# Graphene-Based Functional Hybrid Membranes for Antimicrobial Applications: A Review
## 5 Antimicrobial Applications
### Antibacterial Applications  
As the human microbiome develops, associations between certain bacteria and specific human diseases are gradually being established. _Escherichia coli_ and _Staphylococcus aureus_ are associated with infections in different parts of the body [99]. _Streptococcus mutans_, _Porthymomonas gingivalis_, and _Candida albicans_ have been associated with oral disease [100; 101; 102]. _POAI_ and _Klebsiella pneumoniae_ are typical nosocomial pathogens [103; 104]. _Salmonella typhimurium_ is associated with gastrointestinal disease [105]. Due to its special physicochemical properties and unique antibacterial mechanism, graphene membrane material has great potential to become a novel antibiotic-independent antibacterial drug. The antibacterial mechanisms of graphene nanomaterials are diverse, and the main antibacterial mechanisms against different bacteria may differ. GO and rGO are toxic to both Gram-positive (_Staphylococcus aureus_) and Gram-negative (_Escherichia coli_) bacteria. Previous studies have shown that the inhibition of cell division may be the main antibacterial mechanism against Gram-positive bacteria, and mechanical damage may be the main antibacterial mechanism against Gram-negative bacteria [106]. The study by [PERSON] et al. showed that the antibacterial activity of GO was clearly dependent on the cellular physiology of Gram-negative and positive bacteria [107]. The sensitivity of bacteria to GO is highest in the exponential growth phase (i.e., the physiological growth phase), while cells in the stationary growth phase (i.e., the non-growing phase) have considerable resistance to GO. Importantly, the order of _E. coli_ susceptibility to GO during growth stages is closely related to changes in the ultrastructure of the cell envelope. Graphene-based membranes can be used to effectively inhibit the growth of planktonic _Streptococcus mutans_ and their biofilm formation. For instance, [PERSON] et al. synthesized graphene nanosheets (GR) and found that an oral adhesive filled with a graphene-based film had a significant inhibitory effect on the adhesion, growth and even biofilm formation of Streptococcus mutans [108], and that oral adhesives filled with graphene-based films exhibited a significant inhibition of adhesion. In the study of [PERSON] et al. [109], azide-functionalized AGO was covalently immobilized on the surface of a reverse osmosis membrane (RO) by azide photochemistry. Surface modification was performed by coating the reverse osmosis membrane with an aqueous dispersion of AGO followed by UV exposure under ambient conditions (Figure 6a). This simple process produces a hydrophilic, smooth, antimicrobial membrane with limited reductions in water permeability or salt selectivity. The biofouling of GO-RO membranes was reduced 17-fold after exposure to _E. coli_ for 24 h. [PERSON] et al. developed a simple, one-step probe sonication method to synthesize ZnO nanoparticle-functionalized rGO-ZnO nanocomposites by exfoliating graphite oxide in the presence of Zn\({}^{2+}\)[110]. The antibacterial activity of nanocomposites against Gram-negativebacteria (_Escherichia coli_ and _Serratia marcescens_) and Gram-positive bacteria (_Bacillus subtilis_) is higher than that of GO and rGO. Furthermore, the incorporation of rGO-ZnO nanocomposites into PES membranes can inhibit the growth of biofilms compared to pristine PES membranes. To improve the anti-b biofouling performance of ultrafiltration membranes, [PERSON] et al. synthesized quatermized graphene oxide (QGO) as a membrane modified by the in-situ growth of quaternary ammonium salts on the surface of ultrafiltration membranes. For the first time, QGO with hydrophilic and antibacterial properties was prepared by chemically grafting quaternary ammonium salt groups with carboxyl and epoxy groups on graphene oxide as active sites [111]. Using QGD as a modifier, a hydrophilic antibacterial bifunctional membrane was prepared by the dipping phase inversion method.