## 4 Synergistic Effect Between Antimicrobial Polymers and Carbon Nanostructures  
Carbon-based nanomaterials (CBNs) have become crucial for a wide range of applications, including biomedical applications [133]. Functionalized CBNs have extensively been used as carriers of antibiotics, which would decrease resistance and enhance their bioavailability. Furthermore, CBNs have also shown potent antimicrobial properties by themselves, in particular carbon nanotubes (CNTs) and graphene oxide (GO) [134]. The features of CNTs attracted considerable attention as nano-reinforcernents for advanced polymer nanocomposites in several fields since [PERSON] first discovered them in 1991 [135]. It is well-known that CNTs have outstanding mechanical, electrical and thermal properties due to their remarkable structure [136]. In addition to the above mentioned features, in recent years it has been reported that CNTs have the ability to eliminate bacteria such as _E. coli_ and _S. aureus_ in solution [137, 138]. Figure 9 shows the proposed biocidal effect associated to a physical interaction of nanotubes acting as needles on the surface of microorganisms, which causes highly localized degradation of bacterial cell walls [139]. However, other secondary factors may also contribute to bacteria destruction, such as inhibiting cell \begin{table}
\begin{tabular}{c c c c} \hline \hline
**System** & **Nanoparticle** &
\begin{tabular}{c} **Microorganism** \\ **Tested** \\ \end{tabular} & **Synergistic Effect** & **Ref** \\ \hline CS coatings applied on & Ag (3 to 5 nm) & AgNPs disturbs the permeability, respiration and cell division. ZnO NPs produce ROS. Under light conditions, Ag improved the charge transfer, reducing the chance of electron-hole pairs to recombine and promoting the generation of perhydroxyl radicals and other potent oxidizing radicals. & [125] \\ CS coatings applied on & Ag (3 to 5 nm) & _E. coli, S. aureus_ & \\ cotton and cotton/polyester & ZnO & _E. coli, S. aureus_ & \\ CS/Ag, CS/ZnO, & Ag-doped ZnO & _E. coli, S. aureus_ & \\ CS/Ag-ZnO & (10 to 35 nm) & & \\ \hline CS NPs & Cu, TiO\({}_{2}\) & Negatively charged TiO\({}_{2}\) NPs acts as a copper ion carrier, and its surface can absorb positively charged copper ions. Cu in combination with TiO\({}_{2}\) can increase the amount of copper in bacteria and subsequently enhances antimicrobial activity. & [126] \\ \hline Quaterized CS-clay & Ag (\(\approx\)26 nm) & _E. coli, S. aureus, P. aeruginosa, B. subtilis_ & Exfoliated MMT with a large specific surface area adsorbs and fixes microorganisms. QAS disrupt cell membrane allowing AgNPs infiltrate and react with compounds in the cell wall. & [127] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._growth, oxidative stress and the presence of metal residues [140]. Previous results demonstrated that the antibacterial activity of CNTs depends on some physical factors such as length, diameter, dispersion and concentration [137; 140; 141]. The synergistic performance of CNTs combined with some polycations was explored by [PERSON] and coworkers, who synthesized multiwalled carbon nanotubes (MWCNT) functionalized with poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) [142]. Furthermore, PDMAEMA chains were quantized with bromoethane and the resulting hybrid material clearly revealed an antibacterial effect against _E. coli_ as well as against _S. aureus_. It was shown that the loss of viability of _E. coli_ highly depended on the PDMAEMA content on the MWCNT surface. Various more complex cationic polymers, such as dendrimers, have also been used to functionalize MWCNT aiming at enhanced biocidal properties. [PERSON] and coworkers evaluated the antimicrobial performance of MWCNT modified with an amphiphilic cationic dendrimer and silver nanoparticles [143]. The hybrid nanostructures were synthesized from carboxylated MWCNT (MWCNT-COOH). On the one hand, poly(propylene imine) dendrimer (PPI) were attached to MWCNT-COOH obtaining a MWCNT-PPI nanohybrid.