Abstract
To investigate a potential new antibacterial agent to combat increasing antimicrobial resistance, undoped and 1% manganese doped Zinc Sulphide quantum dots (ZnS and Zn0.99Mn0.01S QDs, respectively) were synthesised by co-precipitation method and capped with increasing amounts of Zinc Oxide and Silica in aqueous media to prepare ZnS@ZnO, Zn0.99Mn0.01S@ZnO, and Zn0.99Mn0.01S@SiO2 nanostructures. P-XRD analysis confirmed the cubic zinc-blende phase of the seed ZnS QDs, Zn0.99Mn0.01S QDs, and Zn0.99Mn0.01S@SiO2 nanostructures, and the wurtzite phase of the ZnO in the ZnS@ZnO and Zn0.99Mn0.01S@ZnO nanostructures, further confirmed using TEM studies, which also revealed the size of the largest nanostructures to be in the range of a hundred nanometres.
FTIR spectroscopy illustrated the quenching of characteristic ZnS peaks with increasing capping material. UV-Visible absorption spectroscopy and subsequent Tauc analysis illustrated the strong size confinement of the synthesised ZnS and Zn0.99Mn0.01S QDs; Brus equation calculations revealed that the particle size of the samples increases with increasing capping material.
Photoluminescent emission spectroscopy illustrated the tuneable emission properties of the prepared nanostructures; manganese doping induced the characteristic orange emission in the Zn0.99Mn0.01S QDs, which was enhanced by ZnO, but quenched by SiO2. The antimicrobial activity of all the prepared samples was qualitatively evaluated using well known Agar well diffusion method against six human pathogenic bacteria: Gram positive Bacillus subtilis and Staphylococcus aureus; Gram negative Salmonella Typhi, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa.
Qualitative antibacterial assay confirmed the high antibacterial potential of the synthesised ZnS and Zn0.99Mn0.01S QDs, especially against E. coli. Increasing ZnO amount improves the antibacterial activity of the nanostructures against different Gram-positive bacterial strains, while increasing SiO2 amount improves the antibacterial activity of the nanostructures against both Gram positive strains and three of the four Gram negative bacterial strains.
Thus, the positive results suggest that the prepared ZnS@ZnO, Zn0.99Mn0.01S@ZnO, and Zn0.99Mn0.01S@SiO2 nanostructures should be further studied as antimicrobial agents.