Plant-mediated synthesis of zinc oxide nanoparticles using extracts and isolated compounds from senecio serratuloides and their biological activity.
Senecio serratuloides DC of the Asteraceae family is a medicinal plant used in South African traditional medicine for the treatment of skin diseases, sexually transmitted infections (STIs) and wounds. Despite the ethnomedicinal significance of the plant, a phytochemical investigation to determine the active components for future pharmacological developments has not been conducted. Nanotechnology is a promising field in the development of biocompatible metal nanoparticles from bio-resources. Zinc oxide nanoparticles (ZnONPs) are of great interest due to their wide range of applications in the field of biomedical sciences. Research on the exploitation of plant materials for the green synthesis of nanoparticles is increasing rapidly. The aim of this study was therefore to extract, isolate and identify secondary metabolites from S.serratuloides, to use the extracts and isolates as reducing agents in the synthesis of zinc oxidenanoparticles, and to compare the antioxidant, antibacterial and anti-quorum sensing activities of extracts, isolated compounds, freestanding and plant-derived zinc oxide nanoparticles. The phytochemical investigation yielded one sesquiterpene (farnesylamine), five tripernoids (sitosterol, α- and β-amyrin, stigmasterol and taraxerone) and two jacaranones (jacaranone and a mixture of jacaranone and methyl-2-(1-hydroxy-4-oxocyclohexyl)acetate). The synthesised zinc oxide nanoparticles were characterised using spectroscopic and microscopic techniques. Spherical zinc oxide nanoparticles were successfully synthesised but exhibited a wide size range. ZnONPs synthesised using jacaranones showed good antioxidant activity whilst ZnONPs synthesised using extracts of S. serratuloides showed moderate antioxidant activity. The extracts, phytocompounds and nanoparticles were tested for antibacterial activity against three Gram-positive bacteria (Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus faecalic) and two Gram-negative bacteria (Escherichia coli and Chromobacterium violaceum). The extracts demonstrated promising antibacterial activityagainst Chromobacterium violaceum. Amongst the isolated phytocompounds, jacaranones showed promising antibacterial activity against two Gram-positive bacteria, Staphylococcus aureus and Enterococcus faecalic, with good antibacterial activity against Chromobacterium violaceum. Nanoparticles did not possess antibacterial activity. The isolated jacaranone, extracts and nanoparticles were further evaluated for quorum sensing inhibitory activity using a qualitative agar-overlay assay. The extracts showed promising anti-quorum sensing activity whilst the jacaranone showed good anti-quorum sensing activity. Nanoparticles did not show anti-quorum sensing activity. This can be attributed to freestanding nanoparticles not possessing activity against the bacterial strains tested and in plant-derived nanoparticles, the amount of the active compound capping particles could be too little to impart activity.
Master’s Degree. University of KwaZulu-Natal, Durban.