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

In the present paper, low-dimensional Ag(2)S 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 Ag(2)S 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 Ag(2)S QDs. We demonstrate that the growth of Gram-positive bacteria (Bacillus subtillus and Staphylococcus aureus), Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa), and a prevalent fungal pathogen (Candida albicans) are inhibited more when the bacterial and fungal cells were nurtured with the synthesized SB-Ag(2)S QDs, compared with TGA-Ag(2)S QDs or free unbound Schiff bases. The minimum inhibitory concentration (MIC) results confirmed that even low concentrations of SB-Ag(2)S QDs were able to inhibit bacterial (MIC 5–75 μg mL(−1)) and fungal growth (MIC 80–310 μg mL(−1)), and in some cases they performed better than streptomycin (8–25 μg mL(−1)). Lethality bioassay results confirmed that SB-Ag(2)S QDs were not toxic to brine shrimp (Artemia salina). The results show that capping agents are essential in the design of functional Ag(2)S QDs, and highlight that Schiff bases provide an excellent opportunity to optimize the biological activities of silver based QDs.