## 3 Results and Discussion  
Some important parameters can be considered in the green synthesis process to obtain highly stable nanoparticles, such as the selection of the best plant and the optimization of the reaction conditions: for example, light, temperature, pH, mixing speed, concentration, etc. The optimization of these crucial factors could affect the morphology and other properties of the nanoparticles [33; 34]. Therefore, we first propose modifying the solvent used to make the extract, the salt concentration, and the pH in order to obtain a synthesis route that provides silver nanoparticles with a monodisperse size, with good catalytic and antibacterial properties. Figure 1a shows the UV-vis analysis of the AgNPs samples synthesized with the three solvents at a 1:2 volumetric ratio of extract salt. When the ethanol _Sargassum_ extract is employed, no absorption peak is observed, suggesting the nonpresence of AgNPs. On the other hand, when the extract obtained with water is used, an absorption peak appears at 441 nm, which is a characteristic surface plasmon resonance (SPR) band for silver. However, this peak is very wide and has a very low intensity, indicating a low concentration of nanoparticles of a wide size range. Finally, the spectrum corresponding to the sample in which the water-ethanol _Sargassum_ extract is used shows an absorption peak at 432 nm. Furthermore, the absorption peak has a high absorbance, suggesting a high concentration of AgNPs. Figure 1b shows the UV-vis spectra by varying the volumetric ratio of extract salt using the water-ethanol _Sargassum_ extract. As seen, as the volume of extract increases, the AgNPs absorption peak increases in intensity, indicating an increase in the concentration of nanoparticles. Regarding the position of the AgNPs absorption peak, there is no significant variation. In all the spectra, this signal is between 422 nm and 430 nm. Although the spectrum corresponding to the 4:1 ratio shows a more intense absorption peak, its position (430 nm) and width suggest the presence of larger AgNPs, with a wide size dispersion. Thus, the spectrum with the absorption peak of greatest intensity, centered at the lowest wavelength (422 nm), is that of the 3:1 volumetric ratio. Therefore, this sample was selected for the subsequent analysis. According to various reports [35, 36, 37], the synthesis of uniform and monodisperse nanoparticles occurs most readily in alkaline conditions. Figure 1c shows the UV-vis spectra of the samples in which 1 mL of NaOH is added at different pH values. It can be seen that, as a NaOH solution of higher alkalinity is used, the absorption peak of the AgNPs increases significantly. Furthermore, as far as its position is concerned, the peak shifts towards shorter wavelengths. In this way, the sample with NaOH (pH = 14) shows the most intense absorption peak, centered at 408 nm, indicating the presence of a high concentration of AgNPs of a small and uniform size. Therefore, the most efficient sample was selected to carry out the characterization and evaluation of the antibacterial and catalytic properties. This sample is named AgNPs-S from here on. The selection was carried out based on the comparison of all UV-vis spectra. As can be seen, AgNPs-S corresponds to the sample in which the water-ethanol-based extract was used with a volumetric extract:precursor salt ratio of 3:1 and a pH=14--that is, the sample corresponding to the dark red spectrum in Figure 1c. Finally, Figure 1d shows the UV-vis spectrum of AgNPs-C obtained by chemical synthesis, using NaBH\({}_{4}\) and PVP as reducing and stabilizing agents, respectively. The spectrum shows a narrow peak, centered at 395 nm, suggesting the presence of small nanoparticles with a narrow dispersion of size. This sample, named AgNPs-C, was used to compare the catalytic and antibacterial activities of the nanoparticles.