HAp crystals were labeled via fluorexon staining. Briefly, 100mg nHAp crystals and 10 mL fluorexon solution (10mg/mL) were mixed and stirred for 30 min at room temperature. Subsequently, the HAp crystals were collected via centrifugation, washed three times with anhydrous ethanol and distilled water to make sure that there was no free fluorexon residue, and dried.