Zinc concentration in the cell was detected by FluoZin-3, AM probe using fluorescence-activated cell sorting analysis (FACS). FACS was carried out according to the protocols described previously.31 (link) Briefly, cells were seeded into six-well plates and treated with designed concentration of DTPA and/or ZnONPs. The probe was added into the cells at the final concentration of 500 nM and incubated in the dark for 30 min. Subsequently, cells were washed by the complete medium followed by an additional incubation for 30 min in the dark. FACS analysis were performed on Becton Dickinson (BD) Influx Cell Sorter (BD Biosciences, San Jose, CA, USA) and the results were analyzed by BD FACS Software (San Jose, CA, USA).