Understanding the Surfaces of Nanodiamonds

Functional groups and their associated charges are responsible for the binding and release of molecules from the surfaces of particles in nanodiamond colloids. In this work, we describe a combined set of experimental and computational techniques that are used to characterize these functional groups quantitatively. The surfaces of the particles examined during this study are amphoteric, as one would expect for surfaces made of carbon, with high concentrations of phenols, pyrones, and sulfonic acid groups; the average 50-nm-diameter nanodiamond aggregate has approximately 22000 phenols, 7000 pyrones, and 9000 sulfonic acids. The aggregates also have at least 2000 fixed positive charges, stabilized within pyrones and/or chromenes. No evidence for a significant concentration of carboxylic acid groups was found, although some are probably present. Hydroxyl and epoxide groups are present on some areas of the surfaces. The surfaces are graphitized, so the presence of phenols and pyrones is not surprising because such groups are common on graphitic surfaces. The sulfonic acid is due to the sulfuric acid treatment used to remove amorphous carbon and graphite during particle cleaning. The fixed charges are also due to the cleaning procedure that includes the use of KMnO4 with the sulfuric acid. Based on titration and zeta potential experiments, elemental and particle size analyses, and modeling using semiempirical quantum mechanics, a model is proposed for the types and concentrations of surface groups. The modeling shows how functional groups form during the bead milling and cleaning used in the preparation of the colloid. It also shows that the pK(a) associated with the phenols and pyrones that are formed (pK(a) = 7.6-10.0) is consistent with that predicted using titration experiments (pK(a) >= 7.3). The positive surface potential means that the latter pK(a) value is significantly larger than a Henderson-Hasselbalch-based estimate. The model is shown to be useful in explaining a number of recent experiments in which nanodiamonds were used to bind and release therapeutic drug and polymer molecules.