**6. Conclusions**

Following the performed analyses, it was found that the biosynthesis of silver nanoparticles is time and pH dependent. UV-Vis analysis shows that basic pH accelerates biosynthesis independently of the ion salt used. This is also supported by TEM analysis in which the appearance in large quantities and smaller size of AgNPs was demonstrated in all the versions where the basic pH was used.

FTIR analysis demonstrated that certain functional groups of the compounds present in the aqueous extracts contributed to the formation of AgNPs, a fact supported by the intense bands around 1384 cm<sup>−</sup>1.

AgNPs tested solutions showed antioxidant activity as well as inhibitory and bactericidal activity. The lowest MBC was obtained for TS4 (0.19 mg/mL) against *E. coli* ATCC 25922. The small nanoparticles at pH = 9 have a lower MIC/MBC compared to those synthesized at pH = 4. This result could be explained by the fact that alkaline pH favors the penetration of AgNPs into the bacterial cell, having an inhibitory or bactericidal effect.

By determining the growth rate, it was observed that for *E. coli*, the tested AgNPs had a bactericidal effect; while for *S. aureus* and *P. aeruginosa* the growth was inhibited.

These results indicate that bark extract of *Fagus sylvatica* L. is suitable for synthesizing stable AgNPs which act as an antimicrobial agent.

**Author Contributions:** Conceptualization, C.T., L.B., and A.M. (Anca Mare); methodology, C.T., L.B., A.M. (Anca Mare), and A.M. (Andrei Mocan); software, N.A.C. and I.R.; validation, C.T., L.B., and A.M. (Anca Mare); formal analysis, N.A.C., I.R., A.M. (Anca Mare), and L.J.-F.; investigation, N.A.C., I.R., A.M. (Anca Mare), and A.M. (Adrian Man); resources, C.T., L.B., A.M. (Anca Mare), A.M. (Andrei Mocan), and D.B.; data curation, N.A.C., I.R., A.M. (Andrei Mocan), L.J.-F., D.B., F.T., and A.M. (Anca Mare); writing—original draft preparation, C.T. and L.B.; writing—review and editing, A.M. (Anca Mare); visualization, C.T. and A.M. (Adrian Man); project administration, C.T.; funding acquisition, C.T.

**Funding:** This work was supported by the University of Medicine and Pharmacy of Târgu Mures, Research grant number 15609/12/29.12.2017.

**Conflicts of Interest:** The authors declare no conflict of interest.
