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Scientists obtained silver nanoparticles by grinding using lichens

9. 3. 2021 | 902 visits

Scientists from the Slovak Academy of Sciences and Pavol Jozef Šafárik University in Košice were able to synthesize silver nanoparticles with antibacterial activity with the help of lichens using mechanochemistry. They thus overcame the problem of the insolubility of lichen metabolites in water, which prevented their use for classical synthesis. The editors of a prestigious scientific journal also appreciated their innovative approach.

Mechanochemistry is a type of chemical synthesis that is a more environmentally acceptable alternative to classical solution chemistry. Its principle is to grind powdered reactants in special mills. Without the use of solvents or increasing temperature and pressure, scientists can often obtain better results than in traditional chemistry. Using this method, nanomaterials can be prepared that can be used in energy industries, biological applications or even in waste recycling.

This time, Slovak scientists were looking for ecological ways to synthesize silver nanoparticles with a stable antibacterial activity using lichens as reducing agents. "Lichens are known for the production of secondary metabolites, which have been shown to have the ability to reduce metal ions as well as their biological potential to be used in pharmacology or microbiology. The problem, however, is that these substances are almost insoluble in water," said Michal Goga from the Department of Botany of the Faculty of Science, UPJŠ in Košice.

"Currently, the trend is to prepare silver nanoparticles by the so-called green synthesis, where the source of silver, most often silver nitrate (AgNO3), dissolves and mixes with the aqueous extract of a plant. Such an approach cannot be used for all organisms and the stability of products obtained in this way - nanosuspensions is lower than it would be in the case of powder,” explained Matej Baláž from the Department of Mechanochemistry at the Institute of Geotechnics SAS in Košice. 

"Silver nitrate and lichen, both components in powder form, were poured into the grinding chamber of the planetary mill and we prepared silver nanoparticles by grinding," the mechanochemist described the method. "In addition to the synthesis, we managed to combine silver nanoparticles and some lichens into a composite with significant antibacterial activity," added the lichenologist.

This innovative approach was published in the prestigious journal ACS Sustainable Chemistry & Engineering.  In the study, the scientists also compared the reduction capacity of four lichens from different geographical areas. The research results showed that lichens from the extreme Antarctic climate (Usnea antarctica and Leptogium puberulum) produced fewer detectable secondary metabolites, as they are thought to spend all their energy defending themselves against external influences. This was also reflected in the time required to convert silver ions into elementary silver nanoparticles. Although grinding took more than 6 hours, the complete conversion was not observed. On the other hand, lichens growing in milder conditions (e.g. Slovak Cetraria islandica or Xanthoria elegans) have enough metabolites and therefore two hours were sufficient for synthesis.

The article published in ACS Sustainable Chemistry & Engineering

Edited by Katarína Gáliková

Foto: Katarína Čižmáriková a Wikimedia Commons