Invetigations On The Preparation Of Graphene Quantum Dots-Silver Nanoparticles And Their Antibacterial Performances

With increasing concerns on bacterial infections,it is highly desirable to develop effective and broad spectrum antibacterial agents.Considered as a viable alternative to antibiotics,nanoparticles appear to exhibit high potential to solve the problem of bacterial multidrug resistance,with silver nanoparticles(Ag NPs)showing particular promise.Nevertheless,the naked Ag NPs show low stability and tend to aggregation,the modification is necessary.Recently,it was found that the oxidase-like nanoparticles demonstrated obviously enhanced antibacterial performance through promoting the generation of ROS.However,according to density functional theory calculations and experimental verification,Ag NPs lack oxidase-like activity.Herein,we report the first attempt of using graphene quantum dot-Ag nanoparticles(GQDs-Ag NPs)as oxidase mimics and antibacterial agents.In Chapter 2,we used a facile green approach for in-situ growth of Ag NPs on the surface of GQDs.GQDs serve as both reducing agent and stabilizer without any reducing agent and stabilizer.It was demonstrated that Ag NPs are highly dispersible and stable in water in the absence of any stabilizing agent.Then,GQDs-Ag NPs were characterized by Fourier transform infrared spectroscopy(FT-IR),ultraviolet and visible spectroscopy(UV-vis),Raman spectroscopy,scanning electron microscope(SEM),transmission electron microscope(TEM),which confirmed that GQDs-Ag NPs prepared successfully by in-situ growth of Ag NPs onto the surface of GQDs.In Chapter 3,the antibacterial performances and antibacterial mechanism of the GQDs-Ag NPs were investigated by three kinds of bacteria as model.Comparison to other silver graphene-based materials,the GQDs-Ag NPs exhibit an unexceptionable oxidase-like catalytic activity,and possess a high activity and stability in weak-acidic medium within the range from 25℃ to 60℃.GQDs-Ag NPs show excellent antibacterial properties against Gram-negative and Gram-positive bacteria as well as drug resistant bacteria,with a low inhibitory concentration(2-4 μg·mL-1)against 107～108μg·mL-1 of Escherichia coli and Staphylococcus aureus.In the presence of the GQDs-Ag NPs,the appearance of green fluorescence after adding 2’,7’-dichlorofluorescin diacetate demonstrated that reactive oxidative species(ROS)existed in the GQDs-Ag NPs antibacterial system.In addition,TEM and SEM images identified that the disruption of the bacterial cell membrane and loss of barrier function during the sterilization process.Furthermore,the bactericidal performance of the GQDs-Ag NPs under the assistance of H2O2 or laser irradiation were investigated using the gram-positive bacteria(E.coli)and gram-negative bacteria(S.aureus)as representative model.It turned out that the bactericidal performance of GQDs-Ag NPs could be improved with the assistance of H2O2 or laser irradiation.In consideration of inconvenience and potential risk,we suggest that the sufficiently high antibacterial efficiency of GQDs-Ag NPs are applied alone without assistance.Therefore,the GQDs-Ag NPs as a novel oxidase-nanomaterial,exhibit vast potentials for serving as highly effective,broad-spectrum antibacterial agent for sterilization use,facilitating their relative ease of use and cost-effectiveness.