## 3 Results and Discussion
Figure 1: UV–vis analysis of the AgNPs synthesized with _Sargassum_ extract, varying: (**a**) the extracting solvent (the volumetric ratio of extracts:salt was 1:2), (**b**) the water–ethanol extract:precursor salt volumetric ratio, and (**c**) the pH of the NaOH solution added, using the water–ethanol extract with an extract:salt volumetric ratio of 3:1. (**d**) The UV–vis spectrum of AgNPs synthesized by the chemical method. Figure 2 shows the X-ray diffraction (XRD) patterns of the prepared AgNPs. Figure 2a shows the XRD pattern of the AgNPs-C obtained from the conventional chemical reduction method, where diffraction peaks at 20 = 38.20\({}^{\circ}\), 44.22\({}^{\circ}\), 64.55\({}^{\circ}\), and 77.91\({}^{\circ}\) were assigned to (111), (200), (220), and (311) planes, respectively, of the face-centered cubic (fcc) lattice of Ag (JCPDS No. 87-0597). The XRD pattern of the AgNPs-S (Figure 2b) shows similar diffraction peaks of Ag with a fcc lattice. However, other diffraction peaks were observed at 20 = 31.76\({}^{\circ}\) and 45.50\({}^{\circ}\), corresponding to the cubic phase of sodium chloride (NaCl, JCPDS No. 75-0306), and a further diffraction peak was observed at 20 = 34.80\({}^{\circ}\), corresponding to the cubic phase of potassium chloride (KCl, JCPDS No. 75-1674). NaCl and KCl phases may be associated with seawater residues in the _Sargassum_, since it was only shade dried, hand crushed, and washed with deionized water. The crystallite size of the nanoparticles was calculated using the Debye-Scherrer equation [38], using the half width of the intense (111) reflection. The results were 10.12 and 23.85 nm for the AgNPs-C and AgNPs-S samples, respectively. The values of the crystallite sizes are close to those measured with STEM images, showing that the particles are single crystal. It is known that the antibacterial effect of AgNPs is closely related to their size and shape [39; 40]. Therefore, an exhaustive characterization is recommended to understand the properties of the nanoparticles obtained. Figure 3 shows the bright field-scanning transmission electron microscopy (BF-STEM) images of the AgNPs obtained. Figure 3a shows the BF-STEM image of the AgNPs-C. As can be observed, the AgNPs-C have homogeneous particle sizes. Figure 3b shows a BF-STEM image of higher magnification. It is clear from the image that there are no agglomerates in the sample and that the morphology of the nanoparticles is almost spherical. The average particle size seen in the BF-STEM images was 11.55 nm (Figure 3c). Figure 3d shows the AgNPs-S, which have different morphologies from the AgNPs-C. In this case, most of the nanoparticles are polyhedral in shape (Figure 3e). In the inset of Figure 3e, a color look-up table (LUT) image clearly illustrates that the nanoparticle (green) is coated by an organic compound (yellow) from the _Sargassum_ extract. The average particle size of the STEM images, corresponding to AgNPs-S, was 26.39 nm (Figure 3f) greater than the AgNPs-C. It has been reported that polyhedral AgNPs (decahedral) have higher bactericidal activity than spherical AgNPs [41], as the (111) surface planes are more catalytic than the (100) surface planes; therefore, despite the size difference, we would expect AgNPs-S to have an enhanced antibacterial effect. Figure 2: X-ray diffraction patterns of: (**a**) AgNPs-C and (**b**) AgNPs-S. The DLS technique was also used to determine the particle size and size distribution profile of the AgNPs. To carry out this analysis, the AgNPs-S and AgNPs-C samples were diluted in order to set the concentration at 0.223 mg/mL. Figure 4 shows that the average particle size of the AgNPs-C was 12.71 nm, with a standard deviation of 0.739 nm. For the AgNPs-S, the average particle size was 27.59 nm with a standard deviation of 3.38 nm. As can be seen, the dispersion of the nanoparticle size corresponds to a single modal distribution for both samples; however, AgNPs-S shows a wide particle size distribution as observed by BF-STEM.