Nanodiamond (ND) and its derivs. have been widely used for drug, protein and gene delivery. Herein, exptl. and theor. methods have been combined to investigate the effect of pH on the delivery of doxorubicin (DOX) from fluorescein labeled NDs (Fc-NDs). In the endosomal recycling process, the nanoparticle will pass from mildly acidic vesicle to pH ≈ 4.8; thus, it is important to investigate DOX release from NDs at different pH values. Fc-NDs released DOX dramatically under acidic conditions, while an increase in the DOX loading efficiency (up to 6.4 wt%) was obsd. under basic conditions. Further theor. calcns. suggest that H+ weakens the electrostatistic interaction between ND surface carboxyl groups and DOX amino groups, and the interaction energies at pH < 7, pH 7 and pH > 7 are 10.4 kcal mol-1, 25.0 kcal mol-1 and 27.0 kcal mol-1 resp. Cellular imaging expts. show that Fc-NDs are readily ingested by breast adenocarcinoma (BA) cells and cell viability tests prove that they can be utilized as a safe drug delivery vehicle. Furthermore, pH triggered DOX release has been tested in vitro (pH 7.4 and pH 4.83) in breast adenocarcinoma (BA) cells.