Scalable Fabrication of Clean Nanodiamonds via Salt-Assisted Air Oxidation: Implications for Sensing and Imaging

Nanodiamonds (NDs) have many potential applications, but their development is dependent on obtaining a well-defined surface through cleaning. Several purification methods, such as the promising air oxidation together with centrifugation, have been developed to remove the surface-adsorbed impurities on NDs. These stochastic impurities mainly consist of both ultrasmall NDs and nondiamond structures (e.g., disordered carbons), which are unavoidably generated during the synthesis and processing of diamond materials. However, it is difficult to eliminate these unwanted parts using existing methods, which restrict the widespread usage of NDs. Here, we developed a simple, reliable, and reproducible purification method, namely, the salt-assisted air oxidation treatment, requiring only one additional prestep, that is, mixing NDs with a proper amount of salt crystals (e.g., sodium chloride) prior to conventional oxidation. The developed method enables scale-up manufacturing of clean NDs, with a rounded shape transformed from the original shard-like shape. Furthermore, we uncovered the exact role of salt crystals in eliminating impurities during oxidation. These findings will significantly enhance the scope of these little gemstones in diverse scientific and industrial fields, particularly in demanding areas such as biomedical and quantum sensing requiring stable and sound surface functionalities.