Recent developments in MAS DNP-NMR of materials
Graphical abstract
Introduction
Solid-state NMR spectroscopy provides unique information on the atomic-level structure and dynamics of materials [1], employed for various technological ends, such as energy, health, mobility, catalysis and construction. As a local characterization technique capable of providing atomic resolution, NMR is especially suitable for the study of amorphous, heterogeneous or disordered materials, including oxide glasses [2], heterogeneous catalysts [[3], [4], [5]] or battery materials [6].
Nevertheless, a major limitation of NMR spectroscopy is its lack of sensitivity. Such low sensitivity is due to the small magnetic moments of nuclear spins, which result in small nuclear magnetizations at thermal equilibrium and slow longitudinal relaxation, two phenomena yielding weak NMR signals. The low sensitivity of NMR limits the observation of (i) surfaces and interfaces, (ii) low-volume samples, such as thin-films or cultural heritage samples, (iii) defects, which control numerous properties of materials, such as the reactivity or the ionic conductivity, (iv) isotopes with low gyromagnetic ratio, γ, or low natural abundance, such as 15N, 17O, 2H, 89Y, etc., or subject to large anisotropic interactions, such as 119Sn, 195Pt or 35Cl.
Therefore, numerous methods have been proposed to enhance the sensitivity of solid-state NMR spectroscopy. These methods use two complementary routes: (i) the design of more sensitive detection schemes and (ii) the enhancement of the nuclear polarization. The sensitivity of the NMR detection has been significantly increased by the introduction of Fourier Transform NMR spectroscopy [7]. More recently, the sensitivity per spin of NMR detection for solids has also been increased using techniques such as Magic-Angle Spinning (MAS) NMR probes with cryogenic detection systems, also termed cryoprobes [8], microcoils [9] and non-uniform sampling of the indirect dimensions of multidimensional NMR experiments [10].
The second route involves increasing the nuclear polarization. Such enhancement can be achieved by using high static magnetic fields, B0 [[11], [12], [13]]. Solid-state NMR experiments up to 26 T have been reported using magnets made of low-temperature superconducting (LTS) outer coils in series with high-temperature superconducting (HTS) inner coils. Solid-state NMR experiments at static B0 fields up to 40 T have been carried out using LTS outer coils in series with resistive inner coils [11,13]. Nevertheless, the high running costs of these resistive magnets limit their use. The nuclear polarization can also be enhanced by polarization transfer, such as cross-polarization, from high-γ isotopes to low-γ ones [14,15]. Such polarization transfers are also employed in the indirect detection via high-γ spins, such as protons [16]. Other strategies to increase the nuclear polarization under MAS include low temperature, which has been decreased down to 5 K [[17], [18], [19]], Dynamic Nuclear Polarization (DNP), which consists of a transfer of polarization from unpaired electrons to the nuclear spins [[20], [21], [22], [23], [24]], xenon-129 gas hyperpolarized by Spin Exchange Optical Pumping [25,26], para-hydrogen [27,28] and photochemically-induced DNP [[29], [30], [31]].
Among the approaches listed above, DNP at B0 ≥ 5 T under MAS conditions offers several advantages: (i) it can be applied for a wide range of systems, including small organic molecules, biomolecules, organic, hybrid and inorganic materials, (ii) it yields sensitivity gains of several orders of magnitude (10-103) whereas a doubling of the B0 field strength only improves the sensitivity by a factor of 2.8 for spin-1/2 nuclei, (iii) it is compatible with the acquisition of high-resolution NMR spectra, (iv) it allows the acquisition of multidimensional NMR experiments and (v) companies supply commercial DNP-NMR spectrometers.
We focus here on DNP-NMR of materials at B0 ≥ 5 T under MAS conditions. The DNP of materials is as old as the DNP technique itself since the DNP phenomenon was first experimentally demonstrated on lithium metal in 1953 by Carver and Slichter (see Fig. 1a) [32]. However, these first experiments were carried out at B0 = 3 mT under static conditions. In 1958, Abragam and Proctor reported the first DNP experiments on a dielectric inorganic material, which was a single crystal of LiF doped with F-centers [33]. DNP was also demonstrated for other dielectric materials, including inorganic single crystals doped with paramagnetic ions, such as Nd3+, Tm2+ or Ce3+ or semi-conductors, such as n-doped silicon [34].
