There is a wide range of methods for the synthesis of silver nanoparticles (Table 1), where the participants we select in the reaction depend, so will the shape and characteristics of the Nanoparticles, then we will detail some of the chemical synthesis methods existing, where in its vast majority from Silver Nitrate are obtained Silver Acetate and Silver Citrate, which are recrystallized and purified to be used as precursors of silver nanoparticles. Different studies show that when silver citrate is used as a silver precursor, the solution was transparent and stable over time (for at least 3 months). While when using silver nitrate or silver acetate the resulting colloidal solution was unstable and silver precipitated after 1 day.⁵

Synthesis method 1: preparation of silver particles using NaBH₄ and Ac. ascorbic as a reducing and stabilizing agent

From Silver Nitrate used as a precursor of silver nanoparticles, using ascorbic acid (99%), sodium borohydride as reducing agents also working silver nanoparticles with a concentration between 250-500mg/dm³ by the addition of silver as a precursor (silver citrate, silver nitrate or silver acetate) drop wise to an aqueous solution of sodium borohydride (Figure 6) silver nanoparticles were obtained by injecting NaBH₄ solution into an aqueous solution of AgNO₃ in the presence of citrate (Figure 7). In addition, a subsequent treatment to these nanospheres, using AgNO₃, but now with ascorbic acid as a reducing agent, nanoparticles are obtained in the form of a rod / cable.⁵ The formation of these can be confirmed by means of a silver spectrum for the colloids that used silver citrate since a strong plasmon band is observed near 396nm, which confirms that the silver ions were reduced to Ag° in phase watery (Figure 8).

Figure 6 Method of chemical synthesis of nanospheres and silver nanobars.⁸

Figure 7 A. Preparation of a silver colloid in aqueous solution using sodium borohydride as a reducing agent.5 B. TEM image of silver nanobars.⁴

Figure 8 UV-VIS spectrum for a yellow silver colloid.⁵

Synthesis method 2: preparation of silver particles using PVP as a reducing

By means of this method of synthesis of silver, particles with various forms are obtained, simply other factors must be controlled (Figure 9). Using an aqueous solution and PVP as a stabilizing agent, the silver precursor is added, for example, to about 150mL of deionized water containing 5g of PVP for the formation of the silver colloid in a concentration of 250-500mg/dm³, adding drop wise to the silver nitrate or silver citrate solution and stirring for 1h.⁵ The poly (vinyl pyrrolidone) polymer (PVP) is one of the most widely used stabilizing agents for metallic nanoparticles. Thus, one of the first syntheses consisted in the photo reduction of AgNO₃ in the presence of PVP as a stabilizing agent using 243nm UV radiation. With this method, silver nanoparticles between 15 and 22nm can be obtained depending on the molar ratio between AgNO₃ and PVP. Subsequently, different methods have been described in which PVP acts as a stabilizing agent for silver nanoparticles synthesized by reducing silver salts with different chemical reducing agents such as potassium bitartrate or DMF and even using microwaves and the PVP itself as a reducing agent. PVP has also been used as a stabilizing agent in the reaction of AgNO₃ with polyols (Polyol method) that leads to the synthesis of nanospheres, nanocubes, nano bars or silver nanowires. In general, the study of the parameters that affect the reactions and, especially, the concentration of the stabilizing polymer, allow exercising a great control over the size and shape of the silver nanoparticles.⁸

Figure 9 Different silver nanostructures synthesized by the reduction of silver nitrate in ethylene glycol, using polyvinylpyrrolidone as a stabilizing agent.⁸

Synthesis method 3: preparation of silver particles using DMF as a reducing

In this case, dimethylformamide (DMF) has been used as a solvent and as a reducing agent against silver salts under different reaction conditions (Figure 10). Thus, nanoparticles of different sizes have been achieved using aminopropyltriethoxysilane (APS) or polyvinylpyrrolidone (PVP) as stabilizing agents. When the PVP polymer is used as the stabilizing agent, silver nanoparticles (spherical or prisms) can be obtained. When APS is used, silver nanoparticles surrounded by a layer of silica are obtained. As mentioned above, the aggregation of the nanoparticles in solution can be prevented by the use of stabilizing agents. The process of forming the nanoparticles is usually similar to that described above, that is reduction of a silver salt in the presence of a reducing agent. However, the use of these agents allows, on the one hand, to avoid the aggregation of nanoparticles in organic solvents and, on the other hand, to exert precise control over their size, shape and monodispersity by modifying the reaction conditions.

Figure 10 Synthesis of silver nanoparticles using DMF as a reducing agent.⁸

Synthesis method 4: green method, preparation of silver particles using natural precursors

Green sources include a wide range of natural precursors that include plant extracts, bacteria, and enzymes. One of the most common sources of antioxidants used for the synthesis of silver nanoparticles is fruit juices, as they can act as coating agents and reducers. One of the examples is the pomegranate that as a rich source of polyphenols has been the target of many studies for the determination of its constituents and the attribution of its properties to its available components in the shell, mesocarp, aril and juice. New biosynthetic sources were used to obtain Ag nanoparticles, in the solution phase, the pomegranate shell extract forms a complex of natural silver, which at high temperature (300°C) is transformed into silver nanoparticles (Figure 11). However, the same complex ends in bulk metallic silver at 600°C resulting in well dispersed silver nanoparticles. In addition, the solid state reaction of a natural precursor with the silver salt exposed to high tem