## 3 Electrospun (Nano)Materials Implementing Antibacterial Properties
### Inclusion of Metal Nanophases as Biocides
One of the major applications for electrospun fibers has long been in the filtration field and polyacrylonitrile (PAN) has been the most used, due to its high mechanical strength and chemical stability. One of the drawbacks in the filtration process is the rapid fouling by bacteria, negatively affecting the permeability and the throughput of the membrane. Moreover, PAN or composite cellulose/PAN fibers are widely employed in biomedical devices, where the bacterial control is an issue. In this respect, CuNPs and AgNPs may add a clear benefit when embedded in electrospun PAN and cellulose fibers. Additionally, in this case, the blending approach is the most investigated; silver and copper salts [103; 104; 105], or preformed ZnO-CuO-NPs [106], are included in the electrospinning solution. An interesting alternative is represented by the cellulose nanofibers, because the high number of -OH groups offers a location for fibers functionalization, thus regulating the nanophases upload and their release. [PERSON] et al. [107] published a comprehensive paper about carboxymethyl cellulose functionalized with copper, iron, and zinc NPs. They reported on the different morphology according to the different MeNPs (Figure 6), along with composite nanofibers release properties and their excellent antibacterial activity. Figure 5: (**a**) Comparative plot representing the antibacterial activity of CuO-ZnO (CZ) loaded nanofibers with different concentrations against GFP-_E. coli_ (G\(-\)) and _S. aureus_ (G+); (**b**) Proposed mechanism of bactericidal action CZ nanofibers; and (**c**) Fluorescence spectra depicting the ROS generation with different concentrations of CZ nanofibers. Reprinted from [102], Copyright 2017, with permission from Elsevier. The _attachment_ and _core-sheath_ preparation methods are far less common and usually involve a two or three-step procedure. Nevertheless, they provide electrospun nanofibers with worthy performance as antimicrobial materials. In particular, when applications involve coatings [108] or barrier membranes [109], those methods offer good results. Moreover, the multi-step procedure offers the possibility to tune the materials properties and morphology, by acting on post-synthesis different parameters. [PERSON] at al. [108] reported on Ag-PAN nanofibers obtained soaking electrospun fibers into silver nitrate solution, and were subsequently performing a reduction reaction. In Figure 7 the SEM characterization of those fibers is reported, where it is evident that the influence of silver salt concentration and reaction time on the adsorption effect and crystallization ability of silver ions. Then, the subsequent silver reduction step is a further independent parameter. Figure 6: On the **(left)**: TEM images of CMC electrospun nanofiber (**a**) and CMC-MNPs electrospun nanofibers CuNPs (**b**), FeNPs (**c**) and ZnNPs (**d**). On the **(right)**: SEM images of CMC electrospun nanofiber (**a**) and CMC-MNPs electrospun nanofibers CuNPs (**b**), FeNPs (**c**) and ZnNPs (**d**). Reprinted by permission from [107], Springer Nature, Copyright 2017. Figure 7: SEM images of pretreating PAN (polyacrylamideonitrile) nanofibers in Ag NO\({}_{3}\) solution at different soaking times, different concentrations, and different reduction reaction time. Reprinted from [108], Copyright 2015, with permission from Elsevier. An increased stabilization of the nanophases into the membranes is always beneficial and could be obtained by means of _encapsulation_ approach, in order to avoid loosely attached nanoparticles to be dispersed in the environment. Few papers have been published so far following this method, which allows also a controlled release of metal species [110, 111]. In particular, [PERSON] et al. [110] reported how silver and copper NPs supported in sepiolite and silica act as reservoirs able to affect microorganism germination and growth, and to reduce fungal metabolic activity.