Antimicrobial Properties of 2D MnO₂ and MoS₂ Nanomaterials Vertically Aligned on Graphene Materials and Ti₃C₂ MXene

Two-dimensional (2D) nanomaterials have attracted considerable attention in biomedical and environmental applications due to their antimicrobial activity. In the interest of investigating the primary antimicrobial mode-of-action of 2D nanomaterials, we studied the antimicrobial properties of MnO₂ and MoS₂, toward Gram-positive and Gram-negative bacteria. Bacillus subtilis and Escherichia coli bacteria were treated individually with 100 μg/mL of randomly oriented and vertically aligned nanomaterials for ∼3 h in the dark. The vertically aligned 2D MnO₂ and MoS₂ were grown on 2D sheets of graphene oxide, reduced graphene oxide, and Ti₃C₂ MXene. Measurements to determine the viability of bacteria in the presence of the 2D nanomaterials performed by using two complementary techniques, flow cytometry, and fluorescence imaging showed that, while MnO₂ and MoS₂ nanosheets show different antibacterial activities, in both cases, Gram-positive bacteria show a higher loss in membrane integrity. Scanning electron microscopy images suggest that the 2D nanomaterials, which have a detrimental effect on bacteria viability, compromise the cell wall, leading to significant morphological changes. We propose that the peptidoglycan mesh (PM) in the bacterial wall is likely the primary target of the 2D nanomaterials. Vertically aligned 2D MnO₂ nanosheets showed the highest antimicrobial activity, suggesting that the edges of the nanosheets were likely compromising the cell walls upon contact.