Antimicrobial activity of silver sulfide quantum dots functionalized with highly conjugated Schiff bases in a one-step synthesis.

In the present paper, low-dimensional AgS QDs were fabricated for the first time, with four different dithiocarbazate derivative Schiff bases (SB) as capping agents in a one-pot synthesis. These SB-capped AgS QDs were almost spherical with an average size range of 4.0 to 5.6 nm, which is slightly smaller than conventional thioglycolic acid (TGA)-capped AgS QDs. We demonstrate that the growth of Gram-positive bacteria ( and ), Gram-negative bacteria ( and ), and a prevalent fungal pathogen () are inhibited more when the bacterial and fungal cells were nurtured with the synthesized SB-AgS QDs, compared with TGA-AgS QDs or free unbound Schiff bases. The minimum inhibitory concentration (MIC) results confirmed that even low concentrations of SB-AgS QDs were able to inhibit bacterial (MIC 5-75 μg mL) and fungal growth (MIC 80-310 μg mL), and in some cases they performed better than streptomycin (8-25 μg mL). Lethality bioassay results confirmed that SB-AgS QDs were not toxic to brine shrimp (). The results show that capping agents are essential in the design of functional AgS QDs, and highlight that Schiff bases provide an excellent opportunity to optimize the biological activities of silver based QDs.