In 1983, Wind reported the first DNP-NMR experiments under MAS conditions [20]. The MAS technique allowed for the acquisition of high-resolution DNP-enhanced NMR spectra of materials, such as coal, diamonds, polymers and organic conductors [22]. The sensitivity gain provided by DNP was notably used to probe the interface of an immiscible mixture of polycarbonate and polystyrene [37] and the surface of chemical vapor deposited (CVD) diamond film (see Fig. 1b) [35,38]. However, these MAS DNP-NMR experiments were constrained to B0 ≤ 1.5 T, i.e. 1H Larmor frequency ν0(1H) ≤ 60 MHz because of the paucity of microwave sources operating above 40–50 GHz.
DNP-enhanced NMR at B0 ≥ 5 T under MAS conditions was pioneered by the Griffin group at MIT in the 1990s. Major developments included (i) the introduction of cyclotron resonance masers, known as gyrotrons, into DNP experiments as a continuous high-power microwave source at frequencies higher than 140 GHz [39], (ii) the design of cryogenic MAS probes that operate at temperatures of 90 K and below [21,40], and (iii) the design of nitroxide biradicals, which generally yield more efficient DNP transfer at high fields than monoradicals [41]. MAS DNP spectroscopy at high fields was initially mainly applied to the study of solid-state biomolecules and notably allowed for the observation of photocycle intermediates of bacteriorhodopsin, a proton pump of Archea [42].
The availability of a commercial MAS DNP-NMR system [43] at B0 = 9.4 T has led to the use of this technique for the characterization of materials. The possibility of applying high-field MAS DNP-NMR to materials was first reported by the groups of Emsley, Copéret and Bodenhausen in 2010 [36]. They showed that DNP at 9.4 T could yield a 50-fold enhancement of 1H→13C Cross-Polarization under MAS (CPMAS) signals of phenol moieties covalently bonded to the surface of mesoporous silica impregnated with an aqueous solution of nitroxide biradicals (see Fig. 1c). Independently, our group demonstrated that DNP can yield a 30-fold enhancement of 29Si signals in direct excitation experiments under MAS of mesoporous silica impregnated with a solution of nitroxide biradicals in a DMSO/water mixture [44,45]. Owing to the availability of commercial MAS DNP-NMR systems, this technique is applied to the characterization of a rapidly increasing number of organic, hybrid and inorganic materials (see Fig. 2).
We review herein the recent publications relating to high-field MAS DNP-NMR experiments on materials since 2010. Hence, we mainly restrict ourselves to articles reporting on materials with this technique, even if some seminal results about MAS DNP-NMR of small molecules or biomolecules in frozen solutions are also mentioned. The interested reader is referred to other recent review articles on that topic [4,23,24,[46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65]]. We will first describe the transfers of polarization in MAS DNP and the various factors contributing to the global sensitivity of MAS DNP-NMR experiments. We then present a selection of the various polarizing agents (PAs), which have been employed for high-field MAS DNP-NMR, and the preparation of material samples for these experiments. We describe the MAS DNP-NMR spectrometers used and the influence of various experimental conditions (B0 field, microwave power, sample temperature, MAS frequency …) on the DNP enhancement. We present the various isotopes and NMR experiments, for which MAS DNP has been employed in the case of materials. Finally, we provide an overview of applications of high-field MAS DNP for the characterization of organic, hybrid and inorganic materials.
