## 3 Results and Discussion
### Evaluation of the Antifouling Behaviour of NPs Doped Coatings Against Green Algae in Freshwater
Three times more deposition was recorded between its inspection at day 15 and that after one-month immersion. Ag-TiO\({}_{2}\)NPs doped coatings (Figure 8f), in turn, did not present changes in relation to immersion time. This NP treatment showed the highest antibiofouling capacity among those investigated in this work. In the case of the ZnO-NPs coating (Figure 8c), the deposition of microalgae observed after one month was twice the value of the optical density (OD) at day 15, and two times higher that of the TiO\({}_{2}\)-NPs coating after both periods. If compared with the Ag-TiO\({}_{2}\)NPs coating, fouling with the ZnO-NPs coating was up to four times higher. Based on the current literature this effect could be related to the nanosize. [PERSON] et al. [52] describe the relationship between the nanoparticles size and the nano-ZnO toxicity. They indicated that 5-nm particles had a five-fold stronger toxic effect than 50-70-nm particles. Then, the standard size selected in this work (close to 100 nm) is likely to decrease the effectiveness of the NPs. On the other hand, a certain nanomaterial aggregation is perhaps acting as a limitation for ion release mechanism due to the loss of surface area. Finally, these effects for the nano-ZnO depend also on the environmental conditions of the test [53]. As already mentioned, the roughness of the NP doped coating could play an important role, because it favors the growth of the microalgae and the accumulation of nutrients. The ZnO-NPs doped coating had an average roughness of 9.8 \(\upmu\)m. This value could be considered very high as compared to the Ag-TiO\({}_{2}\)NPs doped coatings, the most efficient ones, which were characterized with the lowest roughness values (1.46 \(\upmu\)m, Table 3). These results confirm that high roughness is not desirable for antibiofouling capacity [30; 31]. However, in the case of CuNPs incorporated into polymer nanocomposites, it has been shown that the antibiofouling capacity could be improved by increasing the load of CuNPs [54]. This could explain the fact that the CuO-NP-loaded coatings in this work were more effective than ZnO-NPs loaded ones of similar roughness (9.28 and 9.8 respectively). The comparison of all NP loaded coatings here studied (Figures 7 and 8) indicates that the coating based on PVA/GA/Ag-TiO\({}_{2}\)NPs had the highest antifouling capacity against green _Coelastrella_ microalgae in freshwater, in contrast with the coating based on ZnO-NPs, which showed the worst antifouling capacity under the same conditions. In order to confirm that the nanoparticles are still present on the coating after the antifouling experiments, X-ray diffraction (XRD) analyses of PVA/GA/Ag-TiO\({}_{2}\)-NPs and PVA/GA/ZnO-NPs coatings were carried out. Figure 9 shows both X-ray diffraction patterns (20 range from 5\({}^{\circ}\) to 70\({}^{\circ}\)), which confirm the permanence of the nanoparticles. The X-ray diffraction patterns show the presence of the PVA (in blue) in both cases. Figure 9a shows in red the characteristic pattern of anatase in the PVA/GA/Ag-TiO\({}_{2}\)-NPs coating. AgNPs (in green) could not be detected due to the low crystallinity of the sample and the PVA used as a matrix in the coating. Figure 9b shows in red the pattern of ZnO in the PVA/GA/ZnO-NPs coating. These results confirm the viability of the hybrid materials investigated.