## 3 Environmental Applications of Graphene-Based Photocatalysts
### Antimicrobial Properties of Graphene-Based Composite Photocatalysts
45 and 35% inhibition at 24 h for GO and rGO, respectively) in Luria-Bertani (LB) medium due to oxidative stress (the deficiency of Mg\({}^{2+}\) and Ca\({}^{2+}\) ions), but antibacterial effect is negligible in Mueller-Hinton (MH) medium (with 5-10 higher concentration of Mg\({}^{2+}\) and Ca\({}^{2+}\) ions) [90]. [PERSON] et al. have proposed that rGO has no direct bacterial toxicity, i.e., rGO does not show the bactericidal activity in physiological saline solution, even though the aggregation of bacteria and rGO is observed, resulting in the efficient removal of live/dead bacteria by filtration [14]. Moreover, inhibited growth of bacteria in nutrient broth (NB) medium has not been caused by antibacterial activity of rGO, but only because of the adsorption of nutrients on rGO (no cell damages) [91]. It should be mentioned that carbon (not necessary in graphene form) could be also responsible for bactericidal activity. For example, practically same antibacterial activity has been achieved on graphene (rGO)- and carbon-modified titania samples (prepared by same method via laser synthesis) both in the dark and under vis irradiation (three times higher activity under vis than that in the dark) [11]. Interestingly, both samples show also almost the same activity for oxidative decomposition of organic compounds under UV and vis irradiation, whereas rGO/TiO\({}_{2}\) sample exhibits much higher activity only for evolution of hydrogen under UV irradiation, indicating that graphene might work as co-cataly for hydrogen evolution. A total of three main mechanisms of bactericidal action of graphene and its compounds have been considered, as follows: (i) the cell membrane damage [87; 95], (ii) oxidative stress by reactive oxygen species (ROS) [96], and (iii) charge transfer [97]. It should be pointed out that, generally, the efficiency of antimicrobial agents for organisms is quite different not only for different categories (e.g., mammalian and bacteria), but even among the species of the same genera (e.g., _Aspergillus niger_ and _A. melleus_). Indeed, [PERSON] et al. have clearly shown different response for different fungi to G/GO, where G inhibits the growth of _A. flavus_, whereas _A. niger_ is inhibited by GO, indicating that G and GO have different toxicity mechanisms against fungi (e.g., hyphae cell wall damage, and changes in synthesis of enzymes) [98]. Further applications of graphene materials have been examined for the microorganisms that cause chronic wounds and disease (due to prolonged inflammation, persistent microbial infections and multispecies drug-resistant microbial biofilms). For example, antimicrobial and antibofilm properties of GO against bacterial wound pathogens, such as _S. aureus_, _Pseudomonas aeruginosa_, and yeast _Candida albicans_, have been elucidated, and it has been considered that the interactions between GO and microorganisms, i.e., hydrogen bond interactions between carbonyl and hydroxyl groups abundant in polysaccharides and GO and \(\pi\)-\(\pi\) interactions between DNA and RNA bases and graphenic domains in GO, result in the inhibition of microorganisms' aggregation and adhesion on the surfaces [99]. Interestingly, different from the direct killing of microorganisms by GO, it has been proposed that poly(ethylene glycol-amine)-derivatized GO nanosheets (PEG-GO) can stimulate and activate macrophages (M1 and M2) and more efficient phagocytosis of _C. albicans_ is observed than that on unstimulated GO, indicating the possibility of avoiding some chronic infectious diseases [100]. Moreover, GO modification with organic compounds that possess the antimicrobial activity and biocompatibility have been intensively studied very recently, e.g., chitosan (CS) [92; 101; 102; 103], curcumin [104; 105], antimicrobial agents (isoniazid (INH), amikacin (AMK) and linezold (LZD), Inomycin hydrochloride (LMH), chloramphenicol (CPC) and gentamycin sulfate (GMS) [106; 107]) and dyes [108]. Chitosan exhibits high biocompatibility, biodegradability and antibacterial properties [109].