Deaggregation of nanodiamond powders using salt- and sugar-assisted milling

Diamond particles of 5-10 nm in size can be produced in large quantities by denonating oxygen-lean explosives in a closed chamber. They have numerous useful properties and are used in applications ranging from lubricants to drug delivery. Aggregation of diamond nanoparticles is limiting wider use of this important carbon nanomaterial because most applications require single separated particles. We demonstrate that dry media assisted attrition milling is a simple, inexpensive, and efficient alternative to the current ways of deaggregating of nanodiamond. This technique uses water-soluble nontoxic and noncontaminating crystalline compounds, such as sodium chloride or sucrose. When milling is complete, the media can be easily removed from the product by water rinsing, which provides an advantage when compared to milling with ceramic microbeads. Using the dry media assisted milling with subsequent pH adjustment, it is possible to produce stable aqueous nanodiamond colloidal solutions with particles <10 nm in diameter, which corresponds to 1-2 primary nanodiamond particles. The study of milling kinetics and the characterization of the produced nanodiamond colloids led us to conclude that aggregates of less than 200 nm in diameter, observed at the tail of the pore size distribution of milled nanodiamond, are loosely bonded and rather dynamic in nature. Color change observed in ND colloids upon shifting their pH toward the basic end allowed us to demonstrate that the coloration comes from the light interaction with colloidal particles and not from an increase in nondiamond carbon content.