# Silver Nanowires: Synthesis, Antibacterialativity and Biomedical Applications
## 2 Synthesis of AgNWs
### Soft-Template Synthesis  
To overcome the limitations associated with the hard-template method, the soft-template or polyol method was developed [47]. The soft-template, or silver cation reduction mediated by a polyol, is generally a faster process compared to the hard-template method, and it produces a greater yield of AgNPs, but most importantly, it is carried out in solution. This is advantageous when considering biomedical applications, as the resulting AgNPs can be successively readily dispersed in aqueous environments [26]. [PERSON] et al. [25; 26] were the first to demonstrate the synthesis of AgNPs with the use of poly(vinyl pyrrolone) (PVP) as the soft template; this allowed to overcome limitations of previous methods that employed platinum (Pt) nanoparticle seeds to induce heterogeneous nucleation. The Pt-based process required a two-step reaction and the cost of Pt nanoparticles was prohibitory. Instead, the introduction of PVP allowed the development of a much simplified self-seeding method; PVP was initially dissolved in ethylene glycol, and AgNO\({}_{3}\) was carefully injected by a two-syringe pump into the reaction mixture at a controlled rate, the rate of addition of the Ag\({}^{+}\) source was crucial in the control of the size of the nanospecies produced [26]. Despite successful synthesis on a small scale, carefully and accurately controlling the injection rate of AgNO\({}_{3}\) into the reaction mixture was not feasible on an industrial scale. Thus, [PERSON] et al. [27] suggested that the addition of trace amounts of salts was able to slow the reaction rate and more finely control the growth of AgNPs. Initially, CuCl\({}_{2}\) was added to the reaction mixture after the addition of an Ag\({}^{+}\) salt. Cu\({}^{2+}\) was found to remove oxygen atoms blocking growing sites of the AgNPs surface, favoring deposition of more silver, while also preventing oxidative etching. On the other hand, Cl\({}^{+}\) acted as the rate-determining step of the overall reaction, by forming temporary precipitates of AgCl, the slow degradation of which provided control over the reaction rate [28]. [PERSON] et al. [29] exploited the same principle by adding trace amounts of NaCl in place of CuCl\({}_{2}\), this resulted in similar size distribution (50 nm in diameter and 20 \(\upmu\)m length) as assessed by SEM. Other factors able to control the size of the nanowires obtained are the molar ratio between PVP and AgNO\({}_{3}\) and the length of the PVP molecule employed as soft template, as shown by [PERSON] and [PERSON] [24]. [PERSON] et al. [30] studied the effect of reaction time and temperature on the size of the obtained nanowires. They were able to control the length and diameter of the AgNPs produced by carefully selecting the temperature of the seeding and growth phases and by stopping the reaction at predefined time points. Several studies have also been carried out in an attempt to elucidate the molecular mechanisms involved in the formation of silver nanowires; the complexities of this process are well explained in a review by [PERSON] et al. [48]. The soft-template method involves the reduction of a silver salt to metallic silver in a reducing solvent, a polyol. At elevated temperatures, the polyol undergoes oxidation to an aldehyde that is the actual reducing agent [30]. The formed silver crystals assemble to form multiply twinned particles of decahedron structure (Figure 1a,b) [47]. Adsorption of further silver atoms to this structure was shown to force growth in both the axial and radial direction until a critical phasewas reached where radial growth ceases and only axial growth continues over the [111] facets. In this case, PVP is thought to act as a capping agent, thanks to the ability of its carbonyl groups to bind silver atoms on the [100] facets (Figure 1c), which helps to drive the growth solely in the axial direction (Figure 1d) [48; 49; 50; 51]. Controversial evidence is, however, available, as [PERSON] et al.