## 3 Environmental Applications of Graphene-Based Photocatalysts  
### Antimicrobial Properties of Graphene-Based Composite Photocatalysts  
Various antimicrobial materials have been intensively studied to protect humans, animals, and the whole environment, including metals (e.g., silver and copper) [77; 78; 79], titania [80; 81; 82; 83; 84] and organic compounds (e.g., antibiotics and chlorine compounds). Although many of them have been widely applied to daily necessities, the concerns about environmental contamination, the possible toxicity [85], antibiotic resistant bacteria (ARB), and high costs still remain. From these points of view, carbon (graphene) compounds might be the promising antimicrobial materials. It should be pointed out that the study on the antimicrobial activity of graphene species is a new topic as first report on GO/TiO\({}_{2}\) was published by [PERSON] and [PERSON] in 2009 [86]. In this report, though GO/TiO\({}_{2}\) composites have exhibited much higher activity than titania, GO seems to be inactive both under UV-vis irradiation and in dark condition and, thus, it can only accelerate the activity of titania. In 2010, the same authors have proposed that graphene nanosheets in the form of nano-walls have shown bactericidal activity both for gram-negative (_Escherichia coli_) and Gram-positive (_Staphylococcus aureus_) bacteria (several percent to several tens of percent survival) with RNA effluxes, and rGO has higher activity than GO [87]. Since then, many studies have been performed and even summarized in some review papers and, thus, in this section, only the recent reports are presented (summarized in Table 2) and discussed. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **Preparation Method** & **Experimental Conditions** & **Findings** & **Ref.** \\ \hline GO & HM (GO)/ & PA; _Streptococcus_ sp., & & \\ G & GO red. by _Allium capa_ & _Staphylococcus_ sp., _E. coli_ or & activity: GO \textgreater{} G & [88] \\  & extract (G) & _P. aeruginosa_. & & \\ \hline GO & sonication of commercial GO & & & low conc.: enh. growth; high conc.: & \\  & to reduce sample polydispersity & & & inh. growth. & [89] \\ \hline GO & graphite oxidation by BaM (GO) & CC; _E. coli_ cultured in LB or & in LB 35\% and 45\% CFU red.; & \\ rGO & and BrM (rGO) & NB media & for GO and rGO, respectively & & \\  & & & (24 h) & \\ \hline rGO & graphite oxidation by & Fluorescence analysis & in NaCl no effect; & \\ rGO & KMnO\({}_{4}\) (GO); & using GFP-labelled _E. coli_ & in NB inh. growth (nutrient ads.) & [91] \\  & GO red. by L-ascorbic acid (rGO) & & & \\ \hline  & HM (GO); & & & \\ CS6 GO & GO and CSCI mixing, & & & 96\% inhibition for 0.6\% GO; & \\  & evaporating, freeze-drying & & stable after 3 times (>90\%) & [92] \\  & & & & \\ \hline \hline ads.—adsorption; BaM—Bangal method; BrM—Brodie Method; CC—colony-counting method; conc. —concentration; CS—chitosan; enh.—enhanced; HM—Hummers’ method; inh.—inhibited; LB—Luria-Bertani; MH—Mueller-Hinton; NB—nutrient broth; PA—photoabsorption measurements by UV/vis at 500–600 nm; red. —reduction. & [91] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Graphene compounds for microbial disinfection. For example, it has been shown that GO exhibits higher bactericidal activity than graphene (prepared by the reduction of GO by _Allium cepa_ extracts), possibly due to the typical aggregation of graphene [88]. The concentration of GO significantly influences the bacterial viability, i.e., at low concentrations, GO forms floating scaffolds, which enhances bacterial growth, whereas at high concentrations, GO forms scaffolds, which inhibits the bacterial growth [89]. Moreover, it has been suggested that the morphology of GO membrane (roughness, sharpness, and curvature) is important for bactericidal effect, i.e., the rougher or sharper the membrane is, the higher is bactericidal activity [93; 94]. Interestingly, it has been shown that culture media influence the bactericidal effect [90; 91]. For example, bacteria are inactivated on GO and rGO (ca.