S- and N-doped carbon quantum dots: Surface chemistry dependent antibacterial activity

Sulfur and nitrogen-doped carbons quantum dots (S-CQDs and N-CQDs) were obtained using a simple hydrothermal treatment of S- or N-containing organic compounds/polymers. They were evaluated for their bactericidal activity against representative Gram-negative (Escherichia coli, CECT 831) and Gram-positive (Bacillus subtilis subsp. subtilis 168) bacterial strains, using a qualitative estimation approach. Quantitative tests revealed greater effectiveness of N-CQDs compared to S-CQDs. The bactericidal activity of the dots was linked to their specific surface chemistry, and their sizes in the range of nanometers. In the case of the N-CQDs, amides and amines played the most important role in enhancing bactericidal function. They caused a bacterial death which was linked to the electrostatic interactions between their protonated forms and the lipids of the bacterial cell membrane. It is also possible that the ability to activate oxygen species by the CQDs surface played some role. S-CQDs showed a much lower bactericidal activity compared to that of N-CQDs. These dots (S-CQDS), containing mainly a negatively charged surface due to dissociation of sulfonic/carboxylic groups and sulfates, showed a size dependent rather than a chemistry dependent (electrostatic interactions) inhibition of the Gram-positive bacterial growth. This is the first study where the role of different heteroatoms incorporated to CQDs is examined in the context of the bactericidal activity.