## 3 Results and Discussion
### Evaluation of the Antifouling Behaviour of NPs Doped Coatings Against Green Algae in Freshwater  
The surface of the commercial substrate coated only with PVA/GA (Figure 6) was taken as reference for the evaluation of all NP containing coatings. After fifteen and thirty days immersion in the culture tank, the surface was visually inspected. Once the samples had been immersed for fifteen days, green accretions could already be observed covering the surface of the control substrate. After another fifteen days (one month of immersion), the green algae accretions had continued growing and formed a dense layer over the whole exposed surface. In any case after 30 days of immersion the coating remained stable, an indication of the high reliability of the PVA/GA coat when immersed in fresh water. In order to check the long-term stability and antifouling capacity of the MP modified-coatings, their surfaces were also visually inspected after fifteen days and one-month immersion in the _Coelastrella_ microalgae culture in freshwater. To prove the inhibitory effect on microalgae adhesion, images of the different NP coatings were taken by optical microscopy. In addition, the signal intensity of the captured images was digitized using ImageJ-J 1.53t-win-java8 software for all specimens tested. Optical density results are shown in Figure 7. Each value obtained is the mean of the measurements in each specimen (\(n=3\)), and the error bar corresponds to the standard deviation. The low values for the standard deviation confirm the reproducibility of the measurements. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Coating Surface** & \begin{tabular}{c} **Cutoff Length** \\ **(cm)** \\ \end{tabular} & \begin{tabular}{c} **Roughness (Ra) 1** \\ **Average (\(\mu\)m)** \\ \end{tabular} & \
\begin{tabular}{c} **Total Roughness (Ra)** \\ **Average (\(\mu\)m)** \\ \end{tabular} &
\begin{tabular}{c} **Standard Deviation** \\ **Standard Deviation** \\ \end{tabular} \\ \hline PVA/GA & 1.65 & 0,89 & & & \\ PVA/GA & 3.30 & 0,95 & 0.95 & 0.06 \\  & 4.95 & 1,02 & & \\ PVA/GA/AgNPs & 3.00 & 5.12 & 5.0 & 0.2 \\  & 4.95 & 4.94 & & \\ PVA/GA/TiO\({}_{2}\)-NPs & 1.65 & 2.00 & & \\ PVA/GA/TiO\({}_{2}\)-NPs & 3.30 & 2.31 & 2.1 & 0.2 \\  & 4.95 & 1.98 & & \\ PVA/GA/ZnO-NPs & 1.65 & 9.74 & & \\ PVA/GA/ZnO-NPs & 3.30 & 9.91 & 9.8 & 0.1 \\  & 4.95 & 9.68 & & \\ PVA/GA/Ag-TiO\({}_{2}\)-NPs & 3.30 & 1.52 & 1.46 & 0.06 \\  & 4.95 & 1.41 & & \\ PVA/GA/CuO-NPs & 3.30 & 9.35 & 9.28 & 0.07 \\  & 4.95 & 9.26 & & \\ \hline \hline \end{tabular}  
* Roughness average is the result of the measurements that were carried out on three substrate specimens.  
\end{table} 
Table 3: Roughness test results for the different coatings applied to the substrate. Figure 6: Qualitative antifouling test image of the PVA/GA coating surfaces observed at immersion times of 15 days and 30 days. In order to observe biofouling on the NP modified coated surfaces and be able to carry out an optical density (OD) analysis, the color contrast on the images was modified. Figure 8 shows the images obtained by optical microscopy after 15 days and 30 days of immersion times. Figure 8 shows that under the same conditions, coatings based on CuO-NPs (Figure 8d) and Ag NPs (Figure 8e), hardly underwent any changes in their surface between 15 and 30 days of the immersion, keeping the microalga layer deposited on the coating practically constant. Figure 8: Qualitative antifouling assessment by optical microscopy of: (**a**) PVAGA, (**b**) PVAGA/TiO2-NPs, (**c**) PVAGA/ZnO-NPs, (**d**) PVAGA/CuO-NPs, (**e**) PVAGA/AgNPs and (**f**) PVAGA/Ag-TiO2-NPs coatings, after 15 days and 30 days. Figure 7: Optical density analysis of the metallic nanoparticle coatings at immersion times of 15 days (blue) and 30 days (red). The error bars indicate the standard deviation of 3 measurements. For TiO\({}_{2}\) NPs doped coating (Figure 8b), increased adhesion of microalgae was observed in relation to immersion time.