Green synthesis of silver, selenium and zinc oxide nanoparticles using extracts and isolated secondary metabolites of chrysanthemoides monilifera and harpephyllum caffrum and their biological activity.

Abstract

The method of using plant extracts for the synthesis of metal-based nanoparticles is a recently developed technique which is not only cost effective but environmentally friendly. It has very low energy requirements, needs no specialised equipment and reactions are completed within minutes. The resultant materials can be applied to catalysