Surface phosphorylation of nanodiamond was performed by reaction with phosphoryl chloride in dichloromethane. Depending on the reaction conditions, P contents of up to 1.66 mmol/g were reached. Phosphorylation dramatically enhanced the thermal stability of nanodiamond under oxidizing conditions, shifting the oxidation temperature by up to 190 °C and dividing the oxidation rate by a factor of up to 160. The nature of the grafted phosphate species and their evolution during thermal treatment was followed using solidstate NMR.

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...