Size-dependent reactivity of diamond nanoparticles.

Abstract

Diamond nanoparticles of approximately 4 nm diameter are promising candidates for drug delivery and fluorescence monitoring due to their photostability, but aggregate formation limits their practical use. This study presents a method in which aggregated nanodiamond powder is annealed in hydrogen gas, breaking down large aggregates exceeding 100 nm into their constituent ~4 nm core particles. Subsequent high-power ultrasonication and high-speed centrifugation yield a monodisperse colloidal suspension exhibiting long-term stability across a broad pH range and a strongly positive zeta potential above 60 mV. Notably, the pronounced shift in zeta potential following hydrogen gas exposure indicates that the surfaces of ~4 nm nanoparticles react with molecular hydrogen at relatively low temperatures, a reactivity absent in larger 20 nm diamond particles and bulk diamond surfaces, confirming a size-dependent interaction with H2.

Mechanism

Annealing in hydrogen gas causes ~4 nm nanodiamond surfaces to react with molecular H2 at low temperatures, substantially altering surface charge (zeta potential) and preventing aggregation. This reactivity is size-dependent and is not observed in particles of 20 nm or larger.