NMR and EPR Characterization of Functionalized Nanodiamonds

Abstract

We investigated the potential of solid-state NMR using magic angle spinning (MAS) with and without dynamic nuclear polarization (DNP) and electron paramagnetic resonance (EPR) for the characterization of functionalized nanodiamonds (NDs). We showed that conventional 1 H, 31 P, and 13 C solid-state NMR spectra allow differentiating in a straightforward way NDs from commercial sources and custom-made NDs bearing aromatic or aliphatic phosphonate moieties at their surface. Besides, the short nuclear relaxation times prove the close proximity between the endogenous paramagnetic centers of NDs and the grafted organic moieties. EPR spectra confirmed the presence of these paramagnetic centers in functionalized NDs, which are centered on dangling bonds as well as a few N 0 defects, corresponding to the substitution of carbon atoms by nitrogen ones. Hyperfine sublevel correlation spectroscopy indicates that the N 0 paramagnetic centers are mostly located in the disordered shell of NDs. Preliminary DNP-enhanced NMR experiments at 9.4 T and 100 K under MAS have shown a lack of significant DNP enhancement, which can be attributed to the short relaxation times of the unpaired electrons and the nuclei in NDs. When using exogenous polarizing agents, the endogenous unpaired electrons contribute to a leakage of polarization. Furthermore, low temperatures lead to a broadening of NMR signals. It therefore appears that conventional direct excitation remains the NMR method of choice for the characterization of functionalized NDs. ■ INTRODUCTIONNanodiamonds (NDs), that is, diamond particles with a diameter smaller than 100 nm, are receiving considerable attention due to their remarkable hardness and thermal conductivity, luminescence properties, chemical inertness, and biocompatibility. 1−6 For instance, they have been proposed as fillers to reinforce the mechanical properties of polymers and ceramics, 7,8 as catalyst supports for metal nanoparticles (NPs), 9 or as nanovectors for various drugs. 4,6,10 Depending on the preparation and purification methods used, the surface of NDs can be terminated by a variety of functional groups, including carboxylic groups, hydroxyls, ketones, and lactone moieties. 5,11−14 To attain the targeted properties, the surface of nanodiamonds most often needs to be functionalized, so that the interactions between the NPs and their environment are optimal. Several methods have been proposed to tune the surface properties of NDs by covalent grafting of organic moieties. 14−20To date, the characterization of NDs functionalized by organic fragments has involved a variety of methods, such as X-ray photoelectron, 21 UV, 22 and FTIR 16,20,23−26 spectroscopies. Solid-state NMR spectroscopy has been attempted, 27−29 but the use of this technique is still limited despite the high specific surface area of NDs (∼150−300 m 2 ·g −1 ), which should be favorable for detecting the surface functional groups. So far, solidstate NMR studies of NDs have mainly focused on the investigation of the core and surface structure of N...

Recent developments in MAS DNP-NMR of materials

“…As a consequence, the presence in the materials of paramagnetic centers, which do not contribute to the DNP transfer but shorten the electron and nuclear longitudinal relaxation times, T1(Si) and T1(I), respectively, 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 [85,86]. 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 [87,88].…”

Dynamic nuclear polarization at 40 kHz magic angle spinning

“…Although the principles of this effect have been known for many decades, 2a , 3 its transposition to modern high-resolution NMR of solids at magnetic fields of 5 T or higher has become possible only relatively recently, with the introduction of gyrotron sources capable of delivering high-power high-frequency microwaves, 4 and of cryogenic MAS probes for experiments at 100 K or lower temperatures. In this respect, in the past few years, high-field MAS DNP-enhanced solid-state NMR spectroscopy has made great progress 5 and has been implemented successfully on a wide variety of systems, ranging from porous and non-porous materials, 6 colloids, 7 molecular organic solids including pharmaceuticals, 8 polymers, 9 membrane proteins, fibrillar aggregates, biomaterials or cells, 10 thus gaining more and more importance.…”

Nanodiamond: A high impact nanomaterial

“…With all of the unique properties associated with nanodiamonds, they are truly a high impact material with a bright and opportunity filled future. [74], XPS [74], TDMS [75], NMR [74,76], Electrophoretic mobility (f potential) [75], Titration [77] Metal content (surface) PIXE, b ICP-MS [73,78,79] Diamond core Lattice quality XRD [73], HRTEM [ …”