NIR-Light-Active ZnO-Based Nanohybrids for Bacterial Biofilm Treatment

Nanomaterials with antimicrobial properties triggered by external stimuli appear to be a promising and innovative substitute for the destruction of antibiotic-resistant superbugs as they can induce multiple disruptions in the cellular mechanism. This study demonstrates the use of squaraine (SQ) dye as the photosensitive material, activated in the near-infrared tissue-transparent therapeutic window. The dye has been covalently attached to the ZnO nanoparticle surface, forming ZnO-SQ nanohybrids. The formation of the nanohybrids is confirmed using Fourier transform infrared and other optical spectroscopic methods. The photoinduced interfacial electron transfer process (as confirmed using the time-resolved fluorescence technique) from the excited state of SQ to the conduction band of ZnO is responsible for the greater reactive oxygen species (ROS) generation ability of the nanohybrid. The production of photoactivated ROS (especially singlet oxygen species) by ZnO-SQ provides remarkable antimicrobial action against clinically significant Staphylococcus aureus. Detailed investigations suggest synergistic involvement of cell membrane disruption and nanoparticle internalization followed by photoinduced intracellular ROS generation, which result in an unprecedented 95% bacterial killing activity by the nanohybrid. Moreover, the efficacy of the nanohybrid for disruption of bacterial biofilms has been examined. The electron microscopic images suggest significant bacterial cell death following structural alteration and reduced adherence property of the biofilms. Nanodimension-driven greater internalization of ZnO-SQ followed by an improved dissolution of ZnO in an acidic environment of the biofilm as well as red-light-driven interfacial charge separation and ROS generation improves the efficacy of the material for biofilm destruction. An artificial medical implant mimicking titanium sheets coated with ZnO-SQ depicts light-triggered disruption in the adherence property of matured biofilms. The cytotoxicity and hemolysis assays show inherent biocompatibility of the photoactive nanohybrid. This study is notably promising for the treatment of life-threatening drug-resistant infections and eradication of biofilms formed within artificial implants.