Andrew G M Rankin

Julien Trébosc

Frédérique Pourpoint

Jean-Paul Amoureux

Olivier Lafon
email: olivier.lafon@univ-lille.fr

Recent Developments in DNP-NMR of Materials

Keywords: Dynamic Nuclear Polarization, pharmaceuticals, polymers, porous materials, nanoparticles

Solid-state NMR spectroscopy is a powerful technique for the characterization of the atomic-level structure and dynamics of materials. Nevertheless, the use of this technique is often limited by its lack of sensitivity, which can prevent the observation of surfaces, defects or insensitive isotopes. Dynamic Nuclear Polarization (DNP) has been shown to improve by one to three orders of magnitude the sensitivity of NMR experiments on materials under Magic-Angle Spinning (MAS) at static magnetic field B0 ≥ 5 T, conditions allowing for the acquisition of high-resolution spectra. The field of DNP-NMR spectroscopy of materials has undergone a rapid development in the last ten years, spured notably by the availability of commercial DNP-NMR systems. We provide here an in-depth overview of MAS DNP-NMR studies of materials at high B0 field. After a historical perspective of DNP of materials, we describe the DNP transfers under MAS, the transport of polarization by spin diffusion and the various contributions to the overall sensitivity of DNP-NMR experiments. We discuss the design of tailored polarizing agents and the sample preparation in the case of materials. We present the DNP-NMR hardware and the influence of key experimental parameters, such as microwave power, magnetic field, temperature and MAS frequency. We give an overview of the isotopes, which have been detected by this technique, and the NMR methods, which have been combined with DNP. Finally we show how MAS DNP-NMR has been applied to gain new insights into the structure of organic, hybrid and inorganic materials with applications in fields, such as health, energy, catalysis and optoelectronics…

Introduction

Solid-state NMR spectroscopy provides unique information on the atomic-level structure and dynamics of materials [START_REF] Ashbrook | Solid-State Nuclear Magnetic Resonance Spectroscopy[END_REF], 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 [START_REF] Edén | Chapter Four -27 Al NMR Studies of Aluminosilicate Glasses[END_REF], heterogeneous catalysts [START_REF] Zheng | Acidic Properties and Structure-Activity Correlations of Solid Acid Catalysts Revealed by Solid-State NMR Spectroscopy[END_REF][START_REF] Copéret | Active Sites in Supported Single-Site Catalysts: An NMR Perspective[END_REF] or battery materials [START_REF] Pecher | Materials' Methods: NMR in Battery Research[END_REF].

Nevertheless, a major limitation of NMR spectroscopy is its lack of sensitivity. Such low sensitivity is due to the small magnetic moment of nuclear spins, which results in small nuclear magnetization 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 15 N, 17 O, 2 H, 89 Y…, or subject to large anisotropic interactions, such as 119 Sn, 195 Pt or 35 Cl.

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 [START_REF] Ernst | Application of Fourier Transform Spectroscopy to Magnetic Resonance[END_REF]. 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 [START_REF] Mizuno | Development of a magic-angle spinning nuclear magnetic resonance probe with a cryogenic detection system for sensitivity enhancement[END_REF], microcoils [START_REF] Sakellariou | High-resolution, high-sensitivity NMR of nanolitre anisotropic samples by coil spinning[END_REF] and non-uniform sampling of the indirect dimensions of multidimensional NMR experiments [START_REF] Lesot | Fast acquisition of multidimensional NMR spectra of solids and mesophases using alternative sampling methods[END_REF].

The second route involves increasing the nuclear polarization. Such enhancement can be achieved by using high static magnetic fields, B0 [START_REF] Gan | Seeking higher resolution and sensitivity for NMR of quadrupolar nuclei at ultrahigh magnetic fields[END_REF][START_REF] Hashi | Achievement of 1020MHz NMR[END_REF][START_REF] Gan | NMR spectroscopy up to 35.2T using a series-connected hybrid magnet[END_REF]. Solid-state NMR experiments up to 24 T have been reported using magnets made of low-temperature superconducting (LTS) outer coils in series with high-temperature superconducting (HTS) inner coils [START_REF] Hashi | Achievement of 1020MHz NMR[END_REF]. 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 [START_REF] Gan | Seeking higher resolution and sensitivity for NMR of quadrupolar nuclei at ultrahigh magnetic fields[END_REF][START_REF] Gan | NMR spectroscopy up to 35.2T using a series-connected hybrid magnet[END_REF]. Nevertheless, the high running costs of these resistive magnets limits their use. The nuclear polarization can also be enhanced by polarization transfer, such as crosspolarization, from high-γ isotopes to low-γ ones [START_REF] Hartmann | Nuclear Double Resonance in the Rotating Frame[END_REF]. Such polarization transfers are also employed in the indirect detection via high-γ spins, such as protons [START_REF] Ishii | Sensitivity Enhancement in Solid State 15 N NMR by Indirect Detection with High-Speed Magic Angle Spinning[END_REF]. Other strategies to increase the nuclear polarization under MAS include low temperature, which has been decreased down to 5 K [START_REF] Concistre | Magic-angle spinning NMR of cold samples[END_REF][START_REF] Myhre | Magic angle spinning nuclear magnetic resonance near liquid-helium temperatures. Variable-temperature CPMAS studies of C4H7+ to 5 K[END_REF], Dynamic Nuclear Polarization (DNP), which consists of a microwave-driven transfer of polarization from unpaired electrons to the nuclear spins [START_REF] Wind | Dynamic Nuclear Polarization and High-Resolution NMR of Solids[END_REF][START_REF] Michaelis | Topical Developments in High-Field Dynamic Nuclear Polarization[END_REF][START_REF] Lilly Thankamony | Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR[END_REF][START_REF] Wind | Experimental Setup for Enhanced 13 C NMR Spectroscopy in Solids Using Dynamic Nuclear Polarization[END_REF][START_REF] Hall | Polarization-Enhanced NMR Spectroscopy of Biomolecules in Frozen Solution[END_REF], xenon-129 gas hyperpolarized by Spin Exchange Optical Pumping [START_REF] Raftery | Optical Pumping and Magic Angle Spinning: Sensitivity and Resolution Enhancement for Surface NMR Obtained with Laser-Polarized Xenon[END_REF][START_REF] Brunner | Surface NMR Using Laser-Polarized129Xe under Magic Angle Spinning Conditions[END_REF], para-hydrogen [START_REF] Henning | In situ CF MAS NMR study of the pairwise incorporation of parahydrogen into olefins on rhodium-containing zeolites Y[END_REF][START_REF] Arzumanov | Parahydrogen-Induced Polarization Detected with Continuous Flow Magic Angle Spinning NMR[END_REF] and photochemically-induced DNP [START_REF] Zysmilich | Photochemically Induced Dynamic Nuclear Polarization in the Solid-State 15 N Spectra of Reaction Centers from Photosynthetic Bacteria Rhodobacter sphaeroides R-26[END_REF].

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 (10-10 3 ) 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 Figure 1a) [START_REF] Carver | Polarization of Nuclear Spins in Metals[END_REF]. 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 [START_REF] Abragam | Une nouvelle méthode de polarisation des noyaux atomiques dans les solides[END_REF]. DNP was also demonstrated for other dielectric materials, including inorganic single crystals doped with paramagnetic ions, such as Nd 3+ , Tm 2+ or Ce 3+ or semi-conductors, such as n-doped silicon [START_REF] Abragam | Polarisation dynamique des noyaux du silicium 29 dans le silicium[END_REF].

Figure 1. Some milestones in the development of DNP-NMR of materials: (a) 7 Li NMR spectra of lithium metal without (top) and with (bottom) microwave irradiation at 3 mT under static conditions; (b) 1 H→ 13 C NMR spectra of CVD diamond film with (top) and without (bottom) microwave irradiation at 1.4 T and room temperature with a MAS frequency νR = 3.8 kHz at room temperature; (c) 1 H→ 13 C CPMAS NMR spectra of mesoporous silica functionalized with phenol moieties impregnated with TOTAPOL aqueous solution with (top) and without (bottom) microwave irradiation at 9.4 T with νR = 8 kHz. The numbering of the carbon atoms of the phenol moieties is displayed on the left. Adapted with permission from refs. [START_REF] Carver | Polarization of Nuclear Spins in Metals[END_REF][START_REF] Lock | A Study of 13C-enriched Chemical Vapor Deposited Diamond Film by Means of 13C Nuclear Magnetic Resonance, Electron Paramagnetic Resonance, and Dynamic Nuclear Polarization[END_REF][START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF]. Copyright 1953, American Physical Society, 1993, AIP Publishing and 2010, American Chemical Society.

In 1983, Wind reported the first DNP-NMR experiments under MAS conditions [START_REF] Wind | Experimental Setup for Enhanced 13 C NMR Spectroscopy in Solids Using Dynamic Nuclear Polarization[END_REF]. MAS technique allows for the acquisition of high-resolution DNP-enhanced NMR spectra of materials, such as coal, diamonds, polymers and organic conductors [START_REF] Wind | Dynamic Nuclear Polarization and High-Resolution NMR of Solids[END_REF]. The sensitivity gain provided by DNP was notably used to probe the interface of an immiscible mixture of polycarbonate and polystyrene [START_REF] Afeworki | Molecular dynamics of polycarbonate chains at the interface of polycarbonate/polystyrene heterogeneous blends[END_REF] and the surface of chemical vapor deposited (CVD) diamond film (see Figure 1b) [START_REF] Lock | A Study of 13C-enriched Chemical Vapor Deposited Diamond Film by Means of 13C Nuclear Magnetic Resonance, Electron Paramagnetic Resonance, and Dynamic Nuclear Polarization[END_REF]. However, these MAS DNP-NMR experiments were constrained to B0 ≤ 1.5 T, i.e. 1 H Larmor frequency ν0( 1 H) ≤ 60 MHz because of the paucity of microwave sources operating above 40-50 GHz.

DNP-enhanced NMR at B 0 ≥ 5 T under MAS conditions was pioneered by the group of Griffin 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 frequency higher than 140 GHz [START_REF] Becerra | Dynamic nuclear polarization with a cyclotron resonance maser at 5 T[END_REF], (ii) the design of cryogenic MAS probes that operate at temperature of 90 K and below [START_REF] Hall | Polarization-Enhanced NMR Spectroscopy of Biomolecules in Frozen Solution[END_REF][START_REF] Rosay | Two-Dimensional 13 C-13 C Correlation Spectroscopy with Magic Angle Spinning and Dynamic Nuclear Polarization[END_REF], and (iii) the discovery of nitroxide biradicals, which generally yield more efficient DNP transfer at high field than monoradicals [START_REF] Hu | Dynamic Nuclear Polarization with Biradicals[END_REF]. The MAS DNP-NMR spectroscopy at high field 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 [START_REF] Bajaj | Functional and Shunt States of Bacteriorhodopsin Resolved by 250 GHz Dynamic Nuclear Polarization-Enhanced Solid-State NMR[END_REF].

The availability of a commercial MAS DNP-NMR system [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF] at B 0 = 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 [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF]. They showed that DNP at 9.4 T yields a 50-fold enhancement of 1 H→ 13 C 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 Figure 1c). Independently our group demonstrated that DNP can yield a 30-fold enhancement of 29 Si signals in direct excitation experiments under MAS of mesoporous silica impregnated with a solution of nitroxide biradicals in a DMSO/water mixture [START_REF] Amoureux | New solid-state NMR methods[END_REF][START_REF] Lafon | Beyond the Silica Surface by Direct Silicon-29 Dynamic Nuclear Polarization[END_REF]. Owing to the availability of commercial MAS DNP-NMR systems, this technique is applied for the characterization of a rapidly increasing number of organic, hybrid and inorganic materials (see Figure 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 [START_REF] Copéret | Active Sites in Supported Single-Site Catalysts: An NMR Perspective[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF][START_REF] Lee | Is solid-state NMR enhanced by dynamic nuclear polarization?[END_REF][START_REF] Kobayashi | Dynamic Nuclear Polarization Solid-State NMR in Heterogeneous Catalysis Research[END_REF][START_REF] Liao | Dynamic Nuclear Polarization Surface Enhanced NMR spectroscopy (DNP SENS): Principles, protocols, and practice[END_REF][START_REF] Rossini | Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Zhao | DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients[END_REF]. 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 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 (B 0 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.

Polarization transfer and global sensitivity

DNP mechanisms and depolarization

In materials, MAS DNP transfers at B 0 ≥ 5 T have been mainly achieved using the solid effect (SE), cross effect (CE) and Overhauser effect (OE). The measurement of the DNP enhancement as function of the B0 field, i.e. the DNP field profile, is often useful to identify the involved DNP mechanisms [START_REF] Ni | High frequency dynamic nuclear polarization[END_REF][START_REF] Can | Mechanisms of dynamic nuclear polarization in insulating solids[END_REF].

The SE mechanism involves an unpaired electron and a nucleus. It dominates when the width of the EPR lines is much smaller than the Larmor frequency of the polarized isotope, 𝜈 ! 𝐼 . Hence, it has been observed for unpaired electrons with symmetric local environments, such as 1,3-bisdi-phenylene-2-phenylallyl (BDPA) [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF] as well as Mn(II), Gd(III) and Cr(III) ions [START_REF] Corzilius | High-Field Dynamic Nuclear Polarization with High-Spin Transition Metal Ions[END_REF][START_REF] Corzilius | Dynamic Nuclear Polarization of 1 H, 13 C, and 59 Co in a Tris(ethylenediamine) cobalt (III) Crystalline Lattice Doped with Cr (III)[END_REF][START_REF] Chakrabarty | Paramagnetic Metal-Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids[END_REF][START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF]. The SE relies on the microwave irradiation of the forbidden zero-quantum or double-quantum transitions at frequencies 𝜈 ! 𝐼 ± 𝜈 ! 𝑆 , where 𝜈 ! 𝑆 denotes the Larmor frequency of the unpaired electron. A difference under MAS with respect to static conditions is that the frequencies of transitions vary with the rotation of the sample, instead of being time-independent. Hence, these transitions under MAS are irradiated periodically, instead of continuously under static conditions [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF][START_REF] Mentink-Vigier | Fast Passage Dynamic Nuclear Polarization on Rotating Solids[END_REF][START_REF] Thurber | On Mechanisms of Dynamic Nuclear Polarization in Solids[END_REF]. Under MAS, the irradiation of these forbidden transitions can be viewed as adiabatic level crossings and hence, the polarization transfer between the unpaired electron and the nucleus is less efficient at higher B 0 field and MAS frequency. Such decrease can be counteracted by increasing the microwave power.

The CE mechanism is often the most efficient one under the conditions of high-field MAS DNP. It involves two unpaired electrons, Sa and Sb, coupled by J-exchange and/or dipolar interactions and a nucleus, I. It is notably observed when the g-tensor anisotropy of one of the unpaired electrons dominates the EPR linewidth and exceeds the Larmor frequency of the polarized nucleus, 𝜈 ! 𝐼 . It has been shown that the CE mechanism under MAS differs from that under static conditions [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF][START_REF] Mentink-Vigier | Fast Passage Dynamic Nuclear Polarization on Rotating Solids[END_REF][START_REF] Thurber | On Mechanisms of Dynamic Nuclear Polarization in Solids[END_REF][START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF]. Under MAS, during the rotor period, the instantaneous resonance frequency of an unpaired electron, 𝜈 ! 𝑆 ! with i = a or b, is moving across the EPR line, which leads to several crossings and anticrossings of energy levels. These crossings and anti-crossings can produce population exchange between the energy levels, provided they are long enough, i.e., adiabatic [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF]. During the rotor period, when the microwave irradiation is resonant with one of the two unpaired electrons, i.e., its instantaneous EPR frequency is equal to the carrier frequency of microwave, this irradiation can create a difference in polarization between the coupled unpaired electrons, which exceeds the nuclear polarization at thermal equilibrium. Such a difference can only be created if microwave irradiates one electron, without perturbing the other one. The exchange and dipolar couplings between the unpaired electrons allow the polarization difference between them to be maintained. The polarization difference between the unpaired electrons is periodically transferred to the nucleus during CE rotor events, occurring when 𝜈 ! 𝑆 ! -𝜈 ! 𝑆 ! ≈ 𝜈 ! 𝐼 . The microwave crossings and CE anti-crossings do not have to occur simultaneously. The difference in polarization between the unpaired electrons decreases for increasing MAS frequency and B0 field since these parameters reduces the adiabaticity of crossings and anti-crossings [START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF]. Nevertheless, the decrease of DNP enhancement at higher fields is slower for MAS CE than for MAS SE [START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF]. The polarization difference between unpaired electrons can be enhanced by increasing the microwave power and the strength of the exchange and dipolar couplings between electrons. Furthermore, larger exchange and dipolar interactions between unpaired electrons lead to more efficient population exchange between them and hence accelerate the build-up of the nuclear polarization [START_REF] Mentink-Vigier | Fast and accurate MAS-DNP simulations of large spin ensembles[END_REF][START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF]. Nevertheless, for efficient CE transfer, the electron-electron exchange and dipolar interactions must remain smaller than 𝜈 ! 𝐼 [START_REF] Mathies | Efficient Dynamic Nuclear Polarization at 800 MHz/527 GHz with Trityl-Nitroxide Biradicals[END_REF]. In addition, larger microwave intensity is needed in the case of shorter electron longitudinal and transverse relaxation times, T1(Si), in order to obtain the same difference in the polarization of unpaired electrons [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF][START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF].

Furthermore, it has been shown that MAS-DNP simulations are accurate enough to reproduce the DNP field profile of bis-nitroxide PAs (see Figure 4) using as input parameters (i) the g-tensors, (ii) their relative orientation, (iii) the electron-electron dipolar coupling, (iv) the J-exchange interaction, (v) the hyperfine coupling and (vi) the electron relaxation times, which were determined using DFT calculations and high-field EPR measurements, as well as (vii) the nuclear relaxation times, (viii) the MAS frequency and (ix) the microwave frequency [START_REF] Mentink-Vigier | De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization[END_REF]. The ratio between the positive maximum and the negative minimum of the DNP field profile can only be reproduced correctly, when taking into account the dependence of the electron longitudinal relaxation time with the PA orientation [START_REF] Mentink-Vigier | De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization[END_REF][START_REF] Mao | Host-Guest Complexes as Water-Soluble High-Performance DNP Polarizing Agents[END_REF]. Conversely the nuclear relaxation times and the hyperfine coupling values have, within reasonable limits, little effect on the shape of the DNP field profile, but they strongly affect the absolute DNP enhancement. Simulations of the build-up of DNP-enhanced 1 H magnetization have been used to estimate the longitudinal relaxation time of protons close to the PA. Long nuclear longitudinal relaxation time, T1(I), is needed so that the steady state nuclear polarization can become equal to the difference of polarization between the unpaired electrons. As consequence, the presence in the materials of paramagnetic centers, which do not contribute to the DNP transfer but shorten the T1(Si) and T1(I) values, decreases or even cancels the DNP enhancement. This polarization leakage phenomenon has been reported for materials, containing endogeneous paramagnetic centers, such as natural clays and functional nanodiamonds [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF][START_REF] Presti | NMR and EPR Characterization of Functionalized Nanodiamonds[END_REF]. Similarly it has been shown that the removal of the paramagnetic molecular oxygen (O2) dissolved in organic solvents or adsorbed in polymers can improve the DNP enhancement [START_REF] Kubicki | Amplifying dynamic nuclear polarization of frozen solutions by incorporating dielectric particles[END_REF][START_REF] Le | Up to 100% Improvement in Dynamic Nuclear Polarization Solid-State NMR Sensitivity Enhancement of Polymers by Removing Oxygen[END_REF]. It has also been demonstrated that the passivation of the surface of mesoporous silica by methyl or OSi(CH3)3 groups strongly reduces the T1( 1 H) time and the DNP enhancement [START_REF] Zagdoun | Improved Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy through Controlled Incorporation of Deuterated Functional Groups[END_REF]. The replacement of the fast-relaxing protons by deuterons however mitigates this effect.

