The 13 C spin-lattice relaxation measurements provide crucial information on the chemical environments of the two main spatial groups of carbon speciesthe core sp 3 carbons and the aliphatic carbons present in the surface or near-surface layers of the nanoparticles. The 13 C T1 relaxation times of the core sp 3 carbons evaluated by fitting the saturation recovery experimental data to a single-exponential function indicate an increasing trend of 13 C spin-lattice relaxation times with respect to the median particle size that ranges from 18 nm to 75 nm for the solid diamond powders (Figure ). Conversely, the 13 C T1 values (0.6-0.9s) obtained for the crosspolarizable carbons observable at ~ 34 ppm in the 13 C CPMAS spectra of the untreated diamond powders do not indicate such a drastic trend. These sp 3 carbons are expected to be spatially close to protons, thus representing the surface carbons of the nanoparticles . Correspondingly, the chemical treatment of nanoparticles does not affect the dipolar interactions between the carbonbased functional groups and paramagnetic defect centers . 13 C NMR spin-lattice relaxation studies. Median particle sizes are reported by the Microdiamant AG . A typical dataset (Figure ) of a saturation recovery experiment for the determination of 13 C T1 values is displayed in the supporting information.