## 3 Results and Discussion
### Functional Properties Evaluation  
#### 3.2.1 Antibacterial Effect  
The antibacterial effect is a very important parameter, since the survivability of microorganisms on the facemask surface can compromise the protection of the wearer and become a source of finite transmission. Therefore, the development of active facemasks with antibacterial properties is crucial to kill or deactivate microorganisms in addition to particle filtration [45]. The antibacterial effect was assessed by the formation of a halo around the samples, which indicates the absence of bacteria growth, the size of the halo being proportional to antibacterial effect level [28]. The antibacterial tests were performed against Gram-positive (_S. aureus_) and Gram-negative (_E. coli_) bacteria, and the results are described in Table 2.  
As expected, PCL membranes did not present any antibacterial effect [46]. On the contrary, inhibition zones were formed using PCL/NPs membranes, except when using PCL/TiO\({}_{2}\) NPs sample for _E. coli_. In fact, several NPs, including metal and metal oxide NPs, exhibit antibacterial effect against a broad spectrum of bacteria, the main antimicrobial mechanisms proposed being: (i) disruption of the bacterial cell membrane; (ii) generation of reactive oxygen species (ROS); (iii) penetration of the bacterial cell membrane and (iv) interactions with DNA, RNA, proteins and lipids, resulting in bacteria death [18; 20].  
PCL/Ag NPs electrospun membranes exhibited an inhibition zone diameter of 18.3 and 16.5 mm for _S. aureus_ and _E. coli_, respectively. Ag NPs are well recognized as an effective antibacterial agent [28; 47]. In fact, Ag NPs can adhere to the bacterial cell membrane, altering its permeability, causing structural changes and functional damage. Moreover, Ag\({}^{+}\) ions released from NPs can interact with intracellular components of bacteria, leading to their destruction. Ag NPs are also able to produce ROS [48]. [PERSON] et al. also demonstrated that PCL electrospun membranes functionalized with synthetized Ag NPs exhibited antibacterial activity against _Staphylococcus epidermidis_ and _Staphylococcus haematolytica_[46].  
On the other hand, PCL/TiO\({}_{2}\) NPs samples only demonstrated antibacterial effect against Gram-positive bacteria, with the size of the halo at 18.7 mm. This is in accordance with other works, which also found that Gram-positive bacteria were more susceptible to TiO\({}_{2}\) NPs than Gram-negative, which could be due to the distinct structure of the bacteria. Gram-negative bacteria have a thin peptidoglycan layer, which is surrounded by an outer membrane layer composed by proteins, lipids and lipopolysaccharide, whereas Gram-positive bacteria present one thick peptidoglycan layer which contains a mixture of lipoteichoic acid and glucose with amino acids [30; 49]. The higher amount of negatively charged peptidoglycans in Gram-positive bacteria can make them more susceptible to positively charged antimicrobials [27].  
Finally, PCL/MgO NPs membranes exhibited an antibacterial effect against both _S. aureus_ and _E. coli_ bacteria, showing the highest inhibition zone for Gram-positive bacteria (25.3 mm) when compared to the other samples. Once again, the antibacterial effect was more pronounced on Gram-positive compared to Gram-negative bacteria. The main proposed mechanism of MgO NPs for bacterial lysis is the generation of ROS, which results in excessive oxidative stress, causing cell damage. The release of Mg\({}^{2+}\) ions from NPs can also contribute to the ROS production [50]. [PERSON] et al. showed that PCL/gelatin/MgO NPs electrospun membranes presented antibacterial effect against _E. coli_, _S. aureus_ and _S. epidermidis_, being this effect proportional to the amount of MgO NPs incorporated [51]. [PERSON] et al. also demonstrated that MgO NPs addition to polyacrylamide(PAN) electrospun membranes enhanced their antibacterial effect [23].