origin of the project, and the ultimate objective of the R&D efforts undertaken, makes it possible to work without liquid helium for a minimal operating cost (a few dozen euros per day). In terms of performance, it is the most advanced machine in this field, capable of reaching a minimum stabilized temperature of 35 K at the sample holder, regardless of the flow rates used to rotate it. A new milestone is expected shortly with the arrival of SACRYPAN II, which will push back the low temperature limits (< 25 K) never reached with an autonomous cryogenic fluid instrument.

Schematic diagram of the device

These developments began with the development by IRIG researchers of the NUMOC N2 cryostat, which demonstrated its ability to rotate a sample holder with gaseous nitrogen tempered by liquid nitrogen (Figure 1).
Figure 1: NUMOC N2

Following major evolutions, NUMOC became NUMOC He. In its first version, called "lost fluids", NUMOC He was used to take a further step by demonstrating that it was possible to run a helium gas sample in a temperature range of 70 K-25 K (Figure 2).
Figure 2: NUMOC HeII @ PFNC

Since then, this first milestone has given IRIG researchers a real visibility in the field of instrumental research for NMR. Subsequently, the NUMOC He cryostat evolved into a version II that made it possible to prove for the first time that the rotation of a millimeter-size sample holder at several kilohertz down to temperatures below 10 K was possible. Scientific world firsts were thus achieved and published.

In parallel with the scientific exploitation of this cryostat, the IRIG teams designed and finalized SACRYPAN, the latest version of the system, this time completely autonomous in cryogenic fluid (Figure 3).

To complement this saga, another cryostat PAVLOT, the ultra-low temperature NMR-DNP probe, was designed and assembled thanks to the DSBT's know-how in cryogenics. The PAVLOT ultra-low temperature probe is currently being developed in partnership with Bruker, the world leader manufacturer in NMR (Figure 4).
Figure 4: The PAVLOT probe under the NMR magnet

Within the framework of this collaboration, the IRIG researchers aimed to design the entire cryo-mechanical vacuum part of the instrument while the Bruker team was in charge of developing the radio-frequency system for sample analysis. The researchers' task was complex since it involved integrating the Bruker partner's subassembly into an ultra-compact cryostat limited by the size of the NMR magnet bore. A major challenge has been to develop miniature mechanical activators that allow the tuning of the probe resonance frequencies. These activators provide the transitions from the cryogenic box containing the radio frequency device / rotation module to the vacuum chamber isolating the assembly, and then from the vacuum chamber to the outside of the cryostat. The same work had to be done to pass optical and electrical signals through. The very high level of thermal performance demanded required a high vacuum with zero leakage tolerance. These obstacles were overcome and the nominal operation of the equipment as initially defined in the specifications was demonstrated. The integration phase of the Bruker radio frequency system that is now beginning will allow the first DNP spectrum to be realized with this instrument..

A decade, 7 cryostats and no less than 7 national and European grants (ANR, ERC, AGIR-PEPS, PTC, Bottom-up) have been necessary to progress on this project. At the end of this long-distance race, it is a "cryogen-free" instrument protected by 7 international patents that will allow the routine and autonomous use of cryogenic fluids in DNP.
DNP at high magnetic fields relies on the use of a high-power high-frequency microwave source. Such irradiation triggers the transfer of the electron spin polarization of paramagnetic dopants to the surrounding nuclear spins and thus strongly increases the signal detected by NMR. By combining microwave irradiation with a fast pneumatic rotation of the sample (> 10 kHz) at cryogenic temperature, one can thus record high-resolution spectra with a sensitivity increased by several orders of magnitude. No commercial offer allows access to sample temperature below 100 K today.
The IRIG teams involved are those of the Department of Low Temperature Systems (D-SBT) and those of the Modelling and Exploration of Materials (MEM) laboratory.

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