Similar to the efflux assay procedure, 100 µl of ILM suspension was mixed with 150 µl of Buffer A in multiple tubes and kept in the dark at 4°C. At various time points (0, 2, 4, 6, 8, 10, 17, 23, 30, 45, and 60 days), 100 µl of supernatant from the three tubes was transferred into new tubes and kept in the dark at 4°C. After 60 days of storage, the fluorescence intensity of the supernatant was measured in triplicate. Values are presented as mean ± SD. One-way ANOVA with Bonferroni correction was used for the comparison between the supernatant of day 0 and each measurement day. *P<0.05 vs day 0. Abbreviation: AU, arbitrary unit.