## 3 Results and Discussions
### Antibacterial Analysis  
In the literature, there are different reports about the antibacterial activity of GO. For example, some studies reported that the antibacterial activity against _E. coli_ was higher for GO than for reduced GO or graphite [58; 59], while other papers showed a bacterial growth with the addition of GO to the culture medium [60]. Therefore, due to the functionalization capacity of GO, it was tried to increase its antibacterial capacity by introducing different nanoparticles (i.e., Ag, Zn). The antimicrobial mechanism of GO-Ag is based on the fact that the Ag\({}^{+}\) ions released by silver nanoparticles can bind strongly to the thiol (SH) groups from the enzymes and proteins on the cell surface, causing damage to the cell walls through the holes in the membrane and in the cellular wall, thus ensuring the penetration of Ag nanoparticles into the bacteria [50; 61]  
In this study, the antibacterial effect of non-silanized powders (GO, GO-Ag, GO-Zn) and silanized powders (GO-APTES, GO-Ag-APTES, GO-Zn-APTES) against five bacterial strains (Table 1 and Figure 8) was investigated, following the direct contact between the powders and the culture medium. After the end of the incubation period, at 37 \({}^{\circ}\)C, the zones of inhibition (mm) were determined at the tested microbial strains. For all bacterial strains, it was observed that the results from the tested powders varied in the size of the diameter of the inhibition zone depending on the microbial strain tested. Following the performed test, a bacterial inhibition was noticed for all the samples against the tested bacterial strains. The highest values of the diameter of the inhibition zone (ranging between 17 and 22 mm) for all powder samples, including for the reference sample (GO) were recorded against _Streptococcus mutans_. By contrast to _Streptococcus mutans_, against the other bacterial strains, the control GO sample generated a constant antibacterial effect. The highest bacterial inhibition effect against _Streptococcus mutans_ was obtained in the case of GO-ZnO sample. The GO-Zn sample also produced the highest diameter of the inhibition zone (18 mm) against _Escherichia coli_ and _Staphylococcus aureus_, while all other \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample \({}^{\star}\)/Bacterial Strain** & **0** & **1** & **2** & **3** & **4** \\ \hline _Streptococcus mutans_ & 18 & 18 & 18 & 22 & 17 \\ \hline _Porphymomonas gingivalis_ & 14 & 17 & 20 & 19 & 15 \\ \hline _Enterococcus faecalis_ & 14 & 15 & 12 & 17 & 19 \\ \hline _Escherichia coli_ & 14 & 15 & 15 & 18 & 15 \\ \hline _Staphylococcus aureus_ & 14 & 16 & 15 & 18 & 14 \\ \hline \hline \end{tabular} \({}^{\star}\) Legend: 0 = GO, 1 = GO-Ag, 2 = GO-Ag-APTES, 3 = GO-Zn, 4 = GO-Zn-APTES.  
\end{table} 
Table 1: The diameter of inhibition zones (mm) for the tested graphenes. Figure 8: Images of samples (**a**) at time 0 and (**b**) after 18 h of incubation. samples generated rather similar antibacterial effects. Against _Porphyromonas gingivalis_, GO-Ag-APTES sample produced the highest value of the inhibition zone (20 mm); its antibacterial effect is similar to that of the GO-Zn sample. The GO-ZnO-APTES sample developed the largest diameter of the inhibition zone (19 mm) against _Enterococcus faecalis_. Similar results have been also reported in the literature [11; 62]. A comparison of the results obtained for the non-silanized samples (Figure 8 and Table 1) revealed that the GO-Zn sample exerted the highest antibacterial effect against all tested bacterial strains. However, it should be also taken into consideration that the Zn content in the GO-Zn sample is much higher as compared to the Ag content from the GO-Ag sample. Among the silanized samples, the highest antibacterial effect was noticed for the GO-Ag-APTES sample against the tested bacterial strains, except for _Enterococcus faecalis_, against which it developed the smallest diameter of the inhibition zone (12 mm).