5-CFDA-AM is a cell-permeable esterase substrate that can be used as an active probe to measure enzyme activity and cell membrane integrity. 5-CFDA-AM is electroneutral and can enter the cell at a lower concentration than CFDA, where it is hydrolysed by intracellular esterases to produce carboxyfluorescein. Carboxyfluorescein contains an additional negative charge and can be better retained in the cell.
References:
[1]. Elena Gorokhova, et al. A comparison of TO-PRO-1 iodide and 5-CFDA-AM staining methods for assessing viability of planktonic algae with epifluorescence microscopy. J Microbiol Methods. 2012 Jun;89(3):216-21.