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

Abstract

In the present paper, low-dimensional Ag₂S 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 Ag₂S 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 Ag₂S 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-Ag₂S QDs, compared with TGA-Ag₂S QDs or free unbound Schiff bases. The minimum inhibitory concentration (MIC) results confirmed that even low concentrations of SB-Ag₂S 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-Ag₂S QDs were not toxic to brine shrimp (Artemia salina). The results show that capping agents are essential in the design of functional Ag₂S QDs, and highlight that Schiff bases provide an excellent opportunity to optimize the biological activities of silver based QDs.