Zirconia microbead-assisted ball milling andBASD are now mainly used for theformation of single-digit dentonated nanodiamondsfor research, in particular, for adsorption and delivery of insolubleanti-cancer therapeutics. Both techniques needs the use of ?30 ?mZrO2 microbeads. In BASD, for example, the dense ZrO2microbeads, propelled by the energy of cavitation, collide and crush nanodiamonds aggregates trapped in-between (figure5). BASD yields the stable single-digit ND col-loids upto 10 wt% concentration with up to 80% yield relative to the initialND mass. However, BASD, as well as ZrO2 microbead-assisted ballmilling have some disadvantages, such asa high cost (ZrO2 microbeads areexpensive, special mills have to be designed for the process, separation ofmicrobeads from NDs is also costly) and difficultto remove ZrO2 debris (harsh acid or base treatment isrequired to dissolve ZrO2, which have negatively impacts on production safety and contributes to the costof the purified ND). On the other hand, if ZrO2 is not removed completely, then the presence of thiscontaminant in uncontrolled quantities may negatively effect the prospects ofclinical approval for ND enabled theranostic platforms73. Thus, ZrO2 and similar ceramic contaminants may posea serious obstacle on the way to low-cost and safe ND therapeutics.
Onthe contrary, water-soluble dry media-assisted attritor milling and SAUDutilize inexpensive, non-toxic, and non-contaminating crystalline milling mediasuch as sodium chloride or sucrose. Upon completion of the deg-gregationprocess, the milling media can be easily washed out with water, providing aremarkable advantage over a process containing insoluble ceramic beads.However, during the dry media-assisted attritor milling, parts of the millcontaminate nanodiamonds with Fe, Ni, and other components of steel, so itrequired an extra purification step. Moreover, significantlyreducing the aggregate size from micrometerscale down to 50–30 nm, dry media-assisted attritormilling does not yield truly single-digit nanodiamonds unless the dispersion pHis adjusted to ?11upon completion of milling83,84. SAUDuses ultrasonic power transport by a standard lab horn sonicator into suspensionsof different water-soluble crystalline media (e.g.
,sodium chloride, potassium chloride, sodium acetate, etc)to yield single-digit nanodiamonds colloids without any pH adjustments (figure5). Since no ZrO2 is used, SAUDcompletely eliminates zirconia or any other difficult-to-removeimpurities in nanodiamonds85. The mechanical action of salt crystals in SAUDis combined with formation of a correspondingsalt of Na+, K+, etc with COO? groups of nanodiamonds, thus improving the stabilityof single-digit nanodiamonds colloid. In another approach, hydrogen annealingof nanodiamonds at 800 °C–850 °C gives rise to deaggregatedhydrogen and –OH-terminated nanodiamonds. These hydrogenated nanodiamondsshow high colloidal stability in water due to their high positive zetapotential86,87