## 4 Synergistic Effect Between Antimicrobial Polymers and Carbon Nanostructures
On the other hand, AgNPs were deposited on MWCNT-PPI, yielding a silver-dendrimer complex MWCNT-PPI-Ag. The results demonstrated that the antimicrobial activity of both nanohybrids against _B. subtilis_, _S. aureus_, and _E. coli_ was achieved following the order of efficiency in terms of percentages of the kill of MWCNTs-APPI-AgNPs > MWCNTs-PPI > MWCNTs-COOH. As in previous cases, quaternary ammonium groups helped to increase permeability of the cell membrane, and therefore, significantly enhancing the antimicrobial activity against the _B. subtilis_. In the case of MWCNTs-PPI-AgNPs (Figure 10), the cooperative effects of AgNPs deposition onto MWCNTs-PPI lead to an improvement of the action against the microorganisms and, hence, it was able to destroy _B. subtilis_ and _S. aureus_ bacteria more effectively than the other two materials. Graphene is a single atomic plane of hexagonally arranged carbon atoms obtained through the exfoliation of tridimensional graphite [144]. The sp\({}^{2}\) hybrid carbon framework confers to graphene materials (GMs) excellent thermal, mechanical and electrical properties which are applied to the design of nanoelectronics, biosensors and transistors [145]. Several studies have demonstrated the  
Figure 10: MWCNTs-PPI-AgNP hybrid. Reproduced with permission from reference [143]. Copyright 2011 Elsevier. Figure 9: Proposed mechanism of action of CNTs against bacteria. high antimicrobial activity of graphene and graphene oxide derivatives against a wide variety of microorganisms, including Gram-positive and Gram-negative bacteria, with little resistance and a tolerable effect on mammalian cells [146; 147; 148; 149]. Although the mechanism of action of graphene is not entirely elucidated, in general, as in the case of CNTs, it involves both physical and chemical effects where nanosheets are in direct contact with microorganisms [150]. Physical results mainly imply bacterial membrane damage caused by the sharp edges of graphene and the subsequent leakage of intracellular substances, which ultimately leads to bacterial death. Furthermore, the wrapping or trapping mechanisms might be also involved, avoiding bacteria proliferation by reducing the microbial metabolic activity. On the other hand, the chemical effect is principally associated to a charge transfer mechanism or to an oxidative stress mediated by ROS. Modern approaches combining GMs with other biocidal agents as quaternary ammonium [151] and phosphonium salts [152], silver [153] as well as peptides [154] have been successfully used to develop water disinfection membranes, hydrogels or coatings with enhanced properties, which can be applied in biological and medical sciences. Besides, it has been demonstrated that the antimicrobial performance of GMs improves when combined with cationic polymers. For example, a nanocomposite from GO and CS with enhanced antibacterial properties was reported by [PERSON] et al. [155], where an increase in the roughness of CS-GO surface (in comparison to the smooth morphology exhibited by GO) facilitated bacterial cell adhesion, which finally caused the death of a higher number of _B. subtilis_ and _E. coli_ cells. Therefore, CNTs and GMs represent an excellent alternative to generate active dual antibacterial compounds due to the aforementioned intrinsic properties. Their antimicrobial nature strongly depends on the size and dispersion to get contact between the CNTs/GMs and the bacterial cell surface. Nevertheless, the major drawback of these carbon structures is that they tend to self-associate and aggregate owing to strong van der Waals interactions between them, which would prevent direct contact with the bacterium causing a decrease in antibacterial activity. Thus, it is necessary to decrease this physical interaction with an efficient exfoliation process and/or via surface functionalization.