In the absence of microwave irradiation, the CE anti-crossings can also reduce the nuclear polarization, which can then be lower than its value at thermal equilibrium. This depolarization phenomenon has been demonstrated theoretically and experimentally [START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF][START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF]. It has been shown that the magnitude of the depolarization depends on the PA and in particular, the longer T1(Si) times lead to stronger nuclear depolarization. A consequence of the depolarization is that the DNP enhancement factor, εon/off, estimated as the ratio between signal intensities with and without microwave irradiation, does not provide an accurate measure of the polarization enhancement with respect to thermal equilibrium.

Another DNP mechanism which has been employed for high-field MAS DNP of materials is the Overhauser effect (OE) [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Can | Overhauser effects in insulating solids[END_REF][START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF][START_REF] Brownbill | Oxygen-17 dynamic nuclear polarisation enhanced solid-state NMR spectroscopy at 18.8 T[END_REF]. In OE, the EPR transition is saturated by microwave irradiation, while the stochastic modulation of the electron-nuclear interactions leads to cross-relaxation, which enhances the nuclear polarization [START_REF] Lilly Thankamony | Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR[END_REF]. Recently positive OE has been reported in materials containing 1,3-bisdi-phenylene-2-phenylallyl (BDPA) radicals [START_REF] Can | Overhauser effects in insulating solids[END_REF]. Furthermore, the DNP enhancement of protons in those materials increases with increasing B 0 strength. Hence, OE has been mainly applied at B 0 = 18.8 T [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF][START_REF] Brownbill | Oxygen-17 dynamic nuclear polarisation enhanced solid-state NMR spectroscopy at 18.8 T[END_REF]. The origin of the stochastic modulation of the electronnuclear interactions for BDPA radicals incorporated into solids is still under debate [START_REF] Pylaeva | Molecular Mechanism of Overhauser Dynamic Nuclear Polarization in Insulating Solids[END_REF][START_REF] Ji | Overhauser effects in nonconducting solids at 1.2 K[END_REF].

Spin diffusion

The enhancement of nuclear polarization by DNP also often relies on spin diffusion, which transfers the enhanced polarization of I-spin nuclei in the vicinity of the PAs to the remote identical nuclei through the homonuclear dipolar interactions. It has been shown that I nuclei subject to hyperfine interactions can exchange polarization under MAS during homonuclear dipolar rotor events, where their resonance frequencies are equal [START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF][START_REF] Mentink-Vigier | Fast and accurate MAS-DNP simulations of large spin ensembles[END_REF]. This model suggests the absence of a spin diffusion barrier under MAS, contrary to the static case. The characteristic length of spin diffusion is equal to

𝑙 ! = 𝐷𝑇 ! 𝐼 (1)
where D is the spin diffusion coefficient [START_REF] Afeworki | Mechanism of DNP-Enhanced Polarization Transfer across the Interface of Polycarbonate/Polystyrene Heterogeneous Blends[END_REF][START_REF] Rossini | Dynamic nuclear polarization NMR spectroscopy of microcrystalline solids[END_REF][START_REF] Lafon | Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13 C and 29 Si NMR Enhanced by Dynamic Nuclear Polarization[END_REF][START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF]. This coefficient increases with increasing homonuclear dipolar coupling between I nuclei. Hence, lD length increases with larger homonuclear dipolar coupling and slower longitudinal relaxation. For instance, spin diffusion is often more efficient at low temperature, which increases the rigidity of the materials. For protons in organic materials, the lD values typically range from 10 nm to 1 μm at about 100 K, depending on the T1( 1 H) value, which can reach 10 3 s for organic crystals of molecules, which do not possess methyl groups [START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF]. 1 H spin diffusion is always involved in experiments using DNP-enhanced 1 H polarization. Nevertheless, it is especially useful for materials, for which the PAs cannot be brought within a few nanometers of the nuclei to polarize. For instance, 1 H spin diffusion has been employed in microcrystalline solids, including pharmaceuticals, to transport DNP-enhanced polarization from the surface into the interior of the crystals [START_REF] Zhao | DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients[END_REF][START_REF] Rossini | Dynamic nuclear polarization NMR spectroscopy of microcrystalline solids[END_REF][START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF]. 1 H spin diffusion transport has also been applied to non-porous inorganic solids, such as magnesium and calcium hydroxide [START_REF] Blanc | Dynamic Nuclear Polarization Enhanced Natural Abundance 17 O Spectroscopy[END_REF], hydroxyapatite [START_REF] Leroy | Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization[END_REF][START_REF] Lee | Interfacial Ca 2+ environments in nanocrystalline apatites revealed by dynamic nuclear polarization enhanced 43 Ca NMR spectroscopy[END_REF], ammonia borane [START_REF] Kobayashi | Mechanism of Solid-State Thermolysis of Ammonia Borane: A 15 N NMR Study Using Fast Magic-Angle Spinning and Dynamic Nuclear Polarization[END_REF] or proton conductors [START_REF] Blanc | Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3[END_REF]. It has also been employed to porous materials, including mesoporous silica [START_REF] Lafon | Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13 C and 29 Si NMR Enhanced by Dynamic Nuclear Polarization[END_REF][START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF], Metal-Organic Frameworks (MOFs) [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy of Functionalized Metal-Organic Frameworks[END_REF][START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF][START_REF] Guo | Selective Host-Guest Interaction between Metal Ions and Metal-Organic Frameworks Using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF] and zeolites [START_REF] Wolf | NMR signatures of the active sites in Sn-β zeolite[END_REF][START_REF] Gunther | Dynamic Nuclear Polarization NMR Enables the Analysis of Sn-Beta Zeolite Prepared with Natural Abundance 119Sn Precursors[END_REF][START_REF] Wolf | Correlating Synthetic Methods, Morphology, Atomic-Level Structure, and Catalytic Activity of Sn-β Catalysts[END_REF][START_REF] Xiao | Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR[END_REF], for which the PAs are too large to diffuse into the pores. The exclusion of the PAs from the pore can prevent their reaction with the catalytic active sites located within the pores [START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF][START_REF] Wolf | NMR signatures of the active sites in Sn-β zeolite[END_REF][START_REF] Gunther | Dynamic Nuclear Polarization NMR Enables the Analysis of Sn-Beta Zeolite Prepared with Natural Abundance 119Sn Precursors[END_REF][START_REF] Wolf | Correlating Synthetic Methods, Morphology, Atomic-Level Structure, and Catalytic Activity of Sn-β Catalysts[END_REF][START_REF] Xiao | Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR[END_REF]. In the case of aggregated silica nanoparticles functionalized with reactive species, it has also been shown that spin diffusion transports DNP-enhanced 1 H polarization in the interparticle spaces, whereas PAs are excluded from those spaces and hence are not in contact with reactive surface species [START_REF] Pump | Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity[END_REF]. DNP measurements combined with spin diffusion models have also been used to determine the domain size in materials, such as organic microcrystals, mesoporous silica and polymer coatings (see Figure 5) [START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF]. Recently it has also been shown that spin diffusion between spin-1/2 nuclei, other than protons, such as 29 Si, 119 Sn and 113 Cd, at natural abundance contributes to transport the DNP-enhanced polarization from the surface to the interior of inorganic microcrystals [START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF].

When the size, L, of the domain polarized by a PA is small with respect to lD length, spin diffusion is fast enough to equalize the polarization of all nuclei in the domain. A mono-exponential build-up is then observed (β = 1 in Eq. 2) and the polarization buildup time constant, TB, is identical with and without microwave irradiation (see Figure 5b) [START_REF] Mentink-Vigier | Fast and accurate MAS-DNP simulations of large spin ensembles[END_REF][START_REF] Rossini | Dynamic nuclear polarization NMR spectroscopy of microcrystalline solids[END_REF][START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF]. Hence, as seen in Figure 5c, the DNP enhancement depends on the polarization delay. Conversely when L >> lD, the polarization of nuclei closer to the PAs increases faster than that of the remote ones, yielding faster polarization with microwave irradiation than without (see Figure 5d) [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF][START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF]. As a consequence, the polarization build-up cannot be fitted using a single exponential function but follows a stretched exponential build-up of the form:

𝐼 = 𝐼 ! 1 -exp - ! ! ! ! (2)
with β < 1, where I∞ is the asymptotic intensity for t ≫TB. Furthermore, the TB time constant increases with increasing L/lD ratio and the DNP enhancement depends on the polarization delay (see Figure 5e) [START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF]. In subfigure e, an inset shows an expansion of the region corresponding to short polarization delays. In subfigures d and e, the best fit curves to the numerical spin diffusion model are displayed as solid lines and corresponds to L = 2.3 μm, which agrees well with the histidine particle size measured by scanning electron microscopy. Figure adapted from ref. [START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF]. Copyright 2017, American Chemical Society.

Global sensitivity

Besides DNP transfer, depolarization and spin diffusion, several additional factors affect the sensitivity of MAS DNP NMR experiments. These factors depend on the sample (properties of PAs, preparation of the sample…) as well as the DNP NMR system hardware (B0 field, MAS frequency, microwave power in the sample, sample temperature…). The unpaired electrons of the PAs also reduce the apparent transverse relaxation of nearby nuclei, leading to line broadening, known as 'paramagnetic quenching', since it prevents the observation of nuclei distant by less than a few Angstroms from the unpaired electrons [START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF][START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF][START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Kobayashi | Analysis of Sensitivity Enhancement by Dynamic Nuclear Polarization in Solid-State NMR: a Case Study of Functionalized Mesoporous Materials[END_REF]. This quenching has been found to be approximately equal to 15% for a concentration of unpaired electrons of 24 mM and to be independent of the MAS frequency over an interval of 0 to 10 kHz [START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF]. In principle, high MAS frequencies would better average the hyperfine interaction and reduce the paramagnetic quenching. However, such effects have not been reported so far. The unpaired electrons also shorten the lifetime of coherences, which further reduces the sensitivity of NMR experiments employing several delays and pulses [START_REF] Takahashi | Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments[END_REF]. Conversely the unpaired electrons accelerates the nuclear longitudinal relaxation, allowing for shorter longitudinal relaxation delay and hence, enhancing the sensitivity.

As described in section 3, the DNP-NMR samples are often prepared by impregnating or wetting the investigated material with a solution of radicals. Besides the radicals, the presence of the frozen solvent also alters the sensitivity of DNP-NMR experiments. It can (i) dilute the amount of the material of interest in the rotor, (ii) broaden the NMR lines because of the distribution of local environment in the bulk and at the surface of the frozen solvent, (iii) modify the efficiency of the NMR experiments, such as CPMAS, and (iv) change the nuclear relaxation times [START_REF] Lee | Is solid-state NMR enhanced by dynamic nuclear polarization?[END_REF][START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Kobayashi | Analysis of Sensitivity Enhancement by Dynamic Nuclear Polarization in Solid-State NMR: a Case Study of Functionalized Mesoporous Materials[END_REF][START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF].

One approach to estimate the global sensitivity of a DNP-NMR experiment consists of comparing its sensitivity, i.e., its signal-to-noise ratio per unit of time, with that of conventional NMR experiments [START_REF] Lee | Is solid-state NMR enhanced by dynamic nuclear polarization?[END_REF][START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Kobayashi | Analysis of Sensitivity Enhancement by Dynamic Nuclear Polarization in Solid-State NMR: a Case Study of Functionalized Mesoporous Materials[END_REF][START_REF] Takahashi | Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments[END_REF]. However, this approach is only applicable when NMR signals can be detected using conventional NMR experiments.

Polarizing agents and sample preparation

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 Figure 6. All these PAs exhibit an EPR transition at g ≈ 2, since the gyrotron can only deliver microwaves at a frequency which corresponds to that of nitroxide radicals.

Most of high field MAS DNP-NMR experiments on materials have been carried out using exogeneous nitroxide biradicals. These radicals have been designed to provide significant MAS CE at high field. For these PAs, the covalent link between the two nitroxide radicals leads to significant dipolar coupling between unpaired electrons, even at low concentration of PAs, which limits the paramagnetic quenching. The first nitroxide biradical, which was used for high-field MAS DNP of materials, is TOTAPOL [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Song | TOTAPOL: A Biradical Polarizing Agent for Dynamic Nuclear Polarization Experiments in Aqueous Media[END_REF]. It has been shown that the TEMPO rings of TOTAPOL are approximately perpendicular to one another [START_REF] Hu | High-frequency dynamic nuclear polarization using biradicals: A multifrequency EPR lineshape analysis[END_REF]. This favorable orientation results in many CE anticrossings during the rotor period, which facilitates the DNP transfer [START_REF] Mentink-Vigier | Fast Passage Dynamic Nuclear Polarization on Rotating Solids[END_REF]. TOTAPOL is soluble in water as well as in glass-forming water/glycerol and water/DMSO mixtures.

For the study of water-incompatible materials, the first exogeneous nitroxide biradical, which has been used, is bTbk, which is soluble in halogenated solvents [START_REF] Matsuki | Dynamic Nuclear Polarization with a Rigid Biradical[END_REF][START_REF] Zagdoun | Non-aqueous solvents for DNP surface enhanced NMR spectroscopy[END_REF]. In bTbk, a rigid tether constrains the relative orientation of the TEMPO moieties, which are roughly orthogonal. Such constrained relative orientation favors the CE and leads to DNP enhancements, which are 1.4 times larger than those obtained with TOTAPOL under the same conditions (in DMSO/water solvent) [START_REF] Matsuki | Dynamic Nuclear Polarization with a Rigid Biradical[END_REF]. Further improvements consisted of slowing the longitudinal and transverse relaxation of the unpaired electrons, which permits a given DNP enhancement for a lower microwave power to be obtained [START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF]. The replacement of the methyl groups by a cyclohexyl ring lengthens the transverse relaxation time (also called phase memory time). This approach was used to design the bCTbk radical (see Figure 6) [START_REF] Zagdoun | A slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF], which is soluble in halogenated solvents and leads to sensitivity enhancements, which are 2.5 times larger than those of bTbk. A further refinement was to increase the molecular weight of the biradical in order to lengthen the longitudinal relaxation time of the electron since the relaxation via the Raman process, which is the dominant mechanism under DNP conditions used for materials, is less efficient for heavier radicals. This strategy led to the design of TEKPol (see Figure 6) and TEKPol2 [START_REF] Zagdoun | Improved Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy through Controlled Incorporation of Deuterated Functional Groups[END_REF][START_REF] Kubicki | Rational design of dinitroxide biradicals for efficient cross-effect dynamic nuclear polarization[END_REF], which are soluble in halogenated solvents and result in sensitivity gains about 3.3 times larger than those of bTbk. A similar strategy was used to design a water-soluble nitroxide biradical, AMUPol, which resulted in DNP enhancements 3.3 times larger than TOTAPOL [START_REF] Sauvée | Highly Efficient, Water-Soluble Polarizing Agents for Dynamic Nuclear Polarization at High Frequency[END_REF]. Nevertheless, the slower electron relaxation for AMUPol than TOTAPol results in a stronger depolarization since polarization losses of 20% and 60% were measured for TOTAPol and AMUPol, respectively [START_REF] Mentink-Vigier | Nuclear depolarization and absolute sensitivity in magic-angle spinning cross effect dynamic nuclear polarization[END_REF]. AMUPol and TEKPol are commercially available, which explains their wide use in high-field MAS DNP.

In order to limit the depolarization, the water-soluble AsymPolPOK nitroxide biradical was designed [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF]. In such radical, the nitroxide moieties are tethered by a short linker, which leads to sizable exchange and dipolar interactions between the unpaired electrons. These large electron-electron coupling do not only limit the depolarization but also accelerate the build-up of nuclear polarization. At both 9.4 and 18.8 T and temperature of about 100-130 K with MAS frequencies around 8-10 kHz, AsymPolPOK in a glycerol/water mixture yields sensitivity enhancement approximately two-fold higher than AMUPol. AsymPolPOK is currently one of the most efficient watersoluble PAs at 9.4 T and 100 K.

Besides depolarization, the introduction of PAs can have other deleterious effects. When the PAs are close to the observed nuclei, they can produce paramagnetic quenching and reduce the coherence lifetime, which results in enhanced losses in NMR experiments or even react with the observed sites of the material. This limitation has been circumvented by the development of carbosilane-based dendritic nitroxide biradicals, in which the bulky dendrimer attached to the biradical reduces the interaction of the latter with the investigated material [START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF]. The reaction between reactive surface species and nitroxide biradicals can also be prevented by protecting those species inside pores with entrances smaller than the radical size [START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF] or inside the spaces between aggregated nanoparticles [START_REF] Pump | Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity[END_REF].

BDPA dissolved in glass-forming ortho-terphenyl (OTP) solvent has been employed for DNP via the SE and OE [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF]. For increasing B 0 field, the DNP enhancement decreases strongly for SE, whereas it increases slightly for OE [START_REF] Can | Overhauser effects in insulating solids[END_REF]. Nevertheless, the sensitivity enhancement for BDPA is generally smaller than that of TEKPol under similar experimental conditions because the build-up time of the nuclear polarization is by one order of magnitude slower for OE with BDPA than for CE with TEKPol. The DNP enhancement yielded by nitroxide biradicals drastically decreases at higher field. This decrease is related to the broadening of the EPR lines of these biradicals, which are dominated by the g-tensor anisotropy and hence, are proportional to the B 0 field. Such broadening results in faster frequency sweeps during the crossings and the anti-crossings and hence, lower adiabaticity for the population exchange. A strategy to limit the decrease of DNP enhancement at high-field would involve the use of biradicals with narrower EPR lines than nitroxide. Nevertheless, the CE transfer requires that the two electrons of the biradical fulfill the condition ν ! 𝑆 ! -ν ! 𝑆 ! ≈ ν ! H ! during the CE rotor-events (see subsection 2.1). This approach has lead to the development of the water-soluble TEMTriPol-1 biradical composed of a narrow-line trityl radical tethered to a broad-line TEMPO [START_REF] Mentink-Vigier | Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR[END_REF]. This narrow-line trityl radical results in more adiabatic crossings and anti-crossings and so requires notably less microwave power. The difference between the isotropic g values of these two unpaired electrons corresponds approximately to the 1 H Larmor frequency. Hence, an efficient intramolecular CE transfer can be achieved between the trityl and TEMPO unpaired electrons, when the trityl EPR transition is irradiated. Furthermore, TEMTriPol-1 does not induce any depolarization since (i) in the absence of microwave irradiation, the polarization difference between the trityl and TEMPO radicals is approximately equal to the 1 H polarization at thermal equilibrium and (ii) the large exchange interaction, |J| = 73 MHz, between its two unpaired electrons leads to efficient polarization exchange between them during the electron-electron dipolar anti-crossings [START_REF] Mentink-Vigier | Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR[END_REF]. TEMTriPol-1 is currently the most efficient water-soluble PA at 18.8 T and 100 K.

A similar approach was latter applied to develop the HyTEK2 biradical, which is composed of a narrow-line BDPA and a broad-line TEMPO and is soluble in halogenated solvents [START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF]. Here again, the isotropic g values of the two unpaired electrons differ approximately by the 1 H Larmor frequency. Furthermore, BDPA benefits from a slower electron longitudinal relaxation and a narrower EPR line than trityl, and so requires lower microwave power. HyTEK2 also exhibits a significant exchange interaction, |J| ≈ 40 MHz, which limits the depolarization. Furthermore, the nitroxide moiety of HyTEK2 is substituted by cyclohexyl rings with phenyl substituents in order to slow the nitroxide electron relaxation. The HyTEK2 biradical is currently the most efficient PA in halogenated solvents at 18.8 T and 100 K.

