## 3 Environmental Applications of Graphene-Based Photocatalysts
### Antimicrobial Properties of Graphene-Based Composite Photocatalysts
The disinfection properties of GO and GO-based composite photocatalyst might be also enhanced by the modification with metals. For example, copper, one of the most efficient metals against microorganisms, has been introduced into semiconductor/GO in order to obtain enhanced activity in the dark and vis absorption. [PERSON] et al. have revealed that when Cu/ZnO@rGO reacts with bacterial cells, the composite induces the electron donor on the cell surface to react with oxygen molecules to generate and accumulate \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\), causing the oxidative stress (e.g., membrane damage and protein leakage), as shown in Figure 14 [123]. The modification of titania with rGO improves the photocatalytic efficiency due to enhanced charge separation, hindered charge carriers' recombination and vis absorbance (due to the mid-gap states above the valence band of titania) [130; 131]. Interestingly, [PERSON] et al. have revealed the dominant ROS formed on TiO\({}_{2}\)-rGO for the bactericidal effect, i.e., H\({}_{2}\)O\({}_{2}\), \({}^{\bullet}\)OH, and singlet oxygen under UV-vis, and only singlet oxygen under vis [126]. Moreover, Cu\({}_{2}\)O-TiO\({}_{2}\)/rGO exhibits higher antibacterial activity than bare TiO\({}_{2}\) under vis irradiation, i.e., larger zone of inhibition and lower MIC for both Gram-positive and Gram-negative bacteria [45], which is not surprising as titania is usually inactive under vis irradiation. [PERSON] et al. have compared the antimicrobial activity of graphite, GO, ZnO-GO and Ag-GO against _E. coli_, _S. aureus_, _Enterococcusecium_ and _Klebsiella pneumoniae_, and it has been found that though the antimicrobial effect (determined as MIC) does not depend on the bacteria group (gram-positive/negative), but is related to the microorganisms' physiology [132]. Interesting study has been shown by [PERSON] et al. indicating that the mechanism of action is influenced by the degree of contact between GO and bacterial cells [133]. The species of bacteria aggregated into pairs or present as single cells (e.g., Gram-negative _E. coli_) are more sensitive due to larger exposure to GO, whereas bacteria existed as clusters (e.g., gram-positive _S. aureus_) are more resistant due to the benefits from corporate life, which means that only outer bacteria are exposed (screening effect). Recently, interesting application of graphene has also been shown, i.e., the laser-induced graphene (LIG) filter, a microporous conductive graphene foam, formed by direct photothermal conversion of a carbon precursor by a commercial CO2 laser cutter [134]. It has been shown that bacteria are captured and destroyed efficiently by Joule-heating, reaching ca. 380 \({}^{\circ}\)C when a potential is applied. Graphene derivates, have also been proposed as antiviral materials to fight against pseudorabies virus (PRV), porcine epidemic diarrhea virus (PEDV), Herpes simplex virus type 1 (HSV-1), influenza A virus (H9N2) or SARS-CoV-2 [135]. Blocking of the virus binding sites with the graphene to prevent them from entering the target cells is the main mechanism. It has been demonstrated that negatively charged and sharp-edged GO causes the single-layer viral capsid damage by direct contact. High hopes are related to the use of graphene and its composites, e.g., with metals and polymers, for development of electrochemical, piezoelctric and Cluster Regulatory Interspaced Short Polindromic Repeats CRISPR/Cas technology-based biosensors for sensitive, early and fast diagnosis of virus infections. In summary, G/rGO/GO might work as efficient antimicrobial agents, especially when coupled with other antimicrobial agents, e.g., antimicrobial organic compounds, metal nanoparticles and photocatalysts. Many applications of antibacterial properties of graphene-based derivates have already been demonstrated, including bandage, gauze and  
Figure 14: Antibacterial mechanism of CuZnO@RGO nanocomposites, drawn based on reference [123]. Figure 13: SEM images of _B.