Section snippets
DNP mechanisms and depolarization
In materials, MAS DNP transfers at B0 ≥ 5 T have been mainly achieved using the solid effect (SE), cross effect (CE) and Overhauser effect (OE). The thermal mixing is another DNP mechanism, which has been reported in the case of many strongly coupled unpaired electrons exhibiting homogeneously broadened EPR lines [[66], [67], [68]]. However, the EPR lines are generally inhomogeneously broadened during MAS DNP NMR experiments and hence, the thermal mixing will not be discussed further [52]. The
Polarizing agents
Various PAs have been employed for high-field MAS DNP-NMR, including biradicals, BDPA, trityl, paramagnetic ions and endogeneous paramagnetic centers. The structures of some of the radicals used for high-field MAS DNP are displayed in Fig. 6. All these PAs exhibit an EPR transition at a g factor near 2, since commercially available gyrotrons only deliver microwaves at a fixed frequency, which is typically set tuned to that for nitroxides.
Most of high field MAS DNP-NMR experiments on materials
Instrumentation
Most high-field MAS DNP-NMR experiments on materials have been carried out on commercial systems manufactured by the Bruker BioSpin company (see Fig. 9) [43,56]. Therefore the MAS DNP-NMR instrumentation is only briefly discussed here. Further information on that topic can be found elsewhere [24,43,46,56,58]. The microwave source is a continuous-wave gyrotron with a dedicated magnet [39,43,58]. The microwave beam delivered by the gyrotron propagates through a transmission line, which is a
Isotopes and type of transfer
Fig. 13 shows the chemical elements that have been detected in materials using high-field MAS DNP-NMR. The sensitivity enhancement provided by DNP has notably been used to detect insensitive spin-1/2 isotopes, owing to (i) their low natural abundance (NA), such as 29Si (NA = 4.7%) [45,208], 13C (NA = 1.1%) (see Fig. 1c) [36,209] or 15N (NA = 0.37%) [111], (ii) their low gyromagnetic ratio, such as 15N (γ(15N) ≈ 0.1γ(1H)) or 89Y (γ(89Y) ≈ 0.05γ(1H)) [109] or (iii) their large CSA, such as 77Se,
Organic materials
As seen in Fig. 20, high-field MAS DNP-NMR has been used to characterize a broad range of materials, including pharmaceuticals, polymers, catalytists, energy materials, biomaterials etc. We present in this section its use for the characterization of organic materials composed of small molecules or polymers. Organic materials contain protons and so 1H–1H spin diffusion can transport the DNP-enhanced 1H polarization from the surface to the bulk (see section 2.2).
Inorganic and hybrid materials
High-field MAS DNP-NMR has been applied to a wide range of inorganic and hybrid materials used notably in the field of catalysis and energy. The bulk region of inorganic materials often contains no protons and in such cases, indirect DNP can be used for the selective observation of nuclei near the surface (see section 5.1). Furthermore, for most materials, the nuclei located near the surface represent a small fraction of the total number of nuclei and the sensitivity gain provided by DNP
Conclusion
We have presented here the recent developments in high-field MAS DNP-NMR of materials. The development of novel DNP-NMR instruments, the design of tailored PAs, the optimization of sample preparation, and the better understanding of DNP mechanisms under MAS have all significantly improved the sensitivity of this technique and broadened its application fields. Currently, MAS DNP has been applied to a wide range of materials, including organic, hybrid and inorganic materials with applications to
Acknowledgements
The authors would like to thank the anonymous reviewers as well as Gaël De Paëpe, Franck Engelke, Takeshi Kobayashi, Michal Leskes, Frédéric Mentink-Vigier and Giulia Mollica for their comments that greatly contributed to improve the quality of the manuscript. Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur, de la Recherche et de l’Innovation, Hauts-de-France Region and FEDER are acknowledged for supporting and partially funding this work. Financial support from the
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2023, Current Opinion in Colloid and Interface ScienceCitation Excerpt :In the application discussed in In-situ NMR investigation of crystallization from frozen solutions, a MAS DNP spectrometer was employed by modifying the operational conditions so that the temperature could be increased up to ∼200 K, hence enabling in-situ NMR investigation of crystallization from frozen solutions as a function of temperature. However, when using bis-nitroxide radicals as polarizing agents, the DNP sensitivity enhancements sensibly decrease with increasing the temperature [40]. As a consequence, no DNP effect could be exploited for investigating crystallization from frozen solution using in-situ NMR approaches.