## 3 Results and Discussion
The average particle size determined by DLS was slightly larger than that determined by BF-STEM. This is because the DLS technique measures the mean hydrodynamic diameter of the AgNPs coated by the organic layer, and this hydrodynamic diameter is affected mainly by the viscosity and concentration of the medium. On the other hand, BF-STEM only gives the diameter of the nanoparticles obtained from processed images with high contrast. It is clear from the BF-STEM images and DLS values that there were no aggregates. The almost complete nonaggregation of both samples and their good size homogeneity is an outstanding result because these characteristics are usually very difficult to achieve, in particular with green-synthesized nanoparticles. The zeta potential is a key indicator of the stability of colloidal dispersions. This technique gives a measurement of the \"effective\" electric charge on the nanoparticle surface and quantifies the charge stability of the colloidal nanoparticles. Particles with a negative zeta potential will bind to positively charged surfaces, and vice versa. Hence, the zeta potential also suggests how the particle will interact with other nanoparticles. Therefore, this becomes an important parameter when designing a nanoparticle for a specific application. Figure 5 shows the results of the zeta potential analysis. Data collected from the silver nanoparticles show negative potentials, with values of less than 30 mV, which indicates good physical stability of both nanosuspensions and guarantees that the particles do not aggregate. The sign minus is mostly conferred by the radical groups, such as carboxylic and hydroxyl radicals, present at the surface of the nanoparticles. As can be seen, the zeta potential becomes more negative as the solution goes from acid to alkaline. At pH 3 both types of nanoparticles exhibit almost the same zeta potential, but, as the pH is increases, the zeta potential of AgNPs-S falls lower than that of AgNPs-C. This suggests that in alkaline media, the AgNPs-S nanoparticles are more stable and explains why at pH 14, the synthesis of uniform and monodisperse nanoparticles occurred. Figure 3: (**a**,**b**) BF-STEM images of the AgNPs-C, (**c**) corresponding plot of the particle size distribution; (**d**,**e**) BF-STEM images of the AgNPs-S, (**f**) corresponding plot of particle size distribution. There are different methods to determine the concentration of nanoparticles in a solution. Among them, TGA is an easy and common method [42; 43; 44; 45]. TGA measures the mass change in materials, associated with thermal degradation and its transition phase [46; 47]. Thus, TGA can quantitatively resolve complex mixtures because of the characteristic thermal decomposition temperature of each component. Nanoparticle concentration has been determined by the TGA technique and reported previously [42; 43; 44; 45]. So, to determine the concentration of silver in the AgNPs-C and AgNPs-S samples, 40 \(\mu\)L of each were analyzed using TGA. Figure 6a shows the graph obtained for the AgNPs-C. As can be seen, there is a significant weight loss between 30 \({}^{\circ}\)C and 123 \({}^{\circ}\)C. This is due to the evaporation of water and organic compounds from the samples. Above this temperature, there are no changes in the sample weight. Therefore, it can be assumed that the weight only corresponds to Ag. Taking into account that the volume used in the analysis is 40 \(\mu\)L, the concentration of nanoparticles is 3.8 mg/mL for the AgNPs-C sample. Figure 6b shows the analysis of the AgNPs-S sample. The behavior of the TGA curve is very similar. Abrupt weight loss occurs between 30 \({}^{\circ}\)C and 115 \({}^{\circ}\)C. This variation is due to the evaporation of water and  
Figure 4: DLS graph of the AgNPs-C and AgNPs-S in distilled water. The DLS values shown are the mean number to compare with BF-STEM images easily. Figure 5: Analysis of zeta potential of the AgNPs-C and AgNPs-S at 0.223 mg/mL. organic compounds from the _Sargassum_ extract.