Surface interior hydrophobicity of functionalized nanoparticles for enhancing antibacterial activity with reduced toxicity.

While the influence of the exterior hydrophobicity of ligands conjugated to nanomaterial surfaces on biological applications has been extensively studied, little attention has been given to the impact of the interior hydrophobicity of these conjugated ligands. In this study, we systematically functionalized the surfaces of molybdenum disulfide quantum dots (MoS2 QDs) with different ligands having oligo(ethylene glycol) (EG) tethers of varying length. This will expose the interior hydrophobicity, maximum in the case of No-EG and minimum for Tetra-EG. Gram-positive bacteria, methicillin-resistant Staphylococcus aureus (MRSA) and Enterococcus faecalis (E. faecalis), and Gram-negative bacteria, Escherichia coli (E. coli) and Pseudomonas aeruginosa (PA), were employed as model organisms to investigate the impact of exposure of interior ligand hydrophobicity on bacterial membrane interactions. Successful surface modification of MoS2 QDs with the ligands was confirmed via zeta potential measurements and Fourier-transform infrared spectroscopy (FT-IR). Antibacterial assays demonstrated that MoS2 QDs functionalized with ligands possessing higher exposure of interior hydrophobicity (No-EG-MQD) exhibited the most potent antibacterial activity, with an exceptionally low minimum inhibitory concentration (MIC) of 15 ng mL-1. In contrast, MoS2 QDs functionalized with ligands exhibiting the least exposure of interior hydrophobicity (Tetra-MQD) displayed the lowest antibacterial activity, with an MIC exceeding 12 000 ng mL-1. The antibacterial efficacy ranked as follows: No-EG-MQD > Mono-MQD > Di-MQD > Tri-MQD > Tetra-MQD. Mechanistic studies revealed that antibacterial activity was governed by a balance of membrane depolarization, membrane rupture, and cellular internalization. In another surprising observation, the toxicity of No-EG-MQD is much lower than that of EG-functionalized MQDs. These exceptional observations can be implemented in many other systems for related biological applications. After observing successful antibacterial activity and lower toxicity in vitro, we have tested their efficacy in vivo by treating a bacterially infected wound in a mouse to investigate additional biological impacts related to ligand hydrophobicity.