The biradicals presented above were all optimized for the CE polarization transfer between unpaired electrons and protons. As the CE condition 𝜈 ! 𝑆 ! -𝜈 ! 𝑆 ! ≈ 𝜈 ! 𝐼 depends on the Larmor frequency of the polarized nucleus, I, these PAs are not necessarily optimal for the direct CE of other isotopes. The width of the EPR line of the PAs should be lower for decreasing γ values. For instance, it has been shown by the Griffin research group that trityl radicals yield 4-fold larger DNP enhancement for the direct CE of 2 H at 5 T and 90 K with νR = 6 kHz than TOTAPOL with the same concentration of unpaired electrons [START_REF] Maly | 2 H-DNP-enhanced 2 H-13 C solid-state {NMR} correlation spectroscopy[END_REF]. Similarly 15-fold larger enhancements were measured for the direct CE of 17 O using trityl instead of TOTAPOL at 5 T and 90 K under static conditions [START_REF] Michaelis | Dynamic Nuclear Polarization of 17O: Direct Polarization[END_REF]. Nevertheless, at 9.4 T and 100 K with νR = 12.5 kHz, a 4-fold larger sensitivity gain was achieved for 17 O CE of mesoporous silica impregnated with a TEKPOL solution in TCE, instead of aqueous Finland trityl solution [START_REF] Perras | Direct 17 O dynamic nuclear polarization of single-site heterogeneous catalysts[END_REF].

A limitation of the above biradical PAs is that they cannot be easily been incorporated into the bulk of inorganic materials. For such materials and in the absence of an efficient spin diffusion transport, DNP-NMR using these PAs only enhances the polarization of nuclei near the surface. It has been shown that this limitation can be circumvented by doping inorganic or hybrid materials with paramagnetic metal ions (see Figure 25) [START_REF] Corzilius | Dynamic Nuclear Polarization of 1 H, 13 C, and 59 Co in a Tris(ethylenediamine) cobalt (III) Crystalline Lattice Doped with Cr (III)[END_REF][START_REF] Chakrabarty | Paramagnetic Metal-Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids[END_REF][START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF]. Nevertheless, these ions must exhibit a narrow resonance centered at g ≈ 2 so that they can be efficiently irradiated by the microwaves with fixed frequency delivered by the gyrotron. This requirement prohibits the use of ions subject to significant spin-orbit coupling, such as Cu(II) or Fe(III). So far only three paramagnetic metal ions have yielded significant enhancement in high-field MAS DNP experiments: Cr(III), Mn(II) and Gd(III) ions, which have electronic configurations 3d 3 , 3d 5 and 4f 7 , respectively, and ground states with high half-integer-spin S = 3/2, 5/2 and 7/2, respectively. The EPR spectra of Cr(III) and Gd(III) ions display a narrow central transition (CT) between the mS = ±1/2 energy levels at g ≈ 2 and broad satellite transitions (STs) between energy levels mS and mS + 1 with | mS | ≤ S and mS ≠ -1/2. The broadening of the STs results from the first-order Zero-Field Splitting (ZFS) related to the electron-electron dipolar interactions, whereas the CT is only broadened by the second-order ZFS. In the case of Mn(II) ions, the CT consists of a sextet centered at g ≈ 2 owing to the hyperfine coupling with 55 Mn nucleus with spin I = 5/2. For these ions, the dominant DNP mechanism is the SE by irradiation of the CT. Nevertheless, a contribution of CE involving the electrons of STs has also been suggested [START_REF] Lilly Thankamony | Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR[END_REF][START_REF] Kaushik | Gd(III) and Mn(II) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins[END_REF][START_REF] Corzilius | Theory of solid effect and cross effect dynamic nuclear polarization with half-integer high-spin metal polarizing agents in rotating solids[END_REF].

Besides the above exogeneous PAs, which can be introduced into the investigated diamagnetic material, unpaired electrons of paramagnetic materials have also been used as endogeneous PAs. This approach has been used for (i) mesoporous silica functionalized with TEMPO, which is used as a selective oxidation heterogeneous catalysts [START_REF] Lilly Thankamony | Solvent-Free High-Field Dynamic Nuclear Polarization of Mesoporous Silica Functionalized with TEMPO[END_REF], (ii) thermally carbonized mesoporous silica containing carbon dangling bonds [START_REF] Riikonen | Endogenous Stable Radicals for Characterization of Thermally Carbonized Porous Silicon by Solid-State Dynamic Nuclear Polarization 13C NMR[END_REF] and (iii) diamond microcrystals and nanoparticles with a diameter ranging from 45 to 80 nm [START_REF] Bretschneider | On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid-State and Dissolution 13C NMR Spectroscopy[END_REF].

Sample preparation

The DNP-NMR of diamagnetic materials requires the introduction of PAs in the sample, without diluting the sample, in order to preserve the sensitivity. For instance, PAs have been introduced into porous materials using incipient wetness impregnation [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF]. In that technique, the volume of radical solution is just sufficient to fill the pore (see Figure 7a). A similar incipient wetness wetting technique has been used for particulate materials. The volume of radical solution is just enough to uniformly wet the surface of the particles. For colloids, including sols, i.e. colloidal suspensions of solid particles in a liquid, and gels, i.e. a solid three-dimensional network expanded by a liquid, the PA can be dissolved into the liquid phase [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF][START_REF] Protesescu | Unraveling the Core-Shell Structure of Ligand-Capped Sn/SnOx Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mössbauer, and X-ray absorption spectroscopies[END_REF].

For these techniques, the choice of the solvent can strongly affect the DNP enhancement. The employed solvent should fulfill the following criteria: (i) it should dissolve the PA, (ii) it should wet the investigated material, (iii) it should be inert towards both the radical and the material, (iv) the number of magnetically inequivalent nuclei in the solvent must be low in order to avoid masking the spectrum of the material, (v) the solvent must be frozen but non-crystalline at temperature of DNP experiments, (vi) the solvent molecules must be rigid at that temperature and (vi) the concentration of protons in the solvent must be low [START_REF] Rossini | Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Zagdoun | Non-aqueous solvents for DNP surface enhanced NMR spectroscopy[END_REF]. The crystallization of the solvent must be avoided since it leads to aggregation of the PAs along the grain boundary, which decreases the DNP enhancement [START_REF] Barnes | High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR[END_REF]. In practice, crystallization can be avoided by (i) the use of glass-forming solvents, such as water/glycerol and water/DMSO mixtures [START_REF] Hu | Dynamic Nuclear Polarization with Biradicals[END_REF][START_REF] Hu | High-frequency dynamic nuclear polarization using mixtures of TEMPO and trityl radicals[END_REF][START_REF] Rosay | Sensitivity-enhanced nuclear magnetic resonance of biological solids[END_REF][START_REF] Baudot | Glass-Forming Tendency in the System Water-Dimethyl Sulfoxide[END_REF], (ii) the confinement of the solvent into micro-or meso-pores, which prevents the formation of large crystalline domains [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Morishige | The proper structure of cubic ice confined in mesopores[END_REF], and (iii) a rapid freezing of the solvent [START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K[END_REF]. Similarly the aggregation and the precipitation of nanoparticles upon the freezing of sols can result in the segregation between aggregated nanoparticles and PAs [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF]. The aggregation is governed by the steric hindrance of surface groups and their affinity for solvent molecules [START_REF] Pump | Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity[END_REF]. Such aggregation can be prevented by (i) the impregnation of mesoporous silica with a sol, so that PAs and nanoparticles are trapped in the same mesopore and cannot segregate [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF][START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF] and (ii) the dispersion of the nanoparticle into a gel medium, instead of a liquid phase [START_REF] Viger-Gravel | Frozen Acrylamide Gels as Dynamic Nuclear Polarization Matrices[END_REF].

It has been shown that the partial deuteration of the solvent generally increases the DNP enhancement [START_REF] Rosay | Sensitivity-enhanced nuclear magnetic resonance of biological solids[END_REF] since (i) for partially deuterated solvents, the electron polarization is transferred to a reduced number of protons and a larger fraction of them is polarized and (ii) the reduced number of protons slow the electron and nuclear relaxation, which increases the efficiency of the DNP transfer and spin diffusion. For the direct polarization of isotopes other than proton, the total deuteration of the solvent can be advantageous in order to limit the transfer of polarization to the protons [START_REF] Perras | Direct 17 O dynamic nuclear polarization of single-site heterogeneous catalysts[END_REF]. Furthermore, the deuteration of the solvent reduces the intensity of the solvent signal in DNP-NMR experiments, in which the DNP-enhanced proton polarization is transferred to the detected isotope [START_REF] Le | Optimizing sample preparation methods for dynamic nuclear polarization solid-state NMR of synthetic polymers[END_REF][START_REF] Kobayashi | Improved strategies for DNP-enhanced 2D 1 H-X heteronuclear correlation spectroscopy of surfaces[END_REF]. Of course, the optimal concentration of protons depends on the PA, the involved DNP mechanism [START_REF] Corzilius | Solid effect in magic angle spinning dynamic nuclear polarization[END_REF] and the concentration of protons in the investigated sample [START_REF] Kobayashi | Improved strategies for DNP-enhanced 2D 1 H-X heteronuclear correlation spectroscopy of surfaces[END_REF]. Typically the concentration of protons in the solvent must be in order of 10 mol.L -1 [START_REF] Rossini | Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy[END_REF]. For water-soluble PAs, such as AMUPol, [ 2 H8]glycerol/ 2 H2O/H2O (60/30/10 v/v/v) or [ 2 H6]-DMSO/ 2 H2O/H2O (80/10/10 v/v/v) solvents are typically used. For radicals soluble in organic solvents, such as TEKPol, the most widely used solvent is isotopically unmodified TCE, which has a 1 H concentration of 20 mol.L -1 [START_REF] Rossini | Materials Characterization by Dynamic Nuclear Polarization-Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Zagdoun | Non-aqueous solvents for DNP surface enhanced NMR spectroscopy[END_REF]. However, other solvents, such as [ H14]-OTP/OTP have been used [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF][START_REF] Brownbill | Oxygen-17 dynamic nuclear polarisation enhanced solid-state NMR spectroscopy at 18.8 T[END_REF][START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF][START_REF] Zagdoun | Non-aqueous solvents for DNP surface enhanced NMR spectroscopy[END_REF][START_REF] Perras | Direct 17 O dynamic nuclear polarization of single-site heterogeneous catalysts[END_REF][START_REF] Le | Optimizing sample preparation methods for dynamic nuclear polarization solid-state NMR of synthetic polymers[END_REF][START_REF] Ong | Solvent-Free Dynamic Nuclear Polarization of Amorphous and Crystalline Ortho-Terphenyl[END_REF][START_REF] Leskes | Surface-Sensitive NMR Detection of the Solid Electrolyte Interphase Layer on Reduced Graphene Oxide[END_REF].

However, the impregnation or wetting of the material of interest with a radical solution has drawbacks. The radical solution can modify the structure and the dynamics of the sample. For instance, it has been shown that for some forms of asthma drug molecule theophylline, sample grinding and impregnation with radical solution induce polymorphic transitions and desolvation [START_REF] Pinon | Polymorphs of Theophylline Characterized by DNP Enhanced Solid-State NMR[END_REF]. The addition of radical solution can also (i) dilute the investigated material, (ii) reduce the effective amount of sample in the rotor and (iii) broaden the NMR signals owing to the distribution of local environments in the frozen sample and at its surface. The DNP-enhanced signals of the solvent, notably for 1 H and 13 C, can also mask signals from the material of interest [START_REF] Lee | Solvent signal suppression for high-resolution MAS-DNP[END_REF][START_REF] Yarava | Solvent suppression in DNP enhanced solid state NMR[END_REF].

To circumvent the aforementioned issues, techniques have been developed to study materials using DNP-NMR without the presence of a frozen solvent. The first solventfree approach for MAS DNP was polymer solution casting, in which the polymer was dissolved in radical solution and subsequently the solvent was evaporated [START_REF] Le | Optimizing sample preparation methods for dynamic nuclear polarization solid-state NMR of synthetic polymers[END_REF][START_REF] Afeworki | Selective observation of the interface of heterogeneous polycarbonate/polystyrene blends by dynamic nuclear polarization carbon-13 NMR spectroscopy[END_REF][START_REF] Ouari | Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy[END_REF]. It has been shown that this method produces narrower lines than the conventional impregnation with radical solution for the study of semi-crystalline polymers [START_REF] Le | Optimizing sample preparation methods for dynamic nuclear polarization solid-state NMR of synthetic polymers[END_REF]. However, for polymer solution casting, the slow evaporation of the solvent may cause the exclusion of the radicals from the polymer crystalline domains, which results in lower sensitivity than for impregnation methods.

Another solvent-free approach involves the mixing the material of interest with a radical solution, which does not solubilize the material, followed by the evaporation of the solvent [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Kobayashi | Improved strategies for DNP-enhanced 2D 1 H-X heteronuclear correlation spectroscopy of surfaces[END_REF]. The mixing with the radical solution can consist of the preparation of a suspension for nanoparticles [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF] or the impregnation for porous materials [START_REF] Kobayashi | Improved strategies for DNP-enhanced 2D 1 H-X heteronuclear correlation spectroscopy of surfaces[END_REF]. This method is particularly efficient for materials on which the PA adsorbs because this adsorption prevents the aggregation of the PA during the evaporation of the solvent (see Figure 7b andd) [START_REF] Lee | Is solid-state NMR enhanced by dynamic nuclear polarization?[END_REF]. In particular, it has been shown that nitroxide biradicals adsorb on the surface of cellulose, silica or alumina surfaces via hydrogen bonding [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Perras | Optimal sample formulations for DNP SENS: The importance of radical-surface interactions[END_REF].

Nitroxide radicals have also been incorporated into organic materials using solventfree preparation methods, such as co-precipitation [START_REF] Takahashi | Towards Structure Determination of Self-Assembled Peptides using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF], spray drying and hot-melt extrusion [START_REF] Ong | Solvent-Free Dynamic Nuclear Polarization of Amorphous and Crystalline Ortho-Terphenyl[END_REF][START_REF] Ni | In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR[END_REF]. In spray drying, a solution containing the investigated molecules and the radical is sprayed against a flow of warm air, which rapidly evaporates the solvent and yields a powder. In hot-melt extrusion, the solid constituents are mechanically mixed at elevated temperature. Paramagnetic metal ions have been incorporated into the inorganic solids using co-crystallization [START_REF] Corzilius | Dynamic Nuclear Polarization of 1 H, 13 C, and 59 Co in a Tris(ethylenediamine) cobalt (III) Crystalline Lattice Doped with Cr (III)[END_REF] or via solid-state reactions [START_REF] Chakrabarty | Paramagnetic Metal-Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids[END_REF][START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF].

Another important parameter is the concentration of the PA, [PA]. For low [PA], the sensitivity increases with increasing [PA] since (i) each unpaired electron has to polarize a lower number of nuclei and (ii) the build-up of the DNP-enhanced nuclear polarization is faster. However, paramagnetic quenching and the shortening of coherence lifetimes, which produce losses during the pulse sequence, result in a decrease of sensitivity with increasing [PA]. Furthermore, intermolecular electronelectron dipolar couplings tend to equalize the electron polarization through the EPR line [START_REF] Mentink-Vigier | Fast and accurate MAS-DNP simulations of large spin ensembles[END_REF][START_REF] Thurber | Perturbation of nuclear spin polarizations in solid state NMR of nitroxide-doped samples by magic-angle spinning without microwaves[END_REF]. Therefore, there is an optimal concentration, which depends on the nature of PA, the investigated material and the employed pulse sequence [START_REF] Takahashi | Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments[END_REF][START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF][START_REF] Perras | Optimal sample formulations for DNP SENS: The importance of radical-surface interactions[END_REF]. Materials with greater surface areas require higher [PA], when the PAs adsorb on the surface of the material (see Figure 8) [START_REF] Perras | Optimal sample formulations for DNP SENS: The importance of radical-surface interactions[END_REF]. In addition, low [PA] is preferred for long polarization transfer [START_REF] Takahashi | Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments[END_REF]. In the case of material impregnation with nitroxide biradicals, the [PA] producing highest sensitivity can range from a few to 30 mmol.L -1 [START_REF] Perras | Optimal sample formulations for DNP SENS: The importance of radical-surface interactions[END_REF]. Most high-field MAS DNP-NMR experiments on materials have been carried out on commercial systems manufactured by Bruker BioSpin company (see Figure 9) [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF][START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF]. Therefore, the MAS DNP-NMR instrumentation is only briefly discussed here. Further information on MAS DNP-NMR instrumentation can be found elsewhere [START_REF] Lilly Thankamony | Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR[END_REF][START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF][START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF][START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF]. The microwave source is a continuous-wave gyrotron with a dedicated magnet [START_REF] Becerra | Dynamic nuclear polarization with a cyclotron resonance maser at 5 T[END_REF][START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF]. The microwave beam delivered by the gyrotron propagates through a transmission line, which is a corrugated waveguide [START_REF] Bajaj | 250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR[END_REF]. This waveguide terminates close to the radiofrequency (rf) coil of the MAS DNP-NMR probe, where the sample is located. The microwaves are launched between the turns of the sample rf coil. Besides conventional solid-state NMR capabilities and microwave transmission, the MAS DNP-NMR probe must be suitable for MAS at low temperature. This functionality is accomplished using cold bearing and drive gas. A heat exchanger is employed to cool the gas [START_REF] Allen | Apparatus for low-temperature magic-angle spinning NMR[END_REF][START_REF] Albert | Instrumentation for cryogenic magic angle spinning dynamic nuclear polarization using 90L of liquid nitrogen per day[END_REF], which is transported to the MAS stator by thermally insulated transfer lines. Furthermore, the probe is equipped with a pneumatic insert and eject system, which allows the change of the sample at low temperature [START_REF] Barnes | Cryogenic sample exchange NMR probe for magic angle spinning dynamic nuclear polarization[END_REF]. The probe is placed within an NMR magnet. Since commercially available gyrotrons operate at fixed microwave frequency, the NMR magnet is equipped with an additional superconducting sweep coil, in order to tune the static magnetic field at the position of the sample and match the condition for maximal DNP enhancement.

Instrumentation and experimental parameters

Instrumentation

Microwave source and microwave power

As explained in section 2.1, an increase of the microwave power improves the adiabacity of microwave crossings and hence, the efficiency of the DNP transfer. Simulations and experiments show that the DNP signal enhancement increases with increasing microwave field strength up to a plateau [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF][START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF][START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF][START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF]. This plateau corresponds to the saturation of the allowed EPR transition for OE and that of forbidden zero-quantum or double-quantum transitions for SE, i.e., equal populations of the two energy levels of the transition. In the case of CE, this saturation corresponds to the case, where the polarization of one electron is close to zero, whereas the other is at thermal polarization [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF]. The required microwave field strength to reach saturation depends on the DNP mechanism as well as on the longitudinal and transverse electron relaxation times of PA [START_REF] Lilly Thankamony | Dynamic nuclear polarization for sensitivity enhancement in modern solid-state NMR[END_REF][START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF][START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF]. In the case of CE using nitroxide biradicals at 100 K, simulations predict a required microwave field strength on the order of 1 MHz [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF][START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF]. For high-field MAS DNP experiments, the size of sample, which is in the order of few millimeters, is comparable to the wavelength of the microwaves, which ranges from 1.1 to 0.5 mm for electron Larmor frequencies ranging from 263 to 593 GHz. These wavelengths preclude the use of DNP-NMR probes with microwave resonant structure. Therefore, high-power microwave sources are required for MAS DNP-NMR experiments. In the frequency range corresponding to high-field MAS DNP (263-593 GHz), gyrotrons are still the sources producing the highest microwave power [START_REF] Maly | Dynamic nuclear polarization at high magnetic fields[END_REF]. They can deliver high-power microwaves at fixed frequency with power stability better than ±1% for several days, i.e. the time required for long solid-state NMR experiments. A drawback of gyrotrons is the need of superconducting magnet. Nevertheless, the use of microwaves at the second harmonic of the electron cyclotron frequency, instead of the first harmonic, allows the strength of the static magnetic field for the gyrotron magnet to be divided by a factor of two [START_REF] Torrezan | Continuous-Wave Operation of a Frequency-Tunable 460-GHz Second-Harmonic Gyrotron for Enhanced Nuclear Magnetic Resonance[END_REF]. Second harmonic gyrotrons using cryogen-free superconducting magnets of 5, 7, 10 and 11.1 T have been developed by Bruker and CPI to produce continuous microwave at frequencies 263, 395, 527 and 593 GHz, respectively, for DNP-NMR experiments at 9.4, 14.1, 18.8 and 21.1 T, respectively (corresponding to 1 H Larmor frequencies of 400, 600, 800 and 900 MHz) [START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF][START_REF] Blank | Demonstration of a 593 GHz Gyrotron for DNP[END_REF]. These gyrotrons can typically deliver microwaves with a maximal output power of 50 W.

