Biomechanochemical Solid-State Synthesis of Silver Nanoparticles with Antibacterial Activity Using Lichens

A completely solid-state mechanochemical synthesis of silver nanoparticles overcoming the problem with water insolubility of lichen metabolites hampering their use for the classical green synthesis in water is shown herein. Four lichen species (Xanthoria elegans, Cetraria islandica, Usnea antarctica, and Leptogium puberulum) and AgNO3 were used as reducing agents and Ag(0) precursor, respectively. The reaction progress was rapid in the first two cases, whereas in the case of U. antarctica and L. puberulum, a significant amount of AgNO3 could still be detected after 6 h of milling. The products with a higher content of Ag(0) were shown to undergo a backward transformation documented by increasing content of AgNO3 with storage time; however, the repeated formation of Ag(0) could be observed upon remilling. Transmission electron microscopy analysis has shown bimodal nanocrystallite size distribution in all cases. However, the finer fraction was more abundant in the case of silver nanoparticles prepared using lichens with stronger reducing ability (X. elegans and C. islandica). All the products are excellent antibacterial agents. Whereas the as-received products exhibited higher activity against E. coli, the remilled samples were more active against S. aureus.