Nuclear magnetic resonance studies of nanodiamond surface modification

We review Nuclear Magnetic Resonance (NMR) studies of surface modification of nanodiamonds (ND). The spectra of chemically functionalized – fluorinated, chlorinated, hydroxylated, carboxylated and hydrogenated ND show formation of C–F, C–Cl, C–OH, C–COOH and C–H covalent bonds with the surface carbon atoms, respectively. These bonds reveal different chemical shifts that allow distinguishing between them in the NMR spectra. The NMR data are well supported by XPS measurements. Nanodiamond annealing above 600 °C results in a surface graphitization, which increases with increasing the annealing temperature. ND particles with grafted paramagnetic copper, cobalt and gadolinium ions are prepared by mixing of aqueous ND suspension with aqueous solutions of transition metal nitrates. Dissociated cations in this mixture undergo ion exchange with hydrogen atoms of the surface carboxyl groups. To evidence the ion grafting to the surface, we developed an effective approach based on the analysis of nuclear spin-lattice relaxation data. These data show noticeable acceleration of ¹³C and ¹H nuclear spin-lattice relaxation, which results from the interaction of carbon and hydrogen nuclear spins of diamond with the unpaired electron spins of paramagnetic ions. This finding provides clear evidence of the binding of Cu^{2 +}, Co^{2 +} and Gd^{3 +} ions to the ND surface. The aforementioned approach allows calculating distances between the ions and surface. A model of the positioning of transition and rare-earth metal ions on the ND surface is presented. The NMR data obtained are supported by EPR measurements. Biomedical applications of the studied nanomaterials are discussed.