Antibacterial Properties Of G-C₃N₄-based Nanozymes

Nowadays,the problem of bacterial resistance caused by antibiotics has attracted widespread attention and the development of new antimicrobial agents with high antibacterial activity and the ability to effectively avoid bacterial resistance has become a research hotspot.As a new type of artificial enzyme,nanozymes play a crucial role in the production of reactive oxygen species（ROS）because of their peroxidase-like and oxidase-like enzymatic activities.Therefore,ROS-based bactericidal strategies have attracted great attention.However,the high activity and limited diffusion distance of ROS make it difficult to act effectively on bacteria,which affects the antibacterial activity of nanozymes.Currently,the use of photocatalysis to improve the bacterial inhibitory activity of nanozymes and the preparation of nanozymes with light response are the frontiers of research in the field of photocatalytic bacterial inhibition.With exceptional features of superior physicochemical properties and good biocompatibility,graphitic carbon nitride（g-C₃N₄）is an attractive photosensitizer for photocatalysis application.However,the smaller specific surface area,lower visible light utilization,poor electrical conductivity and rapid recombination of electron-hole pairs of g-C₃N₄limit its application in photocatalytic antibacterial applications.Therefore,this thesis focuses on the preparation of g-C₃N₄-based nanozymes with excellent photocatalytic properties,high enzymatic activity,efficient antibacterial properties and good biocompatibility,and explores its photocatalytic antibacterial mechanism by investigating the changes of morphological characteristics,catalytic properties and antibacterial properties before and after its modification,and lays the foundation for its further application in the antibacterial field.The specific research of this thesis is as follows:1.Study on the photocatalytic antibacterial performance of Cu-CDs-g-C₃N₄Cu-CDs-g-C₃N₄was synthesized by doping Cu-CDs with g-C₃N₄through calcination method.The energy band gap of Cu-CDs-g-C₃N₄is significantly lower than that of Cu-CDs and g-C₃N₄,which accelerates the separation efficiency of photoacoustic carriers,so that Cu-CDs-g-C₃N₄has excellent visible light response properties.Enzyme activity assay experiments showed that Cu-CDs-g-C₃N₄could catalyze more ROS production by H₂O₂under visible light irradiation.In vitro antibacterial experiments showed that Cu-CDs-g-C₃N₄showed outstanding broad-spectrum antibacterial effects against Escherichia coli（E.coli）and Staphylococcus aureus（S.aureus）with an antibacterial rate of 99.9%.Finally,by studying the ROS trapping experiments and SEM analysis,it was found that hydroxyl radicals destroyed the bacterial cell membranes to achieve the purpose of killing bacteria.2.Optical-ultrasonic enhancement of the antibacterial properties of g-C₃N₄-Cu-TCPP with peroxidase-like activityAlthough photodynamic therapy has made great progress in antibacterial,the antibacterial effect of the single visible light-excited photodynamic therapy is very limited because of the limited penetration depth of light.Acoustodynamic therapy can compensate well for this deficiency.Herein,sonosensitizer molecules meso-tetra（4-carboxyphenyl）porphyrin（TCPP）-modified graphitic carbon nitride（g-C₃N₄）（g-C₃N₄-Cu-TCPP）was linked by Cu.The obtained g-C₃N₄-Cu-TCPP exhibit highly efficient photoacoustic-augmented catalytic activities.Moreover,the peroxidase-like activity of g-C₃N₄-Cu-TCPP was enhanced by ultrasound-visible light stimulation to enhance ROS production to achieve effective antimicrobial activities against both gram-negative and gram-positive bacteria with killing efficiency up to nearly 99.99%.Furthermore,in vitro antibacterial experiments results indicated that g-C₃N₄-Cu-TCPP exerted a synergistic effect for antimicrobial through disrupting cell integrity and producing ROS.Another possible reason is that a large amount of generation of h⁺and e^-under the photoacoustic-augmented caused charge transfer made a great contribution to the strong antibacterial capacity of the g-C₃N₄-Cu-TCPP.The developed photoacoustic nanozyme system provides a promising strategy for eradication of bacterial infection.3.Study of antibacterial properties of Ce O₂-g-C₃N₄with oxygen vacanciesCompared to visible light,the infrared light has a stronger antimicrobial effect due to its greater tissue penetration and thermal effect.In this study,Ce O₂-g-C₃N₄composite nanozyme with infrared light response properties was synthesized.The morphological characteristics and chemical structure of the nanozyme were confirmed by transmission electron microscope（TEM）,fourier transform infrared spectrometer（FTIR）and x-ray photoelectron spectroscopy（XPS）characterization.In addition,it was found by enzyme activity assay experiments that the composite material has excellent enzyme-like activity activities compared with Ce O₂and g-C₃N₄.Moreover,based on the ability of NIR to enhance the peroxidase activity of Ce O₂-g-C₃N₄,the antibacterial properties of Ce O₂-g-C₃N₄were investigated using a plate counting method.After IR illumination for 10 min,Ce O₂-g-C₃N₄could achieve99.9%antibacterial rate against E.coli,S.aureus,AREC and MRSA.In addition,the antibacterial mechanism was verified by measuring the intracellular ROS production and SEM observation of bacterial morphology,and it was found that Ce O₂-g-C₃N₄produced bactericidal effects mainly through the production of ROS and disruption of bacterial cell membranes.The experimental results showed that the nanozyme had stronger antibacterial effect in synergy with photodynamic therapy and chemodynamic therapy.In summary,the g-C₃N₄materials were modified to obtain Cu-CDs-g-C₃N₄,g-C₃N₄-Cu-TCPP and Ce O₂-g-C₃N₄with good photocatalytic properties,which can realize the application value in various catalytic fields.