## 3 Results
### PCL Polymer Solution, Microfibers and GNPs Impresation Optimization
Moreover, it can also be observed that defect-free membranes were produced. In the first case (Figure 4a), for lower values of both applied voltage and flow rate, the fibers obtained presented a uniform morphology, regular diameters, and good homogeneity. By increasing the flow rate to 2 mL/h (Figure 4b), the fibers began to have more irregular diameters, and the homogeneity was therefore affected. On the other hand, the diameter of the fibers increased for smaller needle-collector distances, higher applied voltage, and higher flow rate (Figure 4d). In the case of the fibers of Figure 4e, similarly to the ones of Figure 4a, a homogeneous distribution and regular and uniform diameters were also observed. These fibers were produced with a voltage of 25 kV and a flow rate of 1 mL/h, but the needle-collector distance was increased for 260 mm, which resulted in a decrease in the average diameter from 2.8 mm to 2.5 mm. By analyzing Figure 4f,h, it is possible to verify that, at bigger needle-collector distances and higher flow rate values, more heterogeneity in the diameters of the fibers produced was verified. Finally, in the case of Figure 4g, it is possible to observe that using a higher voltage and a greater distance caused a rough texture on the surface of the fibers. Therefore, after analyzing the behavior of the polymer solution for the different parameters, it can be concluded that a low flow rate (1 mL/h) allows the production of fibers with uniform diameters. Furthermore, the combination of working with low voltages and high needle-collector distances can be associated with the development of fibers with smaller and more uniform diameters. Thus, it can be concluded that the best conditions for the subsequent studies to be carried out are 25 kV, 1 mL/h, and 260 mm (Figure 4e), being that these conditions allowed the obtention of the finest and most homogeneous and uniform fibers. Finally, PCL membranes with the chosen conditions were produced with a deposition in the drum collector for 5 h.  
Once the electrospinning parameters and the PCL polymeric solution were optimized, GNPs/Ethanol solutions were prepared in order to obtain the GNPs-PCL membranes. Several concentrations of GNPs were added to the fibers by the post-modification method [36], which consists of a treatment of the electrospun fibers after their production. In this case, we used the dipping post-impregnation procedure, as mentioned in the experimental section. After the drying process, a membrane with a deep black (Figure 5) color was obtained. For 5 min, 15 min, and 30 min, of immersion it was observed that the impregnation of the GNPs onto the fibers surface was very heterogeneous. However, for 60 min of immersion, the GNPs were homogeneously coated onto the fibers, regardless of the concentration of GNPs present in the solution. As expected, as the GNPs concentrations increased, the samples became darker. This indicated that, through this approach, GNPs were successfully deposited onto the surface of the PCL membranes. Figure 5: Schematic illustration of the dipping method for GNPs-PCL membranes preparation. The PCL membranes were immersed in an GNPs/Ethanol solution at room temperature for 1 h. Finally, the coated membranes were dried at room temperature. As it can be observed, with increasing concentration of GNPs, the lightness coordinate decreased from 67.95 to 55.54, 48.63, 43.67, 41.50, 40.23, and 39.06, with the addition of 0.05%, 0.1%, 0.2%, 0.5%, 1%, and 2% of GNPs, respectively. These results were expected, as a decrease on the L* parameter means that the samples became darker, indicating a successful incorporation of the GNPs at the membranes surface. Analyzing the standard deviation values, it is possible to observe that the values obtained for each of the five evaluated sites are very similar, which allows us to conclude that the distribution of GNPs onto the membrane surface is quite homogeneous. FESEM was employed to demonstrate the electrostatic adsorption of GNPs onto the surface of the PCL membranes.