Five milligrammes of five-(and six-) carboxyfluorescein diacetate, succinimidyl ester (CFDA-SE) (Beyotime Institute of Biotechnology, China) dissolved in 8969 μL di-methyl-sulphoxide (DMSO) and filtered (0.22 μm) to obtain a stock solution at a concentration of 1mmol/L,was stored at −20 °C until use.