At the end of the waveguide, the microwaves are diffracted and reflected by the rf coil, which has a pitch comparable to the wavelength of the microwaves. Because of reflection of the microwaves, standing waves occur and display patterns with a period related to the wavelength [START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF][START_REF] Nanni | Microwave field distribution in a magic angle spinning dynamic nuclear polarization NMR probe[END_REF]. Hence, the strength of the microwave magnetic field is highly inhomogeneous within the sample space. This inhomogeneous distribution of the microwave field results in an inhomogeneous distribution of the DNP enhancement, the enhancement factors being larger at the center of the rotor, where the microwave beam irradiates the sample (see Figure 10c) [START_REF] Perras | Magnetic resonance imaging of DNP enhancements in a rotor spinning at the magic angle[END_REF]. This result was obtained by mapping the DNP enhancement in the sample using stray-field imaging under MAS (STRAFI-MAS) experiments.

Furthermore, this strength depends on the efficiency of the conversion of the incident microwave power into microwave field amplitude. This conversion depends on several factors, such as the design of the DNP-NMR probe (coating of the inner surface of the stator, pitch of the sample rf coil, use of a lens to focus the microwave beam on the rotor [START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF]…), the rotor and the investigated sample. For instance, sapphire (Al2O3) rotors are more transparent to microwaves than zirconia ones. For 263 GHz DNP-NMR system equipped with 3.2 mm sapphire rotors, it has been shown that the average electron nutation frequency, which is related to the amplitude of the microwave field, is about 0.2-0.3 MHz [START_REF] Mentink-Vigier | De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization[END_REF][START_REF] Kubicki | Amplifying dynamic nuclear polarization of frozen solutions by incorporating dielectric particles[END_REF]. Sapphire rotors also benefit from a higher thermal conductivity, which limits the heating of the sample during microwave irradiation [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF]. Nevertheless, thin-wall zirconia rotors have been utilized to avoid observing 27 Al background signal of sapphire rotors [START_REF] Vitzthum | Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surfaceenhanced aluminium-27 NMR[END_REF][START_REF] Lee | Enhanced Solid-State NMR Correlation Spectroscopy of Quadrupolar Nuclei Using Dynamic Nuclear Polarization[END_REF]. Reduced thickness of the rotor walls and the sample improves the penetration of microwaves and leads to higher DNP enhancements [START_REF] Barnes | High-Field Dynamic Nuclear Polarization for Solid and Solution Biological NMR[END_REF][START_REF] Chaudhari | Dynamic nuclear polarization at 40 kHz magic angle spinning[END_REF]. Furthermore, it has been shown that the addition of particles, such as KBr or sapphire, with high real dielectric and low imaginary constants, give rise to additional diffraction and reflection of the microwaves in the sample space and so amplifies the amplitude of the microwave field in the sample, which increases the DNP enhancement [START_REF] Kubicki | Amplifying dynamic nuclear polarization of frozen solutions by incorporating dielectric particles[END_REF]. However, the addition of these particles significantly reduces the active sample volume and generally does not provide a global sensitivity gain, at least when the available volume of sample is sufficient to fully fill the rotor. Conversely low DNP enhancements can be obtained when the investigated material have unfavorable dielectric properties.

Magnetic field of the NMR magnet

Higher static magnetic fields improve the resolution of NMR spectra, when the linewidth is not dominated by the distribution of isotropic chemical shifts produced by the disorder in the material or the anisotropy of bulk magnetic susceptibility [START_REF] Zorin | Origins of linewidth in 1 H magic-angle spinning NMR[END_REF]. For spin-1/2 nuclei exhibiting field-independent linewidths, the resolution is proportional to B0. For half-integer spin quadrupolar nuclei, the CT linewidth is dominated by the second-order quadrupolar broadening, which is inversely proportional to B0, and the resolution increases as (B0) 2 .

Furthermore, in the case of conventional solid-state NMR experiments, high B0 field generally improves the sensitivity since it enhances the thermal polarization and the induced voltage in the sample coil and also reduces the CT second-order quadrupolar broadening for half-integer quadrupolar nuclei. Hence, even if the electronic noise increases with higher field owing to the skin effect, the signal-to-noise ratio increases as (B0) 7/4 for spin-1/2 nuclei and (B0) 11/4 for half-integer quadrupolar nuclei [START_REF] Hoult | The Signal-to-Noise Ratio of the Nuclear Magnetic Resonance Experiment[END_REF]. The dependence of the sensitivity on B0 may depart from these relationships, when the longitudinal nuclear relaxation time depends on B0 or a significant chemical shift anisotropy (CSA) reduces the sensitivity gain at higher B0 field by enhancing spinning sidebands and lowering the centerband intensity.

In addition, in the case of DNP-NMR experiments, the efficiency of DNP transfer usually depends on the B0 field. As explained in section 2.1, if the efficiency of OE in solids increases with higher B0 field, those of SE and CE decrease with increasing B0 field, the decrease being faster for SE [START_REF] Mentink-Vigier | Theoretical aspects of Magic Angle Spinning -Dynamic Nuclear Polarization[END_REF]. For SE and CE, the decrease of the efficiency at higher field stems from the broadening of the EPR line, which is proportional to B0. This broadening decreases the adiabacity of the level crossings. Despite its unfavorable dependence on B0, CE mechanism still yields the highest sensitivity at B0 = 18.8 T [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF]. Furthermore, for spin-1/2 nuclei in frozen solutions, it has been shown that CE using TEMTriPol-1 nitroxide-trityl bidradicals can yield sensitivity at 18.8 T comparable to that obtained 9.4 T [START_REF] Mentink-Vigier | Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR[END_REF]. Therefore, MAS DNP-NMR experiments on half-integer spin quadrupolar nuclei using TEMTriPol-1 as PA should be more sensitive at 18.8 T than at 9.4 T. Hence, the development of PAs exhibiting narrow EPR lines is desirable for MAS DNP NMR at B0 > 10 T [START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF].

Currently the majority of MAS DNP-NMR experiments on materials has been carried out at B0 = 9.4 T. Nevertheless, some MAS DNP-NMR experiments on materials have been reported at 14.1 T [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF][START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF][START_REF] Blanc | Dynamic nuclear polarization NMR spectroscopy allows high-throughput characterization of microporous organic polymers[END_REF][START_REF] Woodward | Swellable, Water-and Acid-Tolerant Polymer Sponges for Chemoselective Carbon Dioxide Capture[END_REF][START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF][START_REF] Ong | Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS[END_REF][START_REF] Harris | Molecular Structure and Confining Environment of Sn Sites in Single-Site Chabazite Zeolites[END_REF][START_REF] Hope | Surfaceselective direct 17 O DNP NMR of CeO2 nanoparticles[END_REF][START_REF] Moroz | Discerning γ-Alumina Surface Sites with Nitrogen-15 Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine[END_REF][START_REF] Kim | In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca3Al2O6-Stabilized CO2 Sorbents[END_REF], 18.8 T [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF][START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF][START_REF] Brownbill | Oxygen-17 dynamic nuclear polarisation enhanced solid-state NMR spectroscopy at 18.8 T[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF][START_REF] Chaudhari | Dynamic nuclear polarization at 40 kHz magic angle spinning[END_REF] and 21.1 T [START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF][START_REF] Björgvinsdóttir | DNP enhanced NMR with flip-back recovery[END_REF]. DNP-NMR experiments at B0 ≥ 14.1 T benefit notably from a higher spectral resolution for the detection of half-integer quadrupolar nuclei. For instance, the 27 Al signals of tetra-(AlO4), penta-(AlO5) and hexa-coordinated (AlO6) surface sites of γalumina are better resolved at 18.8 T [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF] than at 9.4 T [START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF][START_REF] Vitzthum | Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surfaceenhanced aluminium-27 NMR[END_REF].

As explained in section 2, slow electron and nuclear relaxations improve the efficiency of the DNP transfer. Furthermore, slow nuclear relaxation facilitates the transport of DNP-enhanced magnetization within the sample by nuclear spin diffusion and reduces the losses during the pulse sequence. Therefore, higher DNP enhancements and higher sensitivity are achieved at lower temperature [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF][START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Takahashi | Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments[END_REF][START_REF] Zagdoun | A slowly Relaxing Rigid Biradical for Efficient Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy: Expeditious Characterization of Functional Group Manipulation in Hybrid Materials[END_REF][START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K[END_REF][START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF]. Microwave irradiation and MAS can heat the sample. Currently, almost all MAS DNP-NMR experiments on materials have been carried out at about 100 K using cold N2 gas to cool the sample and spin it. MAS DNP-NMR experiments on materials at lower temperature, down to 35 K, have also been reported using helium instead of nitrogen gas and it has been shown that they are more sensitive than those at ~100 K [START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF]. Nevertheless, the need for low temperature can be a limitation for the characterization of materials, and notably to study their dynamics. Even if the sensitivity enhancement produced by DNP decreases with increasing temperature, it has been shown that DNP can still yield significant sensitivity enhancement at temperatures much higher than 100 K [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Zagdoun | Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K[END_REF][START_REF] Bretschneider | On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid-State and Dissolution 13C NMR Spectroscopy[END_REF]. For instance, a DNP enhancement of ~15 has been reported at room temperature using OE of BDPA in OTP solvent [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF]. Furthermore, DNP enhancements as large as 140 has been measured at room temperature for diamond microparticles containing endogeneous paramagnetic centers [START_REF] Bretschneider | On The Potential of Dynamic Nuclear Polarization Enhanced Diamonds in Solid-State and Dissolution 13C NMR Spectroscopy[END_REF]. Rigid samples with limited molecular motions, and hence, slower electron and nuclear relaxation exhibit higher DNP enhancements at high temperature.

MAS frequency

MAS improves the resolution of solid-state NMR spectra by averaging out the anisotropic interactions. For instance, the suppression of spinning sidebands due to CSA or second-order quadrupolar interaction requires MAS frequencies larger than these interactions. Fast MAS also improves the resolution of 1 H spectra since the width of individual 1 H lines is inversely proportional to the MAS frequency [START_REF] Zorin | Origins of linewidth in 1 H magic-angle spinning NMR[END_REF].

On the other hand, MAS can also affect the efficiency of the DNP transfer and the nuclear spin diffusion. This dependence has so far mainly been investigated for the CE. Under static conditions, CE can only occur if the two unpaired electrons and the nucleus match the CE condition ν ! 𝑆 ! -ν ! 𝑆 ! ≈ ν ! 𝐼 and the microwave irradiation is resonant with one of the electron. Hence, only a limited number of PA molecules contributes then to the CE transfer. MAS sweeps the resonance frequencies of the unpaired electrons and allows a larger number of PA molecules to take part in the CE transfer. Furthermore, as the microwave field is highly inhomogeneous (see Figure 10b), MAS ensures a more uniform microwave irradiation of the sample [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF][START_REF] Rosay | Instrumentation for solid-state dynamic nuclear polarization with magic angle spinning NMR[END_REF]. Additionally, the MAS CE mechanism described in subsection 2.1 is only efficient when the multiple energy level crossings and anti-crossings occur within a time shorter than T1(Si) [START_REF] Thurber | Theory for cross effect dynamic nuclear polarization under magic-angle spinning in solid state nuclear magnetic resonance: The importance of level crossings[END_REF]. As an example for nitroxide biradicals, such as AMUPol, T1(Si) times are about 0.5 ms [START_REF] Mentink-Vigier | De novo prediction of cross-effect efficiency for magic angle spinning dynamic nuclear polarization[END_REF] and as a result, the efficiency of the DNP transfer increases up to a MAS frequency of ~2 kHz (see Figure 12a). As more PAs take part in the CE transfer under MAS conditions than under static ones, the build-up of the DNP-enhanced polarization is faster under MAS [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF].

However, as explained in subsection 2.1, larger MAS frequencies also decrease the adiabaticity of the crossings and anti-crossings and hence can reduce the efficiency of the DNP transfer and produce depolarization, as seen in Figure 12a for AMUPol. It has been shown that such deleterious effect can be avoided by using PA featuring significant exchange and/or dipolar interactions, such TEMTriPol-1, AsymPolPOK or HyTEK2 [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF][START_REF] Mentink-Vigier | Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF]. Furthermore, for several radicals, such as BDPA or HyTEK2, it has been observed that the build-up of the DNP-enhanced polarization is slower for increasing MAS frequency (see Figure 12b) [START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF]. This slower build-up has been ascribed to slower 1 H-1 H spin diffusion. It has also been shown that for 119 Sn nuclei subject to large CSA, the sensitivity can be maximized by using low MAS frequencies during the polarization delay in order to accelerate the spin diffusion and fast MAS frequencies during the acquisition to decrease the intensity of the CSA spinning sidebands [START_REF] Björgvinsdóttir | Maximizing nuclear hyperpolarization in pulse cooling under MAS[END_REF].

From a practical point of view, the increased viscosity of N2 gas and the reduced speed of sound in N2 for the temperatures close to its liquefaction point (77 K at atmospheric pressure) result in significantly reduced maximum MAS frequencies compared to MAS experiments at room temperature. So far the majority of MAS DNP NMR experiments have been carried out using 3.2 mm rotors cooled and spun by cold nitrogen gas. In such cases, at temperature of about 100 K, the maximum MAS frequency of these rotors cannot exceed 15 kHz [START_REF] Rosay | Solid-state dynamic nuclear polarization at 263 GHz: spectrometer design and experimental results[END_REF]. It has been shown that maximum MAS frequency of 25 kHz at 90 K can be reached by cooling and spinning these 3.2 mm rotors using cold He gas (see Figure 11a) [START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF]. Furthermore, MAS DNP NMR probes for 1.3 mm rotors have recently become available and have enabled DNP NMR experiments at MAS frequencies up to 40 kHz at 100 K and 9.4 and 18.8 T [START_REF] Chaudhari | Dynamic Nuclear Polarization Efficiency Increased by Very Fast Magic Angle Spinning[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF][START_REF] Chaudhari | Dynamic nuclear polarization at 40 kHz magic angle spinning[END_REF].

Although this review focuses on MAS DNP-NMR of materials, it is appropriate to mention that high-field DNP NMR experiments on materials under static conditions have been recently reported [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF][START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF][START_REF] Suzuki | Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Kobayashi | DNP-enhanced ultrawideline 207 Pb solid-state NMR spectroscopy: an application to cultural heritage science[END_REF]. These experiments are notably useful to probe the orientation of molecules in aligned samples [START_REF] Suzuki | Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF] or when the magnitude of the anisotropic interactions, such as CSA or second-order quadrupolar interactions, greatly exceed the accessible MAS frequencies [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF][START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF][START_REF] Kobayashi | DNP-enhanced ultrawideline 207 Pb solid-state NMR spectroscopy: an application to cultural heritage science[END_REF]. These experiments have been carried out using specific static DNP-NMR probes with a sample coil orthogonal to the B0 field [START_REF] Suzuki | Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF] or MAS DNP-NMR probe [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF][START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF][START_REF] Kobayashi | DNP-enhanced ultrawideline 207 Pb solid-state NMR spectroscopy: an application to cultural heritage science[END_REF]. However, as explained above, DNP yields lower sensitivity gains when it occurs under static condition, instead of MAS. This issue can be circumvented by applying MAS during the polarization delay and stopping it during the NMR pulse sequence. This approach, called spinning-on spinning-off (SOSO) acquisition, yields a two-fold higher signal enhancement over the acquisition under purely static conditions (see Figure 12c) [START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF]. Figure 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 their low natural abundance (NA), such as 29 Si (NA = 4.7%) [START_REF] Lafon | Beyond the Silica Surface by Direct Silicon-29 Dynamic Nuclear Polarization[END_REF][START_REF] Lelli | Fast Characterization of Functionalized Silica Materials by Silicon-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization[END_REF], 13 C (NA = 1.1%) (see Figure 1c) [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Hoffmann | Directly vs Indirectly Enhanced 13C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems[END_REF] or 15 N (NA = 0.37%) [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy of Functionalized Metal-Organic Frameworks[END_REF], their low gyromagnetic ratio, such as 15 N (γ( 15 N) ≈ 0.1γ( 1 H)) or 89 Y (γ( 89 Y) ≈ 0.05γ( 1 H)) [START_REF] Blanc | Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3[END_REF] or a large CSA, such as 77 Se, 111 Cd, 113 Cd, 119 Sn, 195 Pt or 207 Pb [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF][START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF][START_REF] Protesescu | Unraveling the Core-Shell Structure of Ligand-Capped Sn/SnOx Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mössbauer, and X-ray absorption spectroscopies[END_REF][START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF][START_REF] Kobayashi | DNP-enhanced ultrawideline 207 Pb solid-state NMR spectroscopy: an application to cultural heritage science[END_REF]. However, DNP-NMR has also been employed for receptive spin-1/2 nuclei, such as 31 P [START_REF] Ziarelli | Dynamic nuclear polarisation NMR of nanosized zirconium phosphate polymer fillers[END_REF].

Isotopes and NMR methods

Isotopes and type of transfer

MAS DNP-NMR has also been applied to detect spin-1 quadrupolar nuclei. For instance, the high-sensitivity of DNP has been used to detect 2 H in natural abundance (NA = 0.01%) in organic solids, which gives access to information about hydrogen chemical shifts and 2 H quadrupolar coupling [START_REF] Rossini | High-resolution NMR of hydrogen in organic solids by DNP enhanced natural abundance deuterium spectroscopy[END_REF]. MAS DNP has also been employed to enhance the weak signal of the forbidden overtone transition between the energy levels m = +1 and -1 of 14 N nucleus [START_REF] Rossini | Dynamic nuclear polarisation enhanced 14 N overtone MAS NMR spectroscopy[END_REF]. MAS DNP NMR of 6 Li isotope with NA = 7.59% has also been reported [START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF].

It has also been shown for materials that MAS DNP can enhance the NMR signals of insensitive half-integer spin quadrupolar nuclei with low natural abundance, such as 17 O (I = 5/2, NA = 0.037%, |γ( 17 O)| ≈ 0.14γ( 1 H)) [START_REF] Blanc | Dynamic Nuclear Polarization Enhanced Natural Abundance 17 O Spectroscopy[END_REF], and low gyromagnetic ratio, such as 43 Ca (I = 7/2, NA = 0.14%, |γ( 43 Ca)| ≈ 0.07γ( 1 H)) [START_REF] Lee | Interfacial Ca 2+ environments in nanocrystalline apatites revealed by dynamic nuclear polarization enhanced 43 Ca NMR spectroscopy[END_REF] or with low gyromagnetic ratio and large quadrupolar interaction, such as 35 Cl (I = 3/2) (see Figure 11c) [START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF]. For materials, MAS DNP has also been used to enhance the signals of receptive half-integer quadrupolar nuclei, such as 7 Li (I = 3/2) [START_REF] Chakrabarty | Paramagnetic Metal-Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids[END_REF], 27 Al (I = 5/2) (see Figure 11a) [START_REF] Vitzthum | Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surfaceenhanced aluminium-27 NMR[END_REF], 51 V (I = 7/2) [START_REF] Lilly Thankamony | Characterization of V-Mo-W Mixed Oxide Catalyst Surface Species by 51 V Solid-State Dynamic Nuclear Polarization NMR[END_REF] and 59 Co (I = 7/2) [START_REF] Corzilius | Dynamic Nuclear Polarization of 1 H, 13 C, and 59 Co in a Tris(ethylenediamine) cobalt (III) Crystalline Lattice Doped with Cr (III)[END_REF].

