Synergistic photoactivated antimicrobial effects of carbon dots combined with dye photosensitizers

Background: Carbon quantum dots (CDots) have recently been reported as a new class of visible light activated antimicrobial nanomaterials. This study reports the synergistic photoactivated antimicrobial interactions of CDots with photosensitizers on bacterial cells. Methods: The antimicrobial effects of the CDots with surface passivation molecules 2,2'-(ethylenedioxy) bis(ethylamine) in combination with photosensitizer methylene blue (MB) or toluidine blue (TB) at various concentrations were evaluated against Escherichia coli cells with and without 1-hour visible light illumination. The broth microdilution checkerboard method and isobologram analysis were used for determining if synergistic effect existed between CDots and MB or TB. Results: The results showed that CDots alone at a concentration of 5 mu g/mL did not display antimicrobial effects, 1 mu g/mL MB alone only decreased 1.86 log of viable cell numbers, but the combination treatment with 5 mu g/mL CDots combined with 1 mu g/mL MB completely inhibited bacteria growth, resulted in 6.2 log viable cell number reduction, suggesting synergistic interaction between the two. The antimicrobial effects of CDots/TB combination exhibited similarly synergistic effects on E. coli cells. These synergistic effects between CDots and MB or TB were further confirmed using the checkerboard microdilution methods, where the fractional inhibitory concentration index value (0.5) and the isobologram analyses. The synergistic interactions were also correlated to the increased generation of intracellular reactive oxygen species in E. coli cells upon the combination treatments of CDots/MB or CDots/TB. Conclusion: The study demonstrated the synergistic photoactivated antimicrobial effects of CDots in combination with other photosensitizers. Such synergistic effect may open new strategies for developing highly effective antimicrobial methods.