*3.2. Characterization of AgNPs*

The first sign of nanoparticle formation is the color transformation of the solution. The influence of the extract and of the used pH has an important role and it is observed that the solutions change their color becoming brownish-red at pH = 9 and brown at pH = 4 (Figure 1). The appearance of the opaque aspect is achieved as the biosynthesis time passes.

**Figure 1.** Color transformation of beech bark extract solution with AgNO3 at pH = 4 (test solution 1 (TS1)) and pH = 9 (TS2).
