The Antibacterial Performance And Mechanisms Of Grapheme-based Nanocomposites

With the increasing of environmental pollution,especially pathogenic microorganism pollution,there was an increasing demand for novel antimicrobial materials with improved performance.Nanomaterials have received a world-wide attention due to its high efficiency properties and low risk of secondary pollution.Graphene as a novel nanomaterial with unique physicochemical properties has been developed for antibacterial application.Graphene with bactericidal activity also provides potential application as a substrate for growing various nanomaterials.The uniform distribution of nanoparticles on the surface of graphene sheets could minimize the aggregation of nanoparticles,as well as improve the stability and bactericidal activity of nanoparticles.In this study,graphene-based materials with high stability,high antibacterial efficiency and high reusability were synthesized by loading some active nanoparticles on graphene sheets.The antibacterial performance and mechanisms of grahene-based materials toward two types of bacteria,including Gram positive bacteria S.aureus and Gram negative bacteria E.coli,was investigated.The specific research work and the results could be summarized as follows:The first section described the synthesis and antibacterial performance of graphene-based nanocomposites with separation-convenient property,such as magnetic-graphene oxide(M-GO),melamine sponge decorated with silver nanoparticles-modified graphene(G/AgNPs-MS)and copper nanoparticle s-decorated graphene sponge(Cu-GS).In this study,graphene-based nanocomposites were applied to treat the water contaminated with E.coli and/or S.Aureus.The effect of incubation time,the materials concentration and the initial inoculums concentration on the bactericidal activities was also investigated.The results could be concluded as follows:(1)the superior bactericidal efficiency of graphene-based nanocomposites was ascribed to the coordination of graphene and antibacterial nanoparticles;the inactivation percentage up to 91.49±2.82%at the M-GO concentration of 100 ?g/L;the inactivation efficiency of G/AgNPs-MS was 97.1%for E.coli and 90.9%for S.aureus,respectively,with 10 min incubation;around 3.17-log of viable E.coli(equal to 99.9%)were inactivated after percolating though the Cu-GS with 5 min of hydraulic retention time.(2)the antibacterial activitise of graphene-based nanocomposites were time-and concentration-dependent;the percent of cell viability loss gradually went up with the increased incubation time and the increased concentration of antibacterial materials;efficacy of graphene-based nanocomposites also increased with the decreased bacterial concentration,suggesting the bacterial-concentration dependence of bactericidal activity.(3)graphene-based nanocomposites could be easily separate and recover from contaminated water;magnet or hand compression could be used to separate the M-GO or G/AgNPs-MS from bacterial suspensions;the antibacterial efficiency still up to 87.1%for E.coli and 85.64%for S.aureus at the 12th cycle.The second section focused on the antibacterial mechanisms of separation-convenient graphene-based nanocomposites.The influences of the surface properties,oxidation ability of antibacterial materials and the released ions from nanoparticles on their bactericidal performance have been investigated.The changes of cell morphology,membrane integrity and intracellular ROS were also examined.The results show that(1)the surface properties,including surface roughness and fractal dimension,of materials could significantly influence the attachment of bacteria on the material surface at the period of interaction between materials and bacteria;the antibacterial ability of materials increased with the increase of their oxidation ability;the ions released from nanoparticles,such as Ag+ and Cu2+,had an effect on the antibacterial activity of materials,but was not the primary influence factor.(2)Results from TEM suggested that M-GO nanomaterials were possible to deposit on or penetrate into E.coli cells leading to leakage of intercellular contents and loss of cell integrity;flow cytometric analysis and the generation of intercellular ROS confirmed the involvement of the destruction of bacterial membrane and ROS in the antibacterial process by G/AgNPs-MS and Cu-GS.(3)The inactivation performance of bacteria by graphene-based nanocomposites was supposed to result from both the membrane stress and oxidation stress during the incubation period.The third section investigated the inactivation performance of visible light-responsive graphene-based nanocomposites.Two types of graphene-based nanocomposites,including graphene oxide-Ag3PO4(GO-Ag3PO4)and graphene-CdS(G-CdS),with visible light-responsive property have been prepared for inactivation of E.coli and/or S.aureus in water.The influences of factors such as irradiation time,initial concentration of materials and humic acid(HA)on the photocatalytic inactivation efficiency were examined.The photocatalytic stability and reusability of antibacterial materials were also investigated.Results showed that(1)the adsorption intensity of Ag3PO4 and CdS over the entire visible light range were enhanced in the presence of GO;exposure to G-CdS resulted in 5.3-log reduction of viable E.coli bacteria after 60 min irradiation,and the inactivation efficiency of G-CdS was around 5 times higher than that of pure CdS.(2)the antibacterial activity of graphene-based photocatalytic materials were time-and concentration-dependent;the inactivation efficiency increased with the increased irradiation time and the increased concentration of antibacterial materials.(3)after three repeated experiments,the photocatalytic activities of GO-Ag3PO4 and G-CdS composites displayed a slight decrease,suggesting the good stability.(4)the presence of HA reduced the antibacterial activity of G-CdS composite for E.coli,which could be attributed to that HA absorbed on GO surface acted as a physical barrier to prevent the contact between bacteria cells and G-CdS,and an antioxidant to consume some toxic ROS.In the fourth section,the photocatalytic bactericidal mechanisms of graphene-based nanocomposites with visible light-responsive property were investigated.The visible light-responsive properties and the changes of enzyme activities were also examined.The results could be concluded as follows:(1)the existence of GO in the composite could play a significant role in the enhancement of photocatalytic activity due to the high surface area of GO sheets which offered more active adsorption sites for bacteria,and the excellent electronic conductivity and store electricity which facilitated the transportation and transfer of electrons into GO sheets.(2)the excessive ROS generation in the photocatalytic process overloaded the antioxidant defenses of cell,resulting in the eventual death of cells.