Phenolic Content, Antioxidant, Antibacterial, and Anti-Acetylcholinesterase Activities of Biosynthesized and Characterized Silver Nanoparticles from Tunisian Medicinal Lichen Species.

Lichens produce a myriad of bioactive compounds that can be exploited as reducing and capping agents in the green process of synthesizing nanoparticles. In this study, we exploit a simple, environmentally safe method for synthesizing silver nanoparticles (AgNPs), using aqueous extracts of three lichen species (Lobaria pulmonaria, Ramalina farinacea, and Evernia prunastri) for the first time. Characterization showed that the three lichen species selected could be perfectly suitable as reducing agents to produce AgNPs. First, ultraviolet-visible spectroscopy confirmed the presence of metallic silver with a maximum absorbance at 425 nm. Second, Fourier transform infrared spectroscopy analysis confirmed the chemical compounds involved in reduction. Third, the elemental composition of AgNPs was illustrated by energy dispersive X-ray spectroscopy. Finally, scanning electron microscopy images displayed the size and shape of biosynthesized particles. The prepared extracts containing AgNPs showed high contents of phenolic compounds and high antioxidant activities on three assays (DPPH, ferric reducing power, and chelating power). Moreover, since bacteria are developing resistance to many common antibiotics, AgNPs produced in an environmentally safe method are an interesting replacement. The antibacterial assessment revealed more effective activity against Gram-negative than Gram-positive bacteria and a bactericidal effect against all strains tested. In this study, we present a pioneering evaluation of acetylcholinesterase inhibition by lichen-based AgNPs. The three extracts exhibited strong enzyme inhibition, with IC50 values of 3.46 ± 0.09, 3.59 ± 0.02, and 4.34 ± 0.03 mg/mL for L. pulmonaria, R. farinacea, and E. prunastri, respectively. The green approach presented would encourage the nontoxic production of AgNPs, suggesting pharmaceutical applications.