Effect of ultra-trace Ag doping on the antibacterial performance of carbon quantum dots

Ag,N co-doping carbon quantum dots (Ag,N-CQDs) were synthesized by applying a facile hydrothermal method with green and cheap precursors of ultra-trace silver nitrate, citric acid, and ammonia solution. The as-prepared Ag,N-CQDs deliver effective antimicrobial performance against Gram-negative bacteria of Escherichia coli (E. coli) and Gram-positive bacteria of Staphylococcus aureus (S. aureus). The microstructure of the Ag,N-CQDs was analyzed by applying TEM, AFM, Raman spectra, and XPS spectra. Observations show that the Ag,N-CQDs have a good crystallization structure with sizes of 5-9 nm and are assembled into nanoclusters with sizes of 10-80 nm. Antibacterial activities were determined on the basis of the colonies of E. coli and S. aureus, and the morphologies of the bacteria were observed by scanning electron microscopy. Spread plate tests show that the inhibition effect is sequenced as follows: Ag,N-CQDs > Ag-CQDs > N-CQDs > Ag solution. Thus, the doping of Ag in N-CQDs is a more effective way to enhance the antibacterial performance than the N-CQD and Ag solution. Furthermore, Ag,N-CQDs display significant antibacterial activities against both bacteria colonies, while the N-CQDs counterpart presents inhibitory effect against E. coli only. The lowest prevention concentrations of Ag,N-CQDs are 250 and 200 μg mL^-1toward E. coli and S. aureus, respectively. The enhanced antibacterial performances of the Ag,N-CQDs nanoclusters, especially to the S. aureus, are discussed and can be attributed to the strong adsorption of the polar chemical groups and the perforation effect of Ag atoms to disrupt bacterial membranes.