## 1 Introduction  
Nanodiamonds are diamond nanoparticles; they have most of the unique properties of bulk diamond but at a nanoscale. For this reason, nanodiamonds (NDs) have attracted a great deal of interest and, consequently, active research. These properties [1] include high hardness, the most known characteristic of the diamond-phase; high thermal conductivity and electrical resistivity; chemical stability; biocompatibility; a tunable surface structure; and resistance to harsh environments. These physical properties make NDs suitable for many applications in different fields: tribology, catalysis [2], biomedical applications [3], and drug delivery [4]. In the last decade, fluorescent nanodiamonds (NDs) have received a lot of attention. The fluorescence of NDs is caused by point-defects in the diamond structure, known as color centers. The nitrogen-vacancy (NV) center is the most studied color center. This defect consists of a substitutional nitrogen atom in a diamond lattice site having a carbon vacancy as its nearest neighbor. NV centers exist in two charge states: neutral, NV\({}^{0}\), and negatively charged, NV\({}^{-}\). Of the two states, NV\({}^{-}\) is the most investigated and probably the most interesting in terms of quantum technology applications [5, 6]. This is because of its optical properties. The NV\({}^{-}\) shows bright and stable photoluminescence (PL) under excitation by visible light (usually a 532 nm laser source is used). The NV PL emission has a zero phonon line at 637 nm, with a sideband due to NV-phonon coupling extending to \(\sim\)800 nm. The NV emission is highly suitable for many applications given its photostability (no bleaching is observed at room temperature) and short relaxation time (\(\sim\)10 ns) [7]. However, the most important property of NV optical transitions is their dependence on electron spin-states [8]. This means that NV fluorescence intensity changes in the presence of perturbations acting on the spin-state, such as electromagnetic fields. This, combined with the possibility of accurately manipulating the population of spin levels in the ground state, has allowed NV centers to be used as quantum sensors of environmental physical parameters with nanoscale precision, such as for magnetic [9] or electric fields [10], temperature [11] (from cryogenic to well above 300 K), pressure, as well as in harsh chemical environments. These sensors can be used in biological material given NDs' non-toxicity. Measurements of temperature [12] and magnetic/electric fields [13] with NV-enriched NDs have already been reported. Additionally, as NDs can be chemically attached to a large variety of species (from DNA molecules to proteins) [14], NV-fluorescence can be used to track the position of the macromolecules [15]. Moreover, NDs linked to cell membranes could provide a sensor to study cell activity [16]. Furthermore, spin-dependent fluorescence intensity can be used to read-out the NV spin-state [17]. This property, together with the possibility of optically pumping most of the electrons in specific spin-sublevels with very large spin-coherence lifetimes [18] (as long as milliseconds), makes NV centers an ideal solid-state qubit for quantum information science. Finally, it was shown that the NV spin polarization can be transferred to the surrounding environment [19], such as \({}^{13}\)C nuclear spin, opening the potential for use in nuclear magnetic resonance (NMR) applications [20]. Given the wide application field of NV-enriched NDs, an efficient technique for their synthesis is critical for further development. Many ND production methods have been proposed, the main ones being detonation [1] and milling [21] of macroscopic diamond synthesized either by chemical vapor deposition (CVD) or high pressure, high temperature (HPHT).