Nanocomposite of Ag nanoparticles and catalytic fluorescent carbon dots for synergistic bactericidal activity through enhanced reactive oxygen species generation.

Microwave mediated synthesis of catalytic fluorescent carbon dots (Cdots) has been reported using biodegradable starch as precursor. The as-synthesized Cdots were then characterized using various techniques such as fluorescence spectroscopy, fourier-transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) analysis. Interestingly, Cdots showed high catalytic activity in the photo-reduction of Ag+ to silver nanoparticles (Ag NPs). During the photo-reduction process, no additional surface passivating agents was needed to stabilize the Ag NPs. Further, TEM results indicated the formation of Cdot-Ag NP nanocomposite i.e. Ag NPs surrounded with Cdots, and the emission intensity of Cdots was significantly decreased whereas the lifetime of Cdots remained almost unaltered in the presence of Ag NPs following static quenching. Finally, combination therapy of Cdots and Ag NPs using Cdot-Ag NP nanocomposite was performed which indicated synergistic bactericidal activity against antibiotic resistant recombinant E. coli bacteria. The treatment elevated the reactive oxygen species (ROS) level as compared to its individual components. Additionally, the flow cytometer study demonstrated that combination therapy causing bacterial cell wall perforation that was possibly leading to synergistic bactericidal activity against both Gram positive and Gram negative bacteria. The presence of Cdots on the surface of the Ag NPs due to their ground state complexation, possibly facilitated electrons towards Ag NPs which enhanced the ROS production in comparison to only Ag NPs.