Biosynthesis of silver and selenium nanoparticles using extracts of Commelina Africana and their biological activity.

Abstract

Commelina africana is an indigenous medicinal plant that belongs to the Commelinacaea family. This plant is traditionally used for the treatment of venereal diseases and burns, and it is eaten as a vegetable. Despite the plants medicinal and nutritional significance, it has not been phytochemically investigated. Nanoparticles are currently being investigated for their medicinal value. Different synthesis techniques, especially those that are less toxic, inexpensive and more environmentally friendly are being explored. The use of plant material to synthesise nanoparticles is known to be a greener and safer method. These nanoparticles have potential for applications in the biomedical sciences. Currently, silver nanoparticles (AgNPs) are being used for the dressing of wounds and prevention of infections. Selenium nanoparticles (SeNPs) are being used biologically as antioxidants. This study aimed at extracting, isolating and characterising the secondary metabolites from C. africana. The crude aqueous and methanolic extracts from the plant were used to synthesise AgNPs and SeNPs at varying extract volumes. The crude extracts served as reducing agents and reduced Ag+ and Se4+ to their elemental states. The synthesised nanoparticles were tested for their antioxidant activities (using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) and ferric reducing antioxidant power (FRAP) assays) and antibacterial activities relative to the extracts from which they were synthesised and known standards. Four phytosterols (stigmasterol, ergosterol peroxide, β-sitosterol and campestanol) were isolated and identified from the extracts of C. africana. AgNPs and SeNPs were synthesised using the plant material and characterised using spectroscopic and microscopic techniques. AgNPs consisted of spheres and SeNPs showed a mixture of spheres and rods. Both types of nanoparticles had a wide particle size range.For the antioxidant activity, ergosterol peroxide showed good radical scavenging activity by the DPPH test whilst SeNPs synthesised using the methanol extract showed good radical scavenging activity and reducing potential by the DPPH and FRAP tests, respectively. Crude extracts, phytocompounds and nanoparticles were tested against two Gram-positive bacteria (Enterococcus faecalis and Staphylococcus aureus) and four Gram-negative bacteria (Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae and Chromobacterium violaceum) for their antibacterial activity. Plant synthesised AgNPs showed satisfactory inhibitory activity against the growth of all bacterial strains except E. faecalis. These nanoparticles also exhibited anti-quorum sensing abilities indicating their potential to block cell-to-cell signalling that is used to determine their physiological activities and for co-ordinated gene expression. This study adds to the base of knowledge on the medicinal plant species, C. Africana and highlights the synergistic effects between AgNPs and SeNPs with active plant biomolecules for biological systems. The findings of this study also indicate that medicinal plants can be used as a greener and more environmentally friendly technique for the synthesis of AgNPs and SeNPs.