Highly Efficient Biofilm Eradication by Antibacterial Two-Dimensional Supramolecular Polymers

Abstract

This article describes a generally applicable supramolecular design for construction of ultrathin two-dimensional (2D) nanosheets with a functionalized surface, by aqueous self-assembly of core-substituted naphthalene diimide (cNDI)-derived π amphiphiles, and explores them as an antibacterial material. In cNDI-1 and cNDI-2, the imide positions are substituted with dodecane chains, while the core substitution differs in the nature of the amine group (primary and quaternary, respectively). On the other hand, cNDI-3 has an identical core substitution to that in cNDI-2 but is attached with the C8 hydrocarbon chain. In water, they exhibit spontaneous self-assembly (critical aggregation concentration < 10–5 M) by synchronized π stacking and alkyl-chain packing in the two orthogonal directions, producing single-molecular-layer-thick (height < 1.5 nm) 2D sheets with amine-functionalized cationic surfaces. These 2D nanosheets exhibit broad-spectrum antibacterial activity against both Gram-positive and Gram-negative strains (Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli), respectively) with rather low minimum inhibitory concentration (MIC) in the range of <100 μg/mL. Among the three, cNDI-2 with the C12 chain and a quaternary amine functionality appears to be the most lethal exhibiting the lowest MIC value (30 μg/mL). Hemolysis of red blood cells remains at <5% even at a cNDI concentration of 10× MIC, suggesting excellent hemocompatibility of these 2D nanosheets, which is also evident from the MTT assay showing that >80% mammalian cells (HeLa) remain viable with up to 200 μg/mL of these molecules. Mechanistic investigations by scanning electron microscopy, live/dead fluorescence assay, and inner/outer membrane permeability assays strongly indicate the nonspecific membrane disruption mechanism for killing of bacteria. Furthermore, these 2D nanosheets display a strong ability to eradicate the bacterial biofilm of both S. aureus and E. coli. With less than 100 μg/mL of a given cNDI molecule, a biofilm (aged for 96 h) can be fully eradicated, which makes them exemplary materials for tackling the emerging health concern on bacterial infection and drug-resistant pathogens.