2.2 Assay of antimicrobial photodynamic inactivation
E. coli and S. aureus were used as the model bacteria for the assay of antimicrobial photodynamic inactivation using the spread plate method. A series of the CD solutions at different concentrations were mixed with the E. coli or S. aureus suspension diluted in PBS (106 CFU/mL) at a volume ratio of 1:1 to give a final volume of 2 mL in each well of the 24-well plate. Subsequently, the plate was exposed to a UV lamp (365 nm, 16 W) at a distance of 30 cm from the top of the plate. This condition was applied to all UV irradiation experiments in this work unless specified. After the irradiation, 100 μL of the bacterial suspension (1×) from each well were diluted 5 times in succession with PBS, so that the concentration of the suspension was diluted to 10-5× with an interval of 10-1. Each diluted sample was spread evenly on LB agar plates and incubated at 37°C for 24 h. The photodynamic inactivation of bacteria by the CDs was assessed by the bacterial survival rate, which can be described as follows:
Bacterial survival rate (%) = (Colony Number(CD) / Colony Number(control)) × 100 (1)
where colony number(CD) represents the number of bacterial colony formed in the presence of the CDs, and colony number(control) denotes that in the absence of the CDs. In parallel, the survival rate of the bacteria in the dark was also evaluated.