# Carbon Nanotubes-Based Hydrogels for Bacterial Eradiation and Wound-Healing Applications
## 4 CNT-Based Antibacterial Applications
### Antibacterial Activity of Pristine CNT  
The antibacterial activity of CNTs has been studied widely against numerous bacteria, such as _E. coli_, _Staphylococcus aureus_, _Staphylococcus emipermidis_, and _Bacillus subtilis_. Microbes can rapidly increase their population in the wounded areas and adversely affect the wound-healing process [84]. Hence, antibacterial wound-healing patches are desirable and favorable for protecting the wound from infections and accelerating the healing process. [PERSON] et al., demonstrated the antibacterial effect of SWCNTs against _E. coli_. SWCNTs showed strong antibacterial potential and damaged the bacterial cell membrane [85; 86]. CNTs can directly damage the bacterial cell membrane or increase the reactive oxygen species (ROS) production, causing decrease cell viability [87]. Different conditions, such as pH temperature, retention time, and solute and solvent compositions, also affect bacterial eradication [88]. [PERSON] et al., investigated the antibacterial properties of SWCNTs dispersed in different surfactant solutions, including sodium cholate, sodium dodecyl benzenesulfonate, and sodium dodecyl sulfate against _Salmonella enterica_ (_S. enterica_), _E. coli_, and _Enterococcus ficeium_. SWCNTs concentrations played a significant role in bacterial cell viability [89]. The probable mechanisms for CNTs induced are the inhibition of bacterial growth by impairing the respiratory chain; inhibition of energy metabolism; physical interaction with the cell membrane, formation of cell-CNTs aggregates, and induction of cell membrane disruption. SWCNTs have also shown excellent antimicrobial properties [90; 91; 92]. The size of CNTs contributes a considerable role in the deactivation of microorganisms. The smaller-sized CNTs have a larger surface-to-volume ratio, which helps to make strong bonds with the cell wall or membrane of bacteria, showing better antibacterial potential [93]. SWCNTs primarily aggregate with the cell wall, which is followed by induction of cell membrane rupture, hindering DNA replication [94]. It has also been mentioned that the surface charge of CNTs has a significant role in the inactivation of bacteria by cell membrane interruption [95]. SWCNTs have a specific surface area of approximately 407 m\({}^{2}\)/g, which can remove 3.18 \(\times\) 10\({}^{12}\) CFU/mL [96]. [PERSON] et al., assessed the effect of CNTs' surface charge on bacterial death and found that positive and negative charge dots had antibacterial activity. Reactive oxygen species, such as hydroxyl radicals, are generated by the interaction of CNTs and the cell membrane, which kills the bacteria [97]. [PERSON] et al., assessed that longer (\(\approx\)5 \(\upmu\)m) SWCNTs make better aggregation and show more robust antimicrobial activity. The different activity was observed in the solid and liquid media. In solid media, shorted (<1 \(\upmu\)m) CNTs showed effective antimicrobial activity than the longer ones [98]. The MWCNTs with 50 \(\upmu\)m length wrap on all sides of a microbial cell and cause osmotic lysis. Whereas, in liquid media, longer CNT are more effective in bacterial cell damage. The aggregation or interaction between CNTs and the bacterial cell membrane is unavoidable because of their particular structure and strong van der Waals forces [99]. The tube diameter of CNT also affects the antimicrobial activity. Smaller diameters entail better interactions with the cell wall, mediating determinants to the cell [88]. CNTs with a 1.5 nm diameter act as needles connected to the membrane from one side, and in diameters of 15-30 nm, CNTs are connected to the sidewalls [100]. [PERSON] et al., demonstrated that CNTs have lower activity against _Bacili_ than _Cocci_[101]. The mechanism of these bacteriostatic properties is related to their diameter-dependent penetration and length-dependent wrapping on the disruption of cell walls and membranes of bacteria and intracellular substances such as DNA and RNA.