# Silver Nanowires: Synthesis, Antibacterialativity and Biomedical Applications
## 3 Mechanism of Antibacterial Activity
### Silver Nanowires Mechanism of Action  
As previously stated, AgNPs have been shown to exert their antimicrobial activity in ways similar to that of Ag\({}^{+}\) ions and silver nanoparticles. This section will focus primarily on the differences of mechanism of action researchers have identified for AgNPs on various Gram-positive and Gram-negative bacteria, with an overview of the shape and size effect. [PERSON] et al. [33] performed some of the earliest work that directly compared the antibacterial effects of AgNPs (70-150 nm diameter; 3-8 \(\upmu\)m length) to silver nanospheres (98 nm diameter); in order to do this, they selected silver particles of similar diameter and surface charge and comparable Ag\({}^{+}\) release rate. By using a bioluminescent _E. coli_ strain, the bioluminescence of which decreased in response to disruption of cellular energy production and damage to membrane integrity, resulting in a decrease in overall cell viability; they demonstrated how the release of silver ions dictated toxicity to bacterial cells, and that particle shape had no significant effect. However, they propose that byengineering the size and shape of the particles, antibacterial activity can be controlled, as these factors will affect Ag\({}^{+}\) release rate; they also suggest that in smaller particles, other factors might play a role. Recent studies by [PERSON] et al. [34] have examined the effect of the shape and size of silver nanosheets in relation to their activity against _E. coli_. By altering NaCl concentrations, [PERSON] et al. successfully synthesized silver nanospheres (60 nm diameter), nanocubes (55 nm edge length) and nanowires (60 nm diameter, 2-4 \(\upmu\)m length) via a soft-template microwave-assisted method. It was reported that the AgNPs effectively prevented growth at 1.5 \(\upmu\)g/mL, which was not statistically different from silver nanospheres; however, silver nanocubes inhibited _E. coli_ at concentrations as low as 0.12 \(\upmu\)g/mL. TEM studies showed the higher interaction occurring between the bacterial cell membrane and spheres and cubes compared to wires (Figure 2); the authors suggest that this interaction is critical to the expression of antibacterial activity that also depends on the reactivity of the crystal facets. In fact, silver nanocubes with highly active {100} facets present higher activity than both spheres and wires with exposed {111} facets. The relationship between rate of dissolution and antibacterial activity was demonstrated also by [PERSON] et al. [35] who studied silver nanoparticles (40-70 and 120-180 nm in diameter), nanowires (50-100 nm in diameter and 1-20 \(\upmu\)m in length), nanocubes (140-180 nm in edge length) and nanoplates (20-60 nm in diameter), and found that the activity is directly related to the surface area, and therefore to the Ag\({}^{+}\) release rate. They also observed that this was not the case in eukaryotic cells that are able to internalize the smaller particles as opposed to prokaryotic cells were only an interaction with the cell membrane is observed. They concluded that controlling the release of ions is the main way to control the antibacterial activity. Despite the demonstration that AgNPs have strong antibacterial properties, further enhancement of their application has been pursued. [PERSON] and [PERSON] [36] were the first to load AgNPs onto graphene oxide in order to prevent nanowire aggregation in solution as well as enhance antibacterial activity. Graphene oxide, in a similar manner to AgNPs, has been shown to possess unique optical, chemical and thermal properties as well as antibacterial activity against Gram-positive and Gram-negative bacteria. [PERSON] and [PERSON] compared AgNPs alone, graphene oxide, AgNPs loaded onto graphene oxide, and ampicillin against _E. coli_. Graphene oxide or AgNPs alone were shown to delay and decrease _E. coli_ growth overall, while ampicillin or AgNPs loaded onto graphene oxide completely inhibited _E. coli_.