Most of the MAS DNP experiments on materials employ a coherent transfer of the DNP-enhanced 1 H polarization to the detected isotope. The phase cycle suppresses the direct DNP transfer from unpaired electrons to the detected isotope. This approach, termed coherent indirect DNP, has the advantage that the 1 H-1 H spin diffusion transports the DNP-enhanced magnetization. Such transport acts to homogenize the 1 H polarization within the sample and to accelerate its build-up. In the case of heterogeneous samples, in which protons and detected isotopes are located in different domains, the indirect DNP allows a selective observation of the nuclei near the interfaces since heteronuclear coherent transfers are only effective up to a few angstroms and the efficiency of transfer to remote nuclei is reduced by the dipolar truncation [START_REF] Hologne | 1 H/ 31 P Distance Determination by Solid-State NMR in Multiple-Spin Systems[END_REF]. For instance, this strategy has been used to selectively probe the surface of inorganic materials, for which the bulk region contains no or a limited amount of protons. This approach has been dubbed surface-enhanced NMR spectroscopy (SENS) [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF]. The depth of coherent transfer below the surface depends on several parameters, including the concentration of the detected isotope, which notably depends on its natural abundance, its gyromagnetic ratio, the coherence lifetimes during the transfer and the duration of the coherence transfer [START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF]. For instance, it has been shown that DNP-enhanced 1 H→ 27 Al CPMAS can be used to selectively observe the first layer of γalumina nanoparticles.

It has also been shown that 1 H- 13 C dipolar cross-relaxation can also transfer the DNPenhanced 1 H polarization to nearby 13 C nuclei and produces negative DNP enhancement, i.e. a DNP-enhanced 13 C polarization pointing in opposite direction of the B0 field [START_REF] Kaushik | Gd(III) and Mn(II) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins[END_REF][START_REF] Daube | Heteronuclear Cross-Relaxation under Solid-State Dynamic Nuclear Polarization[END_REF]. Such incoherent indirect DNP transfer was first reported for biomolecules [START_REF] Kaushik | Gd(III) and Mn(II) complexes for dynamic nuclear polarization: small molecular chelate polarizing agents and applications with site-directed spin labeling of proteins[END_REF][START_REF] Daube | Heteronuclear Cross-Relaxation under Solid-State Dynamic Nuclear Polarization[END_REF] and more recently for nonionic surfactants, such as polyethylene glycol (PEG) and solutions of organic molecules, such as cyclohexane, in PEG (see Figure 14a) [START_REF] Hoffmann | Directly vs Indirectly Enhanced 13C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems[END_REF][START_REF] Bothe | Comparative Study of the Magnetic Field Dependent Signal Enhancement in Solid-State Dynamic Nuclear Polarization Experiments[END_REF][START_REF] Hoffmann | Unusual Local Molecular Motions in the Solid State Detected by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy[END_REF]. Local molecular motions, such as the rotation of the methyl groups or cyclohexane ring inversion, promote the 1 H- 13 C dipolar cross-relaxation. Rapid cooling of the samples and the formation of glass help to preserve the local molecular motions. This transfer can be suppressed by the application of a burst of π pulses on the 1 H channel during the polarization delay [START_REF] Hoffmann | Directly vs Indirectly Enhanced 13C in Dynamic Nuclear Polarization Magic Angle Spinning NMR Experiments of Nonionic Surfactant Systems[END_REF]. An alternative to the indirect DNP consists of the direct transfer of polarization from unpaired electrons to the detected heteronucleus, i.e. an isotope distinct from proton. This approach has been termed direct DNP. In the case of materials, it has been applied for the detection of spin-1/2 nuclei, such as 13 C [171], 29 Si [39], 31 P, 113 Cd and 119 Sn [START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF], but also quadrupolar nuclei, such as 6 Li [52], 7 Li [START_REF] Chakrabarty | Paramagnetic Metal-Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids[END_REF], 17 O [79] and 51 V [START_REF] Lilly Thankamony | Characterization of V-Mo-W Mixed Oxide Catalyst Surface Species by 51 V Solid-State Dynamic Nuclear Polarization NMR[END_REF]. Direct DNP complements the indirect version since it allows for the observation of heteronuclei distant from protons. For instance, direct DNP has been used to observe subsurface or bulk sites in materials, for which protons are absent from the bulk (see Figure 14b and Figure 25) [START_REF] Lafon | Beyond the Silica Surface by Direct Silicon-29 Dynamic Nuclear Polarization[END_REF][START_REF] Chakrabarty | Paramagnetic Metal-Ion Dopants as Polarization Agents for Dynamic Nuclear Polarization NMR Spectroscopy in Inorganic Solids[END_REF][START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF][START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF][START_REF] Hope | Surfaceselective direct 17 O DNP NMR of CeO2 nanoparticles[END_REF] as well as non-protonated surface 17 O sites (see Figure 14d) [START_REF] Perras | Direct 17 O dynamic nuclear polarization of single-site heterogeneous catalysts[END_REF]. Furthermore, as the dipolar couplings between heteronuclei are smaller than among protons, spin diffusion among heteronuclei is much slower than among protons. Hence, direct DNP often mainly polarizes the nuclei close to unpaired electrons. This heterogeneous polarization increases the linewidths with respect to conventional NMR spectra or those enhanced by indirect DNP (see Figure 14c) [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF]. Furthermore, the DNP-enhanced polarization cannot be fitted to a monoexponential function but rather to a stretched exponential one since the polarization build-up is slower for heteronuclei at increasing distance from the unpaired electrons [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF][START_REF] Hope | Surfaceselective direct 17 O DNP NMR of CeO2 nanoparticles[END_REF]. However, in the case of slowly relaxing nuclei, it has been shown that nuclear spin diffusion can contribute to the transport of the DNP-enhanced polarization [START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF]. The transfer depth of direct DNP increases with increasing polarization delay and can reach distances of up to 10 nm [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF][START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF][START_REF] Hope | Surfaceselective direct 17 O DNP NMR of CeO2 nanoparticles[END_REF][START_REF] Akbey | Dynamic Nuclear Polarization of Spherical Nanoparticles[END_REF].

NMR methods

Most MAS NMR pulse sequences can be used in combination with DNP. Nevertheless, the efficiency of the sequences can be reduced by the presence of PAs in the sample, which accelerates the relaxation and increases signal losses [START_REF] Takahashi | Optimization of an absolute sensitivity in a glassy matrix during DNP-enhanced multidimensional solid-state NMR experiments[END_REF]. For experiments using coherent indirect DNP, the excited nucleus must be proton and the DNP-enhanced 1 H polarization has to be transferred to the other isotopes of interest. Such transfer is typically performed using the CPMAS sequence. It has been shown that the sensitivity can be further improved by the use of (i) the flip-back recovery method to utilize the residual 1 H magnetization at the end of CW 1 H heteronuclear decoupling [START_REF] Björgvinsdóttir | DNP enhanced NMR with flip-back recovery[END_REF], (ii) multiple contact CP [START_REF] Björgvinsdóttir | Bulk Nuclear Hyperpolarization of Inorganic Solids by Relay from the Surface[END_REF][START_REF] Björgvinsdóttir | Maximizing nuclear hyperpolarization in pulse cooling under MAS[END_REF] or (iii) the CPMG detection scheme for NMR signals inhomogeneously broadened by the distribution of isotropic chemical shifts, CSA or second-order quadrupolar broadening [START_REF] Rossini | One hundred fold overall sensitivity enhancements for Silicon-29 NMR spectroscopy of surfaces by dynamic nuclear polarization with CPMG acquisition[END_REF]. However, the CPMAS technique lacks robustness and efficiency for half-integer quadrupolar nuclei. It has been shown that this issue can be circumvented by using PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order) technique, for which no spin lock is applied to the quadrupolar nucleus (see Figure 15a) [START_REF] Perras | Natural Abundance 17 O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy[END_REF]. This technique has notably been employed to transfer DNP-enhanced 1 H polarization to surface 17 O sites (see Figure 15b). For isotopes exhibiting broad NMR signals, the coherent transfer of polarization can be achieved under static conditions using the BRAIN-CP technique (see Figure 12c) [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF]. As mentioned in section 3.2, a solvent is often added to the investigated material for DNP-NMR experiments. Although this solvent is partially deuterated, its 1 H and 13 C signals can mask those of the investigated materials. These solvent signals can be suppressed by various methods, including (i) the use of short contact times in CPMAS to selectively transfer the DNP-enhanced 1 H polarization to protonated 13 C nuclei of the investigated materials and not to deuterated 13 C sites of the solvent, (ii) the use of relaxation filters, since for heterogeneous samples, such as a solid particles wetted by a PA solution, the nuclei in the solvent relax faster than those of the investigated material owing to their closer proximity to PA and (iii) the reintroduction of 13 C-2 H dipolar couplings [START_REF] Lee | Solvent signal suppression for high-resolution MAS-DNP[END_REF][START_REF] Yarava | Solvent suppression in DNP enhanced solid state NMR[END_REF]. After the coherent polarization transfer from protons to another isotope, additional pulse schemes have been applied to separate the isotropic and anisotropic interactions or to probe the internuclear connectivities and the proximities. For instance, for spin-1/2 nuclei subject to large CSA, such as 113 Cd or 119 Sn, techniques, such as magic-angle turning (MAT) [START_REF] Wolf | NMR signatures of the active sites in Sn-β zeolite[END_REF] and phase adjusted spinning sideband (PASS) [START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF], have been used to refocus the CSA and to correlate the isotropic chemical shifts of the different sites along the indirect dimension to their MAS sideband manifolds, allowing their CSA to be determined (see Figure 24). Similarly multiple-quantum (MQ) MAS (MQMAS) sequences have been used to refocus the second-order quadrupolar broadening of 27 Al and 17 O nuclei and to correlate the isotropic shifts along the indirect dimension with the MAS signals [START_REF] Vitzthum | Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surfaceenhanced aluminium-27 NMR[END_REF][START_REF] Li | Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy[END_REF]. These DNP-enhanced 27 Al MQMAS spectra were recorded by using an initial 1 H→ 27 Al CP step, which excites the desired 27 Al triple-quantum coherences (3Q), whereas the 17 O MQMAS spectrum enhanced by 17 O direct DNP has been acquired using the conventional 3QMAS sequence using z-filter. Indirect DNP has also been combined with pulse sequences to probe homo-and hetero-nuclear connectivities and proximities. The sensitivity gain provided by DNP is notably useful for isotopes with low natural abundance, such as 13 C, 15 N or 29 Si. For instance, the DNP-enhanced refocused incredible natural abundance double-quantum transfer experiment (INADEQUATE) 2D experiments have been used to observe 13 C-13 C [75] and 29 Si-O- 29 Si [START_REF] Lee | Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D 29 Si-29 Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization[END_REF] covalent bonds in isotopically unmodified materials, whereas the probability of these internuclear connectivities are as low as 0.01% and 0.2%, respectively (see Figure 23b). 13 C- 13 C and 29 Si- 29 Si proximities have also been detected at natural abundance in materials using DNP-enhanced 13 C [95] and 29 Si [START_REF] Lee | Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D 29 Si-29 Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization[END_REF] double-quantum (2Q)-singlequantum (1Q) through-space homonuclear correlation 2D spectra. These 2Q-1Q experiments allow for the observation of proximities between sites with close or identical isotropic chemical shifts. The 13 C-13 C and 29 Si- 29 Si dipolar couplings have been reintroduced using symmetry-based 2Q dipolar recoupling schemes, such as POST-C7 (see Figure 17a andb) [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Lee | Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D 29 Si-29 Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization[END_REF], SPC5 [START_REF] Takahashi | Towards Structure Determination of Self-Assembled Peptides using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Kobayashi | Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR[END_REF], SR26 [START_REF] Märker | A New Tool for NMR Crystallography: Complete 13 C/ 15 N Assignment of Organic Molecules at Natural Isotopic Abundance Using DNP-Enhanced Solid-State NMR[END_REF] or S3 [START_REF] Märker | Welcoming natural isotopic abundance in solid-state NMR: probing π-stacking and supramolecular structure of organic nanoassemblies using DNP[END_REF]. An advantage of carrying out these experiments at natural abundance is that the dipolar truncation is significantly reduced with respect to isotopically labeled samples. For instance, it has been shown that 13 C- 13 C distances up to 0.7 nm can be estimated by measuring the build-up of cross-peaks from a series of through-space homonuclear correlation 2D spectra (see Figure 17) [START_REF] Mollica | Quantitative Structural Constraints for Organic Powders at Natural Isotopic Abundance Using Dynamic Nuclear Polarization Solid-State NMR Spectroscopy[END_REF] or the transfer of longitudinal magnetization between two specific sites using selective pulses [START_REF] Märker | Welcoming natural isotopic abundance in solid-state NMR: probing π-stacking and supramolecular structure of organic nanoassemblies using DNP[END_REF]. The second approach only requires the acquisition of a series of 1D spectra. 13 C proximities have also been probed by DNP-enhanced 13 C 1Q-1Q through-space homonuclear correlation 2D spectra, which are usually more sensitive than the 2Q-1Q variants. The 1Q-1Q experiments have been acquired using dipolar assisted rotational resonance (DARR) [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Johnson | Identifying low-coverage surface species on supported noble metal nanoparticle catalysts by DNP-NMR[END_REF] and CHHC [START_REF] Kobayashi | Measuring Long-Range 13 C-13 C Correlations on a Surface under Natural Abundance Using Dynamic Nuclear Polarization-Enhanced Solid-State Nuclear Magnetic Resonance[END_REF] techniques. It has been demonstrated that the latter technique, in which the 13 C-edited 1 H polarization is transported by 1 H-1 H spin diffusion, allows for the detection of long-range correlations between 13 C nuclei that are close to the same network of protons [START_REF] Kobayashi | Measuring Long-Range 13 C-13 C Correlations on a Surface under Natural Abundance Using Dynamic Nuclear Polarization-Enhanced Solid-State Nuclear Magnetic Resonance[END_REF]. 1 H-1 H proximities in aminoacid microcrystals have been observed using 1 H DNPenhanced 2Q-1Q through-space homonuclear correlation experiments employing POST-C7 recoupling [START_REF] Yarava | Solvent suppression in DNP enhanced solid state NMR[END_REF]. For this experiment, 1 H spin locking has been used to filter out the signal of the solvent since solvent protons relax faster than those of the investigated materials. Furthermore, the resolution along the two dimensions of the 2D correlation has been improved by applying eDUMBO-122 homonuclear decoupling during the indirect evolution period, t1, and the acquisition period, t2.

Furthermore, 27 Al- 27 Al proximities have been probed using DNP-enhanced 2Q-1Q through-space homonuclear correlation experiments, in which 27 Al homonuclear dipolar interactions were reintroduced under MAS by the application of BR2 ! ! recoupling [START_REF] Lee | Enhanced Solid-State NMR Correlation Spectroscopy of Quadrupolar Nuclei Using Dynamic Nuclear Polarization[END_REF]. The DNP sensitivity gain compensates for the low transfer efficiency (∼5%) of this technique, which results from the intricate spin dynamics in the presence of MAS and an rf field. 29 Si CPMAS transfer creates the initial DNPenhanced transverse 29 Si magnetization. The spectrum of the subfigure a indicates that Si atoms are covalently connected to AlO4 and AlO5 sites but not to AlO6 ones. Figure adapted from ref. [START_REF] Rankin | Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy[END_REF]. Copyright 2017, American Chemical Society.

DNP experiments have also been used to probe heteronuclear connectivities and proximities in materials. As seen in Figure 18a, 27 Al-O- 29 Si connectivities at the surface of aluminosilicates have been observed using polarization transfer from 29 Si to 27 Al nuclei via 2 J-coupling constants with the help of refocused insensitive nuclei enhanced by polarization transfer (J-R-INEPT) sequence with 27 Al detection (denoted 27 Al{ 29 Si},) [START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF]. One-bond 13 C-14 N covalent bonds have also been observed using DNP-enhanced Jmediated heteronuclear multiple-quantum correlation (J-HMQC) sequence for the indirect detection of 14 N overtone transition via the attached 13 C nuclei [START_REF] Rossini | Dynamic nuclear polarisation enhanced 14 N overtone MAS NMR spectroscopy[END_REF].

1 H-17 O dipolar couplings have been estimated by measuring the build-up of the DNPenhanced 1 H→ 17 O PRESTO signal [START_REF] Perras | Natural Abundance 17 O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy[END_REF]. However, this technique lacks robustness to rf field inhomogeneity, which limits its accuracy. It has been shown that this issue can be circumvented by using the windowed-proton detected local-field (wPDLF) technique combined with 1 H→ 17 O PRESTO scheme [START_REF] Perras | Natural Abundance 17 O DNP NMR Provides Precise O-H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts[END_REF]. DNP-enhanced rotational-echo double resonance (REDOR) technique has been employed to estimate the dipolar couplings between spin-1/2 heteronuclei, such as 13 C- 6 Li [START_REF] Sani | Probing Ionic Liquid Electrolyte Structure via the Glassy State by Dynamic Nuclear Polarization NMR Spectroscopy[END_REF], 13 C-15 N [START_REF] Berruyer | Three-Dimensional Structure Determination of Surface Sites[END_REF], 13 C- 29 Si [START_REF] Sangodkar | Influences of Dilute Organic Adsorbates on the Hydration of Low-Surface-Area Silicates[END_REF], 13 C-119 Sn [START_REF] Conley | Silica-surface reorganization during organotin grafting evidenced by 119 Sn DNP SENS: a tandem reaction of gem-silanols and strained siloxane bridges[END_REF] and 15 N- 29 Si [START_REF] Berruyer | Three-Dimensional Structure Determination of Surface Sites[END_REF] pairs (see Figure 22). Furthermore, dipolar couplings between spin-1/2 and quadrupolar nuclei, such as 13 C- 27 Al, have been estimated using rotational-echo saturation pulse double-resonance (RESPDOR) technique employing simultaneous frequency and amplitude modulation (SFAM) recoupling [START_REF] Pourpoint | Probing 27 Al-13 C proximities in metal-organic frameworks using dynamic nuclear polarization enhanced NMR spectroscopy[END_REF]. This double-resonance 13 C- 27 Al experiment required the use of a frequency splitter owing to the close Larmor frequencies of these isotopes.

Proximities between spin-1/2 heteronuclei, such as 13 C-31 P [172] and 13 C-15 N [START_REF] Märker | A New Tool for NMR Crystallography: Complete 13 C/ 15 N Assignment of Organic Molecules at Natural Isotopic Abundance Using DNP-Enhanced Solid-State NMR[END_REF], have also been probed using through-space heteronuclear correlation (HETCOR) sequence using CP transfer (see Figure 19). The sensitivity gain provided by DNP is particularly useful for detecting proximities within spin pairs having low NA, such as 13 C- 15 N with NA ∼0.004%. Furthermore, DNP-enhanced 13 C{ 111 Cd} through-space heteronuclear correlation have been reported using the dipolar-mediated heteronuclear multiple-quantum correlation (D-HMQC) sequence. This sequence avoids the use of spin lock for the 111 Cd isotope subject to large CSA and so is more robust and efficient than CPMAS in this case [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF]. Similarly, the D-HMQC sequence has been employed to observe proximities between spin-1/2 and quadrupolar nuclei, such as 13 C- 27 Al [START_REF] Pourpoint | Probing 27 Al-13 C proximities in metal-organic frameworks using dynamic nuclear polarization enhanced NMR spectroscopy[END_REF] or 13 C- 35 Cl [START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF]. 27 Al- 29 Si proximities have also been identified using 27 Al{ 29 Si} D-R-INEPT sequence using R 3 [START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF] or REDOR [START_REF] Rankin | Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy[END_REF] schemes to reintroduce the 27 Al- 29 Si dipolar couplings (see Figure 18b).

Organic materials

As seen in Figure 20, high-field MAS DNP-NMR has been used to characterize a broad range of materials, including pharmaceuticals, polymers, catalytic and energy materials, biomaterials… We present its use for the characterization of organic materials composed of small molecules or polymers. Organic materials contain protons and so 1 H-1 H spin diffusion can transport the DNP-enhanced 1 H polarization from the surface to the bulk (see section 2.2).

