2.3 Cytoplasmic membrane permeability
The permeability of cytoplasmic membrane was assessed by measuringβ -galactosidase activity in bacteria using ONPG, a substrate for cytoplasmic β- galactosidase.[21] Briefly,E. coli or S. aureus were cultured separately in MHB medium containing 2% lactose until the logarithmic growth phase. Then, the E. coli or S. aureus suspension was centrifuged and washed with HEPES (5 mM, pH 7.4) containing 20 mM glucose and 2 mM ONPG. Next, 100 μL of the bacterial suspension (106 CFU/mL) was mixed with the CDs (100 μL) at a predetermined concentration in a 96-well plate. The absorbance of the mixture at 420 nm was subsequently measured at different times with a 5 min interval.