Pharmaceuticals and materials composed of small organic molecules

The studies of pure and formulated active pharmaceutical ingredients using MAS DNP have been recently reviewed [START_REF] Zhao | DNP-enhanced solid-state NMR spectroscopy of active pharmaceutical ingredients[END_REF]. DNP-NMR has been used to characterize pure active pharmaceutical ingredients (API), such as indomethacin [START_REF] Ong | Solvent-Free Dynamic Nuclear Polarization of Amorphous and Crystalline Ortho-Terphenyl[END_REF], an anti-inflammation drug, ibuprofen [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF], a common analgesic, ambroxol hydrochloride (see Figure 12c) [START_REF] Lelli | Solid-State Dynamic Nuclear Polarization at 9.4 and 18.8 T from 100 K to Room Temperature[END_REF][START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF], isoxsuprine hydrochloride [START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF], a vasodilator, cetirizine dihydrochloride [START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF], an antihistamine, an theophylline [START_REF] Pinon | Polymorphs of Theophylline Characterized by DNP Enhanced Solid-State NMR[END_REF][START_REF] Mollica | Quantitative Structural Constraints for Organic Powders at Natural Isotopic Abundance Using Dynamic Nuclear Polarization Solid-State NMR Spectroscopy[END_REF], an asthma drug (see Figure 17), acebutolol hydrochloride [START_REF] Veinberg | Natural abundance 14 N and 15 N solid-state NMR of pharmaceuticals and their polymorphs[END_REF], a beta blocker and nicardipine hydrochloride [START_REF] Veinberg | Natural abundance 14 N and 15 N solid-state NMR of pharmaceuticals and their polymorphs[END_REF], used for the treatment of high blood pressure and angina… DNP-NMR has also been applied for the characterization of pharmaceutical formulations, containing API, such as cetirizine dihydrochloride API [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF] or posaconazole, an antifungal drug [START_REF] Ni | In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR[END_REF] and polymeric excipients. DNP has also been used to characterize drug delivery systems, such as lipid nanoparticles containing small interfering RNA and messenger RNA [START_REF] Viger-Gravel | Structure of Lipid Nanoparticles Containing siRNA or mRNA by Dynamic Nuclear Polarization-Enhanced NMR Spectroscopy[END_REF]. As mentioned in section 3.2, PAs have been incorporated into the pharmaceutical materials by wetting with PA solution [START_REF] Rossini | Dynamic nuclear polarization NMR spectroscopy of microcrystalline solids[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF]. However, such procedure can induce polymorphic transitions and desolvation [START_REF] Pinon | Polymorphs of Theophylline Characterized by DNP Enhanced Solid-State NMR[END_REF]. As a result, solvent-free procedures, such as spray drying and hot-melt extrusion [START_REF] Ong | Solvent-Free Dynamic Nuclear Polarization of Amorphous and Crystalline Ortho-Terphenyl[END_REF][START_REF] Ni | In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR[END_REF], has been developed to overcome these issues.

The sensitivity gain provided by DNP is particularly advantageous for detecting diluted APIs (with loadings often below 10%) in pharmaceutical formulations. Furthermore, the sensitivity of DNP allows the problem of signal overlap in pharmaceutical formulations to be circumvented by (i) the acquisition of 13 C throughbond and through-space homonuclear correlation 2D spectra [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF][START_REF] Ni | In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR[END_REF] and (ii) the detection of insensitive isotopes, such as 15 N [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF][START_REF] Veinberg | Natural abundance 14 N and 15 N solid-state NMR of pharmaceuticals and their polymorphs[END_REF] and 35 Cl [START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF], only present in APIs. 1 H-15 N through-space heteronuclear correlation 2D experiments have also been employed to probe the interactions between APIs and excipients [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF]. In addition, DNP measurements allow the API domain size to be estimated using spin diffusion models (see Figure 5) [START_REF] Pinon | Measuring Nano-to Microstructures from Relayed Dynamic Nuclear Polarization NMR[END_REF][START_REF] Rossini | Dynamic Nuclear Polarization Enhanced NMR Spectroscopy for Pharmaceutical Formulations[END_REF]. Besides APIs, DNP-NMR has been applied to study other materials made of small organic molecules, such as self-assembled 2'-deoxyguanosine derivative (see Figure 19) [START_REF] Märker | A New Tool for NMR Crystallography: Complete 13 C/ 15 N Assignment of Organic Molecules at Natural Isotopic Abundance Using DNP-Enhanced Solid-State NMR[END_REF], nanotubes made from cyclic diphenylalanine peptides with potential applications for self-cleaning surfaces or ultra-capacitors [START_REF] Takahashi | Towards Structure Determination of Self-Assembled Peptides using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Märker | Welcoming natural isotopic abundance in solid-state NMR: probing π-stacking and supramolecular structure of organic nanoassemblies using DNP[END_REF] and CO2-based organic frameworks [START_REF] Leclaire | Structure elucidation of a complex CO2-based organic framework material by NMR crystallography[END_REF]. DNP-NMR experiments have permitted the assignment of the 13 C and 15 N NMR signals of these self-assembled supramolecular solids by the acquisition of 13 C homonuclear and 13 C-15 N correlation 2D spectra at natural abundance. The observation of 13 C- 13 C proximities has also provided information about the formation of hydrogen bonds and π-π stacking. DNP-NMR has also been used to study the dynamics of frozen non-ionic surfactants [START_REF] Hoffmann | Unusual Local Molecular Motions in the Solid State Detected by Dynamic Nuclear Polarization Enhanced NMR Spectroscopy[END_REF] and the distribution of Li + ions in frozen ionic liquid salt solution [START_REF] Sani | Probing Ionic Liquid Electrolyte Structure via the Glassy State by Dynamic Nuclear Polarization NMR Spectroscopy[END_REF].

Polymers

DNP-NMR has been applied to the characterization of organic polymers. As seen in Figure 21, the sensitivity provided by DNP has notably been used to observe the 13 C NMR signals of chain-ends of living polystyrene (PS) and poly(ethylene oxide) [START_REF] Le | Up to 100% Improvement in Dynamic Nuclear Polarization Solid-State NMR Sensitivity Enhancement of Polymers by Removing Oxygen[END_REF][START_REF] Ouari | Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy[END_REF]. DNP has also been used to probe the surface of cellulose nanocrystals or fibers. For instance, for cellulose nanocrystals functionalized with rhodamine spiroamide groups, DNP was used to determine the structural changes of immobilized groups after treatment with external stimuli [START_REF] Zhao | Multiresponsive cellulose nanocrystal-rhodamine conjugates : an advanced structure study by solid-state dynamic nuclear polarization ( DNP ) NMR[END_REF]. For imidazole-doped cellulose, an excellent proton conductor, DNP was also employed to observe 15 N signals of imidazole molecules at the surface of cellulose fibers [START_REF] Zhao | Imidazole-Doped Cellulose as Membrane for Fuel Cells: Structural and Dynamic Insights from Solid-State NMR[END_REF]. DNP has also been employed to probe the surface of paper fibers functionalized with poly(benzyl methacrylate) [START_REF] Gutmann | Dynamic Nuclear Polarization Signal Amplification as a Sensitive Probe for Specific Functionalization of Complex Paper Substrates[END_REF]. The atomic-level structure of microporous organic polymers has also been characterized by DNP-NMR [START_REF] Blanc | Dynamic nuclear polarization NMR spectroscopy allows high-throughput characterization of microporous organic polymers[END_REF][START_REF] Woodward | Swellable, Water-and Acid-Tolerant Polymer Sponges for Chemoselective Carbon Dioxide Capture[END_REF]. The enhancement of surface signals using DNP has been used to prove the core-shell structures of polymer nanoparticles [START_REF] Schäfer | Selective DNP Signal Amplification To Probe Structures of Core-Shell Polymer Hybrid Nanoparticles[END_REF]. Insights into the atomic-level structure of raw and catalytically treated biomass samples as well as amorphous photocatalytic polymers have also been obtained using DNP-enhanced 13 C INADEQUATE experiments at natural abundance [START_REF] Perras | Atomic-Level Structure Characterization of Biomass Pre-and Post-Lignin Treatment by Dynamic Nuclear Polarization-Enhanced Solid-State NMR[END_REF][START_REF] Brownbill | Structural Elucidation of Amorphous Photocatalytic Polymers from Dynamic Nuclear Polarization Enhanced Solid State NMR[END_REF]. The sensitivity provided by DNP has also permitted the characterization of the backbone conformation and the intermolecular stacking arrangement of thin-films made of donor-acceptor conjugated copolymers, whereas conventional NMR experiments were unfeasible owing to the very small amount of material used [START_REF] Chaudhari | Donor-acceptor stacking arrangements in bulk and thin-film high-mobility conjugated polymers characterized using molecular modelling and MAS and surface-enhanced solid-state NMR spectroscopy[END_REF]. 13 C CPMAS spectrum with microwave irradiation of the same polymer doped with 0.5 wt% bCTbk using the film casting procedure at 105 K. Both spectra were acquired at 9.4 T with νR = 10 kHz. Figure adapted from ref. [START_REF] Ouari | Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy[END_REF]. Copyright 2013, American Chemical Society.

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). Therefore, DNP has been mainly applied to the analysis of high-surface area inorganic materials, including micro-and meso-porous materials as well as nanoparticles. Such DNP SENS experiments have previously been reviewed in ref. [START_REF] Rossini | Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF].

Mesoporous materials

Mesoporous materials are materials containing pores with diameters between 2 and 50 nm. Hence, the pores of these materials are generally accessible to the PAs, except for the materials with the smallest pore sizes when using large PAs, such as TEKPol, for which the largest dimension is ~2 nm [START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF] or when the pores are filled with surfactants [START_REF] Lafon | Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13 C and 29 Si NMR Enhanced by Dynamic Nuclear Polarization[END_REF].

DNP has been used to characterize mesoporous silica functionalized by organic species (see Figure 1c) [START_REF] Lesage | Surface Enhanced NMR Spectroscopy by Dynamic Nuclear Polarization[END_REF][START_REF] Lafon | Mesoporous Silica Nanoparticles Loaded with Surfactant: Low Temperature Magic Angle Spinning 13 C and 29 Si NMR Enhanced by Dynamic Nuclear Polarization[END_REF][START_REF] Kobayashi | Analysis of Sensitivity Enhancement by Dynamic Nuclear Polarization in Solid-State NMR: a Case Study of Functionalized Mesoporous Materials[END_REF][START_REF] Lelli | Fast Characterization of Functionalized Silica Materials by Silicon-29 Surface-Enhanced NMR Spectroscopy Using Dynamic Nuclear Polarization[END_REF][START_REF] Kobayashi | Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR[END_REF][START_REF] Kobayashi | Measuring Long-Range 13 C-13 C Correlations on a Surface under Natural Abundance Using Dynamic Nuclear Polarization-Enhanced Solid-State Nuclear Magnetic Resonance[END_REF][START_REF] Werner | Synthesis and solid state NMR characterization of novel peptide/silica hybrid materials[END_REF][START_REF] Hamzaoui | Atomic-level organization of vicinal acid-base pairs through the chemisorption of aniline and derivatives onto mesoporous SBA15[END_REF] or metal complexes [START_REF] Berruyer | Three-Dimensional Structure Determination of Surface Sites[END_REF][START_REF] Conley | A well-defined Pd hybrid material for the Z-selective semihydrogenation of alkynes characterized at the molecular level by DNP SENS[END_REF][START_REF] Samantaray | Evidence for Metal-Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru-NHC Alkene Metathesis Catalysts[END_REF][START_REF] Eedugurala | Mesoporous Silica-Supported Amidozirconium-Catalyzed Carbonyl Hydroboration[END_REF][START_REF] Romanenko | Iridium(I)/N-Heterocyclic Carbene Hybrid Materials: Surface Stabilization of Low-Valent Iridium Species for High Catalytic Hydrogenation Performance[END_REF][START_REF] Liu | Revealing Structure Reactivity Relationships in Heterogenized Dirhodium Catalysts by Solid-State NMR Techniques[END_REF]. The sensitivity provided by DNP is particularly useful for the detection of insensitive nuclei, such as 15 N [194,208,209,212,214,215]. For instance, DNP-enhanced 1 H→ 15 N CPMAS experiments have supported the formation of O3SiOZr(NMe2)3 species with Me = CH3 by reaction between Zr(NMe2)3 and mesoporous silica [START_REF] Eedugurala | Mesoporous Silica-Supported Amidozirconium-Catalyzed Carbonyl Hydroboration[END_REF] or have permitted the distinction of covalently bound and adsorbed peptides at the surface of carboxylate-functionalized mesoporous silica [START_REF] Werner | Synthesis and solid state NMR characterization of novel peptide/silica hybrid materials[END_REF]. DNP-enhanced 1 H→ 13 C and 1 H→ 15 N CPMAS experiments have enabled the determination of the binding modes of dirhodium(II) complexes anchored on the surface of mesoporous silica functionalized by amine and carboxylate groups [START_REF] Liu | Revealing Structure Reactivity Relationships in Heterogenized Dirhodium Catalysts by Solid-State NMR Techniques[END_REF][START_REF] Gutmann | Natural Abundance 15 N NMR by Dynamic Nuclear Polarization: Fast Analysis of Binding Sites of a Novel Amine-Carboxyl-Linked Immobilized Dirhodium Catalyst[END_REF]. DNP-enhanced 1 H→ 13 C and 1 H→ 29 Si CP-HETCOR 2D spectra have also revealed interactions between the silica surface and ligands of supported complexes of Pd, Ir and Ru metals [START_REF] Conley | A well-defined Pd hybrid material for the Z-selective semihydrogenation of alkynes characterized at the molecular level by DNP SENS[END_REF][START_REF] Samantaray | Evidence for Metal-Surface Interactions and Their Role in Stabilizing Well-Defined Immobilized Ru-NHC Alkene Metathesis Catalysts[END_REF][START_REF] Romanenko | Iridium(I)/N-Heterocyclic Carbene Hybrid Materials: Surface Stabilization of Low-Valent Iridium Species for High Catalytic Hydrogenation Performance[END_REF]. As seen in Figure 22, 13 C{ 29 Si} and 29 Si{ 15 N} REDOR experiments have been used to determine the 3D structure of a silica-supported Pt complex [START_REF] Berruyer | Three-Dimensional Structure Determination of Surface Sites[END_REF]. The determined structure is consistent with an interaction between the Pt metal center and the silica surface oxygen atoms.

The sensitivity gain of DNP has also allowed probing the 13 C- 13 C and 29 Si- 29 Si proximities at the surface of mesoporous silica at natural abundance. For instance, CHHC 2D experiment has also been used to probe proximities between phenyl and mercaptopropyl moieties grafted at the surface of mesoporous silica [START_REF] Kobayashi | Measuring Long-Range 13 C-13 C Correlations on a Surface under Natural Abundance Using Dynamic Nuclear Polarization-Enhanced Solid-State Nuclear Magnetic Resonance[END_REF]. Furthermore, 29 Si 2Q-1Q through-space homonuclear correlation experiments have revealed that post-synthesis grafting leads to more homogeneous distribution of grafted organic moieties than co-condensation [START_REF] Kobayashi | Spatial distribution of organic functional groups supported on mesoporous silica nanoparticles: a study by conventional and DNP-enhanced 29Si solid-state NMR[END_REF]. DNP has also enabled the detection of 17 O NMR signals in natural isotopic abundance of silanol groups at the surface of mesoporous silica [START_REF] Perras | Natural Abundance 17 O DNP Two-Dimensional and Surface-Enhanced NMR Spectroscopy[END_REF][START_REF] Perras | Probing Surface Hydrogen Bonding and Dynamics by Natural Abundance, Multidimensional, 17 O DNP-NMR Spectroscopy[END_REF]. These DNP-enhanced 17 O experiments have shown that drying leads to the selective dehydroxylation of silanols involved in strong hydrogen bonds.

Besides mesoporous silica, DNP has been applied for other mesoporous materials, including mesoporous alumina [START_REF] Lee | Enhanced Solid-State NMR Correlation Spectroscopy of Quadrupolar Nuclei Using Dynamic Nuclear Polarization[END_REF], periodic mesoporous organosilicate and its postsynthesis functionalized organometallic derivatives [START_REF] Grüning | Molecular-level characterization of the structure and the surface chemistry of periodic mesoporous organosilicates using DNP-surface enhanced NMR spectroscopy[END_REF] as well as Ca3Al2O6-stabilized CaO [START_REF] Kim | In Situ XRD and Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy Unravel the Deactivation Mechanism of CaO-Based, Ca3Al2O6-Stabilized CO2 Sorbents[END_REF]. For this latter material, which is a CO2 sorbent, DNP-enhanced 27 Al NMR has been employed to show the formation of Al2O3 at the surface of the mesopores after around 10 cycles of CO2 capture and sorbent regeneration. [START_REF] Berruyer | Three-Dimensional Structure Determination of Surface Sites[END_REF].

Copyright 2017, American Chemical Society.

Microporous materials

Microporous materials contain pores with diameters less than 2 nm. Hence, the pores of these materials may not be accessible to PAs and the polarization is transported into the particles via 1 H-1 H spin diffusion, notably via the protons of the frozen solvent contained within the micropores [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy of Functionalized Metal-Organic Frameworks[END_REF][START_REF] Guo | Selective Host-Guest Interaction between Metal Ions and Metal-Organic Frameworks Using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF][START_REF] Wolf | NMR signatures of the active sites in Sn-β zeolite[END_REF][START_REF] Gunther | Dynamic Nuclear Polarization NMR Enables the Analysis of Sn-Beta Zeolite Prepared with Natural Abundance 119Sn Precursors[END_REF].

DNP has been applied to detect the 15 N signals of peptide-functionalized MOFs at natural abundance [START_REF] Rossini | Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy of Functionalized Metal-Organic Frameworks[END_REF][START_REF] Todorova | Molecular Level Characterization of the Structure and Interactions in Peptide-Functionalized Metal-Organic Frameworks[END_REF]. The comparison of experimental 15 N chemical shifts with those calculated using density functional theory shows that the conformation of the grafted peptide is constrained by hydrogen bonds with the MOF framework. DNPenhanced 1 H→ 15 N CPMAS experiments have also been used to probe the interactions between Pt 2+ and Cu 2+ ions and amine-functionalized zirconium-based MOF [START_REF] Guo | Selective Host-Guest Interaction between Metal Ions and Metal-Organic Frameworks Using Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy[END_REF]. For these materials, 1 H→ 195 Pt Brain-CP experiments enabled to distinguish signals from cis and trans-coordinated Pt 2+ ions [START_REF] Kobayashi | DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks[END_REF]. Furthermore, the estimate of 13 C- 27 Al dipolar couplings in aluminium-based MOF has demonstrated that the impregnation with a radical solution and the freezing of the sample down to 100 K do not alter the MOF structure [START_REF] Pourpoint | Probing 27 Al-13 C proximities in metal-organic frameworks using dynamic nuclear polarization enhanced NMR spectroscopy[END_REF]. DNP has also been employed for the characterization of zeolites. DNP-enhanced 119 Sn NMR experiments have been used to characterize the various Sn sites in zeolites substituted by Sn atoms in hydrated and dehydrated states [START_REF] Wolf | NMR signatures of the active sites in Sn-β zeolite[END_REF][START_REF] Gunther | Dynamic Nuclear Polarization NMR Enables the Analysis of Sn-Beta Zeolite Prepared with Natural Abundance 119Sn Precursors[END_REF][START_REF] Wolf | Correlating Synthetic Methods, Morphology, Atomic-Level Structure, and Catalytic Activity of Sn-β Catalysts[END_REF][START_REF] Harris | Molecular Structure and Confining Environment of Sn Sites in Single-Site Chabazite Zeolites[END_REF]. DNP-enhanced 29 Si refocused INADEQUATE 2D experiments have been used to locate the silanol groups at the interlayer surface of the calcined form of the zeolite SSZ-70 (see Figure 23) [START_REF] Smeets | Well-Defined Silanols in the Structure of the Calcined High-Silica Zeolite SSZ-70: New Understanding of a Successful Catalytic Material[END_REF]. DNP-NMR has also been applied to investigate the organic species in the zeolite micropores [START_REF] Xiao | Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR[END_REF][START_REF] Ben Abda | Investigating the interaction of two asymmetric dioxins with FAU 13X zeolites using calorimetry and solid-state NMR[END_REF][START_REF] Mance | A DNP-supported solid-state NMR study of carbon species in fluid catalytic cracking catalysts[END_REF]. DNP has notably provided information about the localization of coke in commercial fluid catalytic cracking catalysts [START_REF] Mance | A DNP-supported solid-state NMR study of carbon species in fluid catalytic cracking catalysts[END_REF] and the structure of carbocations on zeolites and their interactions with the frameworks [START_REF] Xiao | Fast detection and structural identification of carbocations on zeolites by dynamic nuclear polarization enhanced solid-state NMR[END_REF].

Nanoparticles and nanosheets

Nanoparticles are particles between 1 and 100 nm in size, whereas nanosheets are two-dimensional nanostructures with a thickness in a scale ranging from 1 to 100 nm. The surface of these nanostructures is generally accessible to the PAs, except when the nanostructures aggregate [START_REF] Pump | Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity[END_REF][START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF]. DNP has been applied to characterize the surface of nanoparticles used for catalysis [START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF][START_REF] Vitzthum | Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surfaceenhanced aluminium-27 NMR[END_REF][START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF][START_REF] Conley | Silica-surface reorganization during organotin grafting evidenced by 119 Sn DNP SENS: a tandem reaction of gem-silanols and strained siloxane bridges[END_REF][START_REF] Lilly Thankamony | Insights into the Catalytic Activity of Nitridated Fibrous Silica (KCC-1) Nanocatalysts from 15 N and 29 Si NMR Spectroscopy Enhanced by Dynamic Nuclear Polarization[END_REF], biomaterials [START_REF] Leroy | Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization[END_REF][START_REF] Lee | Interfacial Ca 2+ environments in nanocrystalline apatites revealed by dynamic nuclear polarization enhanced 43 Ca NMR spectroscopy[END_REF], cements [START_REF] Kumar | The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate[END_REF], polymer fillers [START_REF] Lafon | Indirect and direct 29 Si dynamic nuclear polarization of dispersed nanoparticles[END_REF][START_REF] Ziarelli | Dynamic nuclear polarisation NMR of nanosized zirconium phosphate polymer fillers[END_REF] and optoelectronics devices [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF][START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF].

DNP has notably been used to probe the structure of the surface sites of functionalized silica nanoparticles. DNP-enhanced 1 H→ 13 C CPMAS and HETCOR experiments have been used to show the formation of tantalum alkoxides by reaction between silica-supported tantalum alkyl complexes and oxygen [START_REF] Mohandas | From single-site tantalum complexes to nanoparticles of TaxNy and TaOxNy supported on silica: elucidation of synthesis chemistry by dynamic nuclear polarization surface enhanced NMR spectroscopy and X-ray absorption spectroscopy[END_REF]. DNP-enhanced 1 H→ 15 N CPMAS experiments at natural abundance have been used to identify the surface amine sites formed during the reaction of dendritic fibrous silica nanoparticles with ammonia and hence, to explain the activity of this solid base [START_REF] Lilly Thankamony | Insights into the Catalytic Activity of Nitridated Fibrous Silica (KCC-1) Nanocatalysts from 15 N and 29 Si NMR Spectroscopy Enhanced by Dynamic Nuclear Polarization[END_REF]. DNP-enhanced 1 H→ 89 Y CPMAS experiments have also been applied to characterize the silica-supported Y sites [START_REF] Delley | Local Structures and Heterogeneity of Silica-Supported M(III) Sites Evidenced by EPR, IR, NMR, and Luminescence Spectroscopies[END_REF]. DNP-enhanced 119 Sn NMR has allowed the observation of silica-supported Sn sites and hence, demonstrated the tandem reaction between allyltributylstannane and geminal silanol groups and strained siloxane bridges [START_REF] Conley | Silica-surface reorganization during organotin grafting evidenced by 119 Sn DNP SENS: a tandem reaction of gem-silanols and strained siloxane bridges[END_REF]. DNP-enhanced 29 Si through-space and through-bond 2Q-1Q homonuclear correlation experiments have shown the lateral polymerization of siloxanes at the surface of silica nanoparticles [START_REF] Lee | Untangling the Condensation Network of Organosiloxanes on Nanoparticles using 2D 29 Si-29 Si Solid-State NMR Enhanced by Dynamic Nuclear Polarization[END_REF]. The structure of reaction intermediates for silica-supported tungsten metathesis catalysts has been determined using 13 C through-space and trough-bond 2Q-1Q homonuclear correlation experiments [START_REF] Ong | Atomistic Description of Reaction Intermediates for Supported Metathesis Catalysts Enabled by DNP SENS[END_REF].

The surface Al and O sites of γ-alumina nanoparticles have also been observed using DNP-enhanced 27 Al and 17 O MQMAS experiments (see Figure 16) [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF][START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al 3+ Ions Are Only Located at the Surface of Hydrated γ-Alumina[END_REF][START_REF] Vitzthum | Dynamic nuclear polarization of quadrupolar nuclei using cross polarization from protons: surfaceenhanced aluminium-27 NMR[END_REF][START_REF] Li | Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy[END_REF]. DNPenhanced 1 H→ 13 C CPMAS experiments also permitted the detection of Pd 2+ complexes at the surface of γ-alumina [START_REF] Mouat | Synthesis of Supported Pd0 Nanoparticles from a Single-Site Pd 2+ Surface Complex by Alkene Reduction[END_REF]. DNP-enhanced 1 H→ 15 N CPMAS experiments have also been employed to measure the 15 N isotropic chemical shift of pyridine adsorbed on γalumina via hydrogen bonds or coordination to the surface Lewis acid sites [START_REF] Moroz | Discerning γ-Alumina Surface Sites with Nitrogen-15 Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine[END_REF]. These results show that the 15 N signal of pyridine is more shielded for higher adsorption energy. DNP-enhanced 13 C through-space and through-bond homonuclear correlations as well as 13 C{ 27 Al} RESPDOR experiments have been applied to identify the products from the oxidative degradation of methionine adsorbed on γ-alumina-supported Pd nanoparticles [START_REF] Johnson | Identifying low-coverage surface species on supported noble metal nanoparticle catalysts by DNP-NMR[END_REF][START_REF] Perras | Characterizing Substrate-Surface Interactions on Alumina-Supported Metal Catalysts by Dynamic Nuclear Polarization-Enhanced Double-Resonance NMR Spectroscopy[END_REF]. DNP-enhanced NMR experiments have also been used to observe the surface sites of amorphous silica alumina and the deposition of silica over alumina as well as that of alumina over silica [START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF][START_REF] Rankin | Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy[END_REF][START_REF] Perras | Natural Abundance 17 O DNP NMR Provides Precise O-H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts[END_REF][START_REF] Mouat | Highly Dispersed SiOx/Al2O3 Catalysts Illuminate the Reactivity of Isolated Silanol Sites[END_REF]. DNP-enhanced 29 Si- 27 Al through-bond and through-space RINEPT experiments have notably revealed that SiO4 sites form covalent bonds with AlO4 and AlO5 species (see Figure 18) [START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF][START_REF] Rankin | Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy[END_REF]. DNP-enhanced 17 O NMR has also allowed the observation of Brønsted acid sites at the surface of silica alumina [START_REF] Perras | Natural Abundance 17 O DNP NMR Provides Precise O-H Distances and Insights into the Brønsted Acidity of Heterogeneous Catalysts[END_REF]. Furthermore, the H-O distances determined from the measured 1 H- 17 O dipolar couplings were correlated to the pH of the zero point of charge of those materials.

DNP has also been applied for the characterization of ceria nanoparticles, pristine or functionalized with surfactants [START_REF] Hope | Surfaceselective direct 17 O DNP NMR of CeO2 nanoparticles[END_REF]229]. DNP-enhanced 1 H→ 29 Si CP-HECTOR and 29 Si refocused INADEQUATE experiments have been used to observe 1 H- 29 Si proximities and 29 Si-O- 29 Si connectivities in calcium silicate hydrate, which form nanoparticles and nanosheets depending on Ca:Si ratio [START_REF] Kumar | The Atomic-Level Structure of Cementitious Calcium Silicate Hydrate[END_REF]. These structural constraints in combination with atomistic simulations and density functional theory calculations of chemical shifts have been permitted to solve the three-dimensional structure of these materials, which are the primary binding component of concrete. 13 C-31 P CP-HETCOR experiments have also been used to evidence the formation of P-O-C chemical bonds at the surface zirconium phosphate nanoparticles used as polymer fillers [START_REF] Ziarelli | Dynamic nuclear polarisation NMR of nanosized zirconium phosphate polymer fillers[END_REF]. Moreover, DNP has been employed to characterize hydroxyapatite nanoparticles, which are the key minerals in bones and teeth [START_REF] Leroy | Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization[END_REF][START_REF] Lee | Interfacial Ca 2+ environments in nanocrystalline apatites revealed by dynamic nuclear polarization enhanced 43 Ca NMR spectroscopy[END_REF]. For those nanoparticles, DNP-enhanced 1 H→ 43 Ca CPMAS and CP-HETCOR experiments have allowed for the detection of 43 Ca nuclei at natural abundance and the differentiation of surface and core species [START_REF] Lee | Interfacial Ca 2+ environments in nanocrystalline apatites revealed by dynamic nuclear polarization enhanced 43 Ca NMR spectroscopy[END_REF]. Furthermore, DNP-enhanced 13 C 2Q-1Q through-space homonuclear correlation and 31 P→ 13 C CP-HETCOR experiments indicate that there is a local ordering for the substitution of HO - and PO4 3-sites by carbonates and the hydroxyapatite nanoparticles is made mainly of domains containing clustered carbonates substituting neighboring PO4 3-sites and to a lower extent, neighboring HO -and PO4 3-sites [START_REF] Leroy | Hydroxyapatites: Key Structural Questions and Answers from Dynamic Nuclear Polarization[END_REF]. DNP-enhanced 1 H→ 119 Sn CPMAS experiments with CPMG detection have also permitted the detection of the amorphous SnO2 layer at the surface of ligand-capped partially oxidized Sn nanoparticles, which are used as high-density anode materials for rechargeable Li-ion batteries [START_REF] Protesescu | Unraveling the Core-Shell Structure of Ligand-Capped Sn/SnOx Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mössbauer, and X-ray absorption spectroscopies[END_REF].

DNP has also been used to study crystalline semiconductors, such as CdSe, CdS or InP, in the form of nanoparticles (also called quantum dots) and nanosheets (also called nanoplatelets) [START_REF] Piveteau | Structure of Colloidal Quantum Dots from Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy[END_REF][START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF]. These materials are of interest due to their electronic and photophysical properties and emerging commercial applications, such as television display. DNP-enhanced 31 P and 113 Cd NMR spectra of those materials have allowed signals of surface and core sites to be distinguished. Furthermore, PASS technique has been applied to discern the isotropic chemical shifts of the various 113 Cd sites and to estimate the 113 Cd CSA (see Figure 24). 113 Cd PASS 2D spectrum enhanced by phase-incremented echo-train acquisition (PIETA) of the oleate-capped core/crown CdSe/CdS nanoplatelets sol in TCE mixed with 50 mM TEKPOL solution in TCE impregnating a mesoporous silica at 14.1 T and 100 K with νR = 10 kHz. Four Cd species can be resolved and assigned to cadmium in the CdS core (57 ppm), the CdSe core (-61 ppm), and to Cd-oleate at the surface of CdS (-281 ppm) and CdSe (-310 ppm). Figure adapted from ref. [START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF]. Copyright 2018, American Chemical Society.

Microparticles

Microparticles are particles between 0.1 and 100 μm in size. Hence, the nuclei in the microparticle core are distant from the PAs, except when the core is doped with PAs [START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF]. Nevertheless, for proton-containing microparticles, the 1 H-1 H spin diffusion can transport the DNP-enhanced 1 H polarization (see section 2.2) and significant DNP enhancements have been reported for several proton-containing inorganic or hybrid microparticles [START_REF] Kobayashi | Mechanism of Solid-State Thermolysis of Ammonia Borane: A 15 N NMR Study Using Fast Magic-Angle Spinning and Dynamic Nuclear Polarization[END_REF][START_REF] Blanc | Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3[END_REF][START_REF] Leskes | Surface-Sensitive NMR Detection of the Solid Electrolyte Interphase Layer on Reduced Graphene Oxide[END_REF][START_REF] Kobayashi | DNP-enhanced ultrawideline 207 Pb solid-state NMR spectroscopy: an application to cultural heritage science[END_REF][START_REF] Pourpoint | Probing the aluminum complexation by Siberian riverine organic matter using solid-state DNP-NMR[END_REF]. For instance, DNP-enhanced 1 H-89 Y CP-HETCOR 2D spectrum has been used to assign the 1 H and 89 Y signals of hydrated yttrium-doped BaZrO3 microparticles with diameter up to 45 μm, which are potential proton conductors for intermediate temperature solid oxide fuel cells [START_REF] Blanc | Dynamic Nuclear Polarization NMR of Low-γ Nuclei: Structural Insights into Hydrated Yttrium-Doped BaZrO3[END_REF]. DNP-enhanced 15 N NMR has also been employed to probe the N environments in ammonia borane samples treated at various temperatures with diameter up to 8 μm [START_REF] Kobayashi | Mechanism of Solid-State Thermolysis of Ammonia Borane: A 15 N NMR Study Using Fast Magic-Angle Spinning and Dynamic Nuclear Polarization[END_REF]. These NMR data provided insights into the dehydrogenation mechanism of ammonia borane, which is a promising material for hydrogen storage. DNP-enhanced 13 C{ 27 Al} RESPDOR experiments have also been used to prove the complexation of Al 3+ ions by carboxylate groups of dissolved organic matter (DOM) of Kochechum river located in central Siberia [START_REF] Pourpoint | Probing the aluminum complexation by Siberian riverine organic matter using solid-state DNP-NMR[END_REF]. Such complexation explains the parallel variation of the concentration of DOM and Al 3+ ions in the course of the hydrological year. DNP has also been used to enhance the 13 C signals of the outer layers of solid electrolyte interphase for reduced grapheme oxide anode cycled in Li-ion cells [START_REF] Leskes | Surface-Sensitive NMR Detection of the Solid Electrolyte Interphase Layer on Reduced Graphene Oxide[END_REF]. These NMR data indicate a difference in the chemical composition of the inner and outer layers of solid electrolyte interphase. DNP has also permitted the detection of 207 Pb signals of (i) lead white, a lead-based pigment, (ii) lead palmitate, a lead soap, i.e. a degradation product of lead-based oil painting produced by the reaction between lead white and palmitic acids, and (iii) lead soap in aged painted film [START_REF] Kobayashi | DNP-enhanced ultrawideline 207 Pb solid-state NMR spectroscopy: an application to cultural heritage science[END_REF]. Furthermore, in the case of inorganic microparticles, which do not contain proton, the indirect DNP has been used to detect the signals of nuclei near the particle surface. A challenge is the low surface area of microparticles, which leads to a small fraction of nuclei in the vicinity of surface. Nevertheless, DNP has been successfully employed to probe the molecular-level interactions between the surface sites of triclinic Ca3SiO5 microparticles, a major component of aluminosilicate cement, with a surface area of 1 m 2 .g -1 and hydration inhibitors (sucrose or methylenephosphonic acid) at a loading of 0.1% by weight of solids in the presence of water [START_REF] Sangodkar | Influences of Dilute Organic Adsorbates on the Hydration of Low-Surface-Area Silicates[END_REF].

DNP has also been applied to enhance the NMR signals of the core region of inorganic microparticles. Such enhancement is possible, when the core of the microparticles contains PAs. DNP has particularly been used to detect the distinct 17 O sites of crystalline inorganic microparticles doped with Mn(II) ions, including battery anode materials Li4Ti5O12 and Li2ZnTi3O8, as well as the materials NaCaPO4 and MgAl2O4 (see Figure 25) [START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF].

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, the better understanding of DNP mechanisms under MAS have significantly improved the sensitivity of this technique and broaden its application fields. Currently MAS DNP has been applied to a wide range of materials, including organic, hybrid and inorganic materials with applications to fields, such as catalysis, polymers, health, optoelectronics… The DNP sensitivity enhancement has notably provided unique insights for the characterization of pharmaceutical formulations and the surface of inorganic particles and mesoporous materials.

Nevertheless, the polarization enhancements produced by DNP with respect to thermal equilibrium are still much lower than the maximum theoretical enhancement given by the ratio of the gyromagnetic ratios of the unpaired electron and the polarized isotope, γ(S)/γ(I). For instance, for a frozen solution of 20 mM [ 13 C]-urea and 10 mM AsymPolPOK in glycerol/ 2 H2O/H2O (60/30/10 v/v/v) at 9.4 T and 105 K with νR = 10 kHz, DNP yields a 83-fold enhancement of the nuclear polarization, which is still 8 times smaller than γ(S)/γ( 1 H) ≈ 660 [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF]. Furthermore, the DNP enhancement decreases at higher B0 field. For a frozen solution of 20 M [ 13 C]-urea and 10 mM TEMTriPol-1 in glycerol/ 2 H2O/H2O (60/30/10 v/v/v) at 18.8 T and 125 K with νR = 10 kHz, the polarization enhancement is only equal to 51 [START_REF] Mentink-Vigier | Efficient cross-effect dynamic nuclear polarization without depolarization in high-resolution MAS NMR[END_REF].

In the future, promising approaches to further improve the absolute sensitivity of MAS DNP include the developments of (i) optimized PAs, notably for high magnetic fields [START_REF] Mathies | Efficient Dynamic Nuclear Polarization at 800 MHz/527 GHz with Trityl-Nitroxide Biradicals[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF], (ii) MAS at temperatures below 77 K with closed-loop cold helium system [START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF][START_REF] Thurber | Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source[END_REF][START_REF] Matsuki | Helium-cooling and -spinning dynamic nuclear polarization for sensitivityenhanced solid-state NMR at 14T and 30K[END_REF][START_REF] Lee | Ultra-low temperature MAS-DNP[END_REF][START_REF] Sesti | Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K[END_REF][START_REF] Thurber | Low-temperature dynamic nuclear polarization with helium-cooled samples and nitrogen-driven magic-angle spinning[END_REF], (iii) improved MAS DNP-NMR probes with better coupling between the incident microwave and the sample space [START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF], (iv) frequency-swept DNP microwave sources, which can improve electron decoupling, and hence, reduce linewidth and enhance signal intensity [START_REF] Sesti | Electron decoupling with cross polarization and dynamic nuclear polarization below 6 K[END_REF][START_REF] Saliba | Electron Decoupling with Dynamic Nuclear Polarization in Rotating Solids[END_REF], and (v) DNP microwave sources capable of phase coherent pulses with a length of nanoseconds to microseconds [START_REF] Nanni | Photonic-Band-Gap Traveling-Wave Gyrotron Amplifier[END_REF] combined with pulsed DNP techniques [START_REF] Can | Time domain DNP with the NOVEL sequence[END_REF][START_REF] Jain | Off-resonance NOVEL[END_REF][START_REF] Can | Frequency-Swept Integrated Solid Effect[END_REF][START_REF] Can | Frequency-Swept Integrated and Stretched Solid Effect Dynamic Nuclear Polarization[END_REF][START_REF] Tan | Time-optimized pulsed dynamic nuclear polarization[END_REF]. These latter DNP techniques may improve the sensitivity at high fields since their efficiency is a priori independent of the B0 field. Furthermore, they do not require biradical PAs contrary to CE mechanism and may be applicable at higher temperatures than continuous wave DNP.

DNP experiments at higher MAS frequencies are also desirable for (i) the acquisition of high-resolution 1 H NMR spectra and (ii) the use of indirect detection via protons [START_REF] Ishii | Sensitivity Enhancement in Solid State 15 N NMR by Indirect Detection with High-Speed Magic Angle Spinning[END_REF]. Higher MAS frequencies can be achieved by the development of (i) DNP-NMR probes for rotors with outer diameter smaller than 1.3 mm and (ii) spinning systems using helium gas (see section 4.5) [START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF]. The small diameter rotors will also be useful for the study of volume-limited samples [START_REF] Chaudhari | Donor-acceptor stacking arrangements in bulk and thin-film high-mobility conjugated polymers characterized using molecular modelling and MAS and surface-enhanced solid-state NMR spectroscopy[END_REF].

The advent of lower-cost DNP-NMR instrumentations, including more economic microwave sources, such as klystrons [START_REF] Yau | Synthesis and evaluation of nitroxide-based oligoradicals for low-temperature dynamic nuclear polarization in solid state NMR[END_REF] or solid-state sources [START_REF] Thurber | Low-temperature dynamic nuclear polarization at 9.4 T with a 30 mW microwave source[END_REF], closed-loop cryogenic system for MAS of the sample [START_REF] Bouleau | Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning[END_REF][START_REF] Lee | Ultra-low temperature MAS-DNP[END_REF], and DNP-NMR probes for narrowbore NMR magnets, will further increase the availability of this technique and the number of users and potential applications.

The DNP-NMR study of materials also requires the optimization of the PAs and the sample preparation, notably to limit the reaction between the investigated material and the PAs [START_REF] Pump | Reactive surface organometallic complexes observed using dynamic nuclear polarization surface enhanced NMR spectroscopy[END_REF][START_REF] Pump | Predicting the DNP-SENS efficiency in reactive heterogeneous catalysts from hydrophilicity[END_REF][START_REF] Liao | Dendritic polarizing agents for DNP SENS[END_REF], to avoid the use of solvents [START_REF] Takahashi | Rapid Natural-Abundance 2D 13 C-13 C Correlation Spectroscopy Using Dynamic Nuclear Polarization Enhanced Solid-State NMR and Matrix-Free Sample Preparation[END_REF][START_REF] Ouari | Improved Structural Elucidation of Synthetic Polymers by Dynamic Nuclear Polarization Solid-State NMR Spectroscopy[END_REF][START_REF] Ni | In Situ Characterization of Pharmaceutical Formulations by Dynamic Nuclear Polarization Enhanced MAS NMR[END_REF] or to enhance the signals of the bulk region of inorganic materials [START_REF] Corzilius | Dynamic Nuclear Polarization of 1 H, 13 C, and 59 Co in a Tris(ethylenediamine) cobalt (III) Crystalline Lattice Doped with Cr (III)[END_REF][START_REF] Wolf | Endogenous Dynamic Nuclear Polarization for Natural Abundance 17 O and Lithium NMR in the Bulk of Inorganic Solids[END_REF].

Further improvement in the sensitivity of DNP-NMR will open new avenues for the characterization of materials. It will particularly allow for the observation of diluted sites corresponding to amounts of matter in the nanomole-scale range or below. In particular, these amounts are those of the surface sites of materials with low surface area, such as silicon wafers used in microelectronics with surface area in the order of 10 -3 mm 2 .g -1 . High DNP enhancements at very high magnetic fields, B0 > 17 T, will notably offer the new possibilities for the detection of quadrupolar nuclei [START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF][START_REF] Brownbill | Oxygen-17 dynamic nuclear polarisation enhanced solid-state NMR spectroscopy at 18.8 T[END_REF] and notably those with low gyromagnetic ratio, such as 25 Mg, 33 S, 35 Cl, 67 Zn, 95 Mo… The above developments will expand the scope of the application of MAS DNP-NMR.

Figure 2 .

 2 Figure 2. Number of publications per year relating to MAS DNP-NMR spectroscopy of organic (red) and hybrid or inorganic (green) materials since the introduction of this technique.

Figure 3 .

 3 Figure 3. Evolution of the polarizations of unpaired electrons, Pi(t) with i = a or b (top), the energy levels (middle) and the nuclear polarization, Pn(t) (bottom), of a spin system containing two coupled unpaired electrons and a nucleus during a steady-state rotor period at B 0 = 9.4 T, a temperature of 100 K with νR = 3 kHz. The rotor-events marked on the figure are the (1) the electron-electron dipolar anti-crossing, (2) the CE anti-crossings and (3) the microwave crossings. Adapted with permission from ref. [54]. Copyright 2012, Elsevier.

Figure 4 .

 4 Figure 4. (a) Experimental EPR spectrum (solid line) of 15 mM frozen solution of TEKPol PA (see Figure 6) in a mixture of CHCl3/1,1,1,2-tetrabromoethane/[ 2 H4]-methanol at 100 K with a microwave frequency of 285 GHz. The best-fit simulation (dashed line) of the experimental spectrum is also displayed. The grey overlay corresponds to the error bar for the best-fit simulation. (b) Experimental (black circles) and simulated (red squares) MAS DNP field profile of the same sample as in subfigure a at 9.4 T and 110 K with νR = 8 kHz. Adapted with permission from ref. [60]. Copyright 2019, Royal Society of Chemistry.

Figure 5 .

 5 Figure 5. 1 H→ 13 C CPMAS experiments of microcrystalline powder of L-histidine hydrochloride monohydrate impregnated with 16 mM TEKPol (see Figure 6) in 1,1,2,2-tetrachloroethane (TCE) at 9.4 T and 105 K with νR = 8 kHz. (a) Spectrum with microwave irradiation and polarization delay of 100 s. (b,d) Normalized build-up and (c,e) DNP enhancement of the (b,c) TCE and (d,e) histidine signals as function of the polarization delay. In subfigure e, an inset shows an expansion of the region corresponding to short polarization delays. In subfigures d and e, the best fit curves to the numerical spin diffusion model are displayed as solid lines and corresponds to L = 2.3 μm, which agrees well with the histidine particle size measured by scanning electron microscopy. Figure adapted from ref. [77]. Copyright 2017, American Chemical Society.

Figure 6 .

 6 Figure 6. Structures and names of some of the organic radicals used as PA for high-field MAS DNP of materials. Figure adapted with permission from refs. [58,105,107-111]. Copyright 2011, Elsevier, 2013 and 2018 American Chemical Society, 2016, Wiley, 2018 Royal Society of Chemistry,

Figure 7 .

 7 Figure 7. Schemes depicting the spatial distributions of material, PAs and solvent for DNP samples of (a, b) porous and (c-e) particulate materials, in which PAs are incorporated using (a,c) incipient wetness impregnation or wetting,(c, d) the adsorption of PAs on material surface and (e) the doping of material bulk with PAs.

Figure 8 .

 8 Figure 8. (a) Plot of the relative sensitivity of 1 H→ 29 Si CPMAS experiments on non-porous silica nanoparticles with a BET surface area of 93.3 m 2 .g -1 wetted by a TEKPol solution in TCE as function of the TEKPol concentration. (b) Plot of the TEKPol concentration yielding the highest sensitivity as function of the surface area. (c) The most stable adsorption structure of a TEMPO molecule on β-SiO2 (111) surface calculated using DFT. The H, C, N, O and Si atoms are represented using white, gray, blue, red and brown spheres, respectively. Figure adapted with permission from ref. [138]. Copyright 2018, Elsevier.

Figure 9 .

 9 Figure 9. High-field MAS DNP-NMR system with gyrotron microwave source (gyrotron tube in red), microwave transmission line (cyan) and low-temperature NMR probe (green). Figure adapted with permission from ref. [141]. Copyright 2016, Elsevier.

Figure 10 .

 10 Figure 10. (a) Normalized DNP enhancement as function of microwave power for OE, SE and CE DNP. (b) Simulated spatial distribution of the microwave magnetic field in a radial plane (top) and an axial plane (bottom) of 3.2 mm sapphire rotor in 263 GHz MAS DNP-NMR system. The microwave electric field is polarized parallel to the rotor axis and is depicted as a green cross in the radial plane and a green arrow in the axial plane. (c) 1 H→ 31 P CP-STRAFI-MAS 2D image of DNP enhancement (left) and cross-section of the image along the rotor axis (right) for Na3PO4•12H2O wetted with a 16 mM TEKPol solution in TCE at 9.4 T and 100 K with νR = 10 kHz. Figure adapted from refs. [19,141,148]. Copyright 2016 and 2017, Elsevier.

Figure 11 .

 11 Figure 11. (a) 1 H→ 27 Al CPMAS spectra of γ-alumina impregnated with 10 mM AMUPOL solution in [ 2 H6]-DMSO/ 2 H2O/H2O (78/14/8 w/w/w) at 9.4 T and 78 (top, purple) and 36 K (bottom, orange) with MAS frequencies of 13 and 21 kHz, respectively. Asterisks denote spinning sidebands. (b) 1 H DNP enhancement as function of temperature for 32 mM BDPA or 16 mM TEKPOL solution in [ 2 H14]-OTP/OTP (95/5 mol/mol) mixture at 9.4 T with νR = 8 kHz. Figure adapted from refs. [48,147]. Copyright 2015, Royal Society of Chemistry and American Chemical Society.

Figure 12 .

 12 Figure 12. (a) Enhancement of the 1 H polarization compared to thermal one as function of the MAS frequency for 10 mM AsymPolPOK (red) and AMUPol (blue) solutions in [ 2 H8]-glycerol/ 2 H2O/H2O (8/1/1 v/v/v) mixture at 9.4 T and 105 K. (b) Enhancement of 1 H polarization with and without microwave irradiation, build-up times of 1 H polarization with (TB,ON) and without microwave irradiation (TB,OFF) as function of MAS frequency for 32 mM HyTEK-2 solution in TCE at 9.4 T and 125 K. (c) 1 H→ 35 Cl broadband adiabatic inversion cross-polarization (BRAIN-CP) spectra with Carr-Purcell Meiboom-Gill (CPMG) detection using wideband uniform-rate smooth truncation (WURST) pulses of finely ground ambroxol hydrochloride, a drug used in the treatment of respiratory diseases, impregnated with 15 mM TEKPol solution in TCE at 9.4 T and 100 K. These spectra were acquired with microwaves and MAS rotation during most of polarization delay but static conditions during the BRAIN-CP-WURST-CPMG sequence (SOSO acquisition, red), with microwaves and stationary sample (blue), and without microwaves and with stationary sample (black). Figure adapted from refs.[START_REF] Mentink-Vigier | Computationally Assisted Design of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR: The AsymPol Family[END_REF][START_REF] Wisser | BDPA-Nitroxide Biradicals Tailored for Efficient Dynamic Nuclear Polarization Enhanced Solid-State NMR at Magnetic Fields up to 21[END_REF][START_REF] Hirsh | 35 Cl dynamic nuclear polarization solid-state NMR of active pharmaceutical ingredients[END_REF]. Copyright 2016, Royal Society of Chemistry, 2018, American Chemical Society.

Figure 13 .

 13 Figure 13. Periodic tables showing the spin-1/2 (blue) and quadrupolar (red) isotopes, which have been detected using high-field MAS DNP-NMR experiments using either (a) coherent transfer of DNP-enhanced 1 H polarization to the detected isotope or (b) direct excitation experiments.

Figure 14 .

 14 Figure 14. (a) 13 C spectra of 20 mmol.kg -1 AMUPol solution in nonionic surfactant with chemical formula HO[CH2CH2O]6C10H21 with microwave irradiation at 9.4 T and 107 K with νR = 8 kHz. The black spectrum was recorded using a 13 C single-pulse sequence and is the sum of the direct DNP transfer and the incoherent indirect one. The red spectrum is only enhanced by direct DNP since the incoherent indirect transfer is suppressed by the application of π pulses on the 1 H channel during the polarization delay. For the blue spectrum, 2π pulses, instead of π pulses, are applied on the 1 H channel, which reintroduces the incoherent indirect transfer. (b,c) Comparison of the 1 H→ 29 Si CPMAS and single-pulse 29 Si spectra of (b) mesoporous silica impregnated with 15 mM TOTAPOL solution in [ 2 H6]-DMSO/ 2 H2O/H2O (78/14/8 w/w/w) and (c) laponite synthetic clay dispersed in 20 mM TOTAPOL solution in the same mixture with microwave irradiation at 9.4 T and 100 K with νR = 10 kHz. In both subfigures, the spectra are scaled to the same maximum intensity. In subfigure c, the single-pulse experiments were acquired with two distinct polarization delays, 120 s [direct DNP (120)] and 2300 s [direct DNP (2300)]. In the CPMAS and single-pulse spectra, the 29 Si signals are enhanced by direct DNP and coherent indirect DNP, respectively. In subfigure b, the CPMAS and single pulse spectra are dominated by the signals of surface (SiO)3SiOH (Q 3 ) sites and subsurface (SiO)4Si (Q 4 ) sites, respectively. The subfigure c shows that direct DNP leads to increased linewidth with respect to indirect DNP but the line broadening of directly enhanced signals decreases with increasing polarization delay. (d) 17 O NMR spectra of single-site catalysts, made of Zr(NCH3)n species supported on mesoporous silica nanoparticles impregnated with 15 mM TEKPol solution in [ 2 H2]-TCE with microwave irradiation (top) and without microwave irradiation (bottom) at 9.4 T and 100 K with νR = 12.5 kHz. The signal of the 17 O CT was enhanced by the application of a hyperbolic secant (HS) pulse before the CT-selective π/2 pulse. Figure adapted from refs. [39,62,111,171]. Copyright 2011, Wiley, 2013 and 2018, Royal Society of Chemistry, 2017, American Chemical Society.

Figure 15 .

 15 Figure 15. (a) 1 H→ 17 O PRESTO-QCPMG pulse sequence, where τR denotes the rotor period. (b) Comparison of 1 H→ 17 O PRESTO-QCPMG and CP-QCPMG spectra of Ca(OH)2 wetted with 16 mM TEKPol solution in TCE with

Figure 16 .

 16 Figure 16. 1 H→ 27 Al CP-3QMAS 2D spectrum of γ-alumina impregnated with a solution of 5 mM AsymPolK and 2 M [ 13 C]-urea in [ 2 H8]-glycerol/ 2 H2O/H2O (6/3/1 v/v/v) mixture with microwave irradiation at 18.8 T and 125 K with νR = 24 kHz. Figure adapted from ref. [58]. Copyright 2018, American Chemical Society.

Figure 17 .

 17 Figure 17. (a,b) 13 C 2Q-1Q through-space homonuclear correlation 2D spectra of the anhydrous polymorph of theophylline impregnated with 66 mM TEKPol solution in 1,1,2,2-tetrabromoethane with microwave irradiation at 9.4 T and 105 K with νR = 8 kHz. POST-C7 scheme was used to reintroduce the 13 C-13 C dipolar interactions. The length of the excitation and reconversion POST-C7 blocks was (a) 0.5 and (b) 2 ms. (c) Experimental build-up curves of the cross-peak intregated intensities (black disks) between the (top) C2 and C6+7 and (bottom) C5 signals. The best-fit curves used for the determination of the 13 C-13 C distances are displayed as solid lines. Figure adapted from ref. [184]. Copyright 2015, Wiley.

Figure 18 .

 18 Figure 18. 27 Al{ 29 Si} (a) J-and (b) D-R-INEPT 2D spectra of 29 Si-enriched silicated γ-alumina (1.5% wt Si) wetted with 15 mM TEKPol solution in TCE at 9.4 T and 100 K with νR = 10 kHz. A 1 H→29 Si CPMAS transfer creates the initial DNPenhanced transverse29 Si magnetization. The spectrum of the subfigure a indicates that Si atoms are covalently connected to AlO4 and AlO5 sites but not to AlO6 ones. Figure adapted from ref.[START_REF] Rankin | Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy[END_REF]. Copyright 2017, American Chemical Society.

Figure 19 .

 19 Figure 19. 13 C{ 15 N} CP-HETCOR 2D spectrum of an isotopically unmodified of a self-assembled 2'-deoxyguanosine derivative with ribbon-like structure with potential application in fields ranging from medical chemistry to nanotechnology wetted with a 10 mM AMUPol solution in [ 2 H8]-glycerol/ 2 H2O (6/4 v/v) mixture with microwave irradiation at 9.4 T and 100 K with νR = 12.5 kHz. Molecules A in red and B in blue denote the resonances of propanoyl and guanine, respectively. Figure adapted from ref. [187]. Copyright 2015, American Chemical Society.

Figure 20 .

 20 Figure 20. Proportions of publications reporting high-field MAS DNP NMR results for various classes of materials.

Figure 21 .

 21 Figure 21. (a) Conventional 1 H→ 13 C CPMAS spectrum of a living PS sample with a number average molar mass Mn = 13,5 kg.mol -1 at 285 K. (b) 1 H→ 13 C CPMAS spectrum with microwave irradiation of the same polymer doped with 0.5 wt% bCTbk using the film casting procedure at 105 K. Both spectra were acquired at 9.4 T with νR = 10 kHz. Figure adapted from ref. [137]. Copyright 2013, American Chemical Society.

Figure 22 .

 22 Figure 22. (a,c) Experimental 13 C{ 29 Si} and (b,d) 29 Si{ 15 N} REDOR signal (black dots) as function of recoupling time of mesoporous silica immobilizing Pt complexes selectively labeled with 15 N on sites (a,b) N1 and (c,d) N2 impregnated with 16 mM solution of TEKPol2 in TCE with microwave irradiation at 9.4 T and 110 K with νR = 8 kHz. The best fit curves are shown as solid green lines. (e) Best-fit 3D structure of the surface species. Figure adapted from ref. [194]. Copyright 2017, American Chemical Society.

Figure 23 .

 23 Figure 23. (a) Schematic structure of the interlayer region of the calcined form of the zeolite SSZ-70. The terminal and interlayer-bridging O atoms are shown in red, whereas the others have been omitted. (b) DNP-enhanced 29 Si refocused INADEQUATE 2D spectrum of isotopically unmodified SSZ-70 zeolite at 9.4 T and ~100 K with νR = 8 kHz. The DNP-enhanced 1 H→ 29 Si CPMAS 1D spectrum acquired under the same conditions is shown along the 1Q dimension. The blue and red lines indicates the connectivity paths to isolated silanol (on the left of subfigure a) and nested silanol groups (on the right of subfigure b), respectively. Figure adapted from ref. [219]. Copyright 2017, American Chemical Society.

Figure 24 .

 24 Figure 24. (a) Atomic model of Cd-terminated core/crown CdSe/CdS nanoplatelets. (b)113 Cd PASS 2D spectrum enhanced by phase-incremented echo-train acquisition (PIETA) of the oleate-capped core/crown CdSe/CdS nanoplatelets sol in TCE mixed with 50 mM TEKPOL solution in TCE impregnating a mesoporous silica at 14.1 T and 100 K with νR = 10 kHz. Four Cd species can be resolved and assigned to cadmium in the CdS core (57 ppm), the CdSe core (-61 ppm), and to Cd-oleate at the surface of CdS (-281 ppm) and CdSe (-310 ppm). Figure adapted from ref.[START_REF] Piveteau | Resolving the Core and the Surface of CdSe Quantum Dots and Nanoplatelets Using Dynamic Nuclear Polarization Enhanced PASS-PIETA NMR Spectroscopy[END_REF]. Copyright 2018, American Chemical Society.

Figure 25 .

 25 Figure 25. (a) Supercell for one of the two configurations of Li4Ti5O12 exhibiting the lowest energy. (b) 17 O NMR spectrum of Li4Ti5O12 doped Mn(II) ions at 0.5% molar fraction with (black solid line) and without (grey solid line) microwave irradiation at 9.4 T and around 100 K with νR = 10 kHz. Figure adapted from ref. [52]. Copyright 2019, American Chemical Society.

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Acknowledgements

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 funding partially this work. Financial support from the IR-RMN-THC FR-3050 CNRS for conducting the research is gratefully acknowledged. Authors also thank contracts ANR-17-ERC2-0022 (EOS) and ANR-18-CE08-0015-01 (ThinGlass). This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 731019 (EUSMI). OL acknowledge financial support from Institut Universitaire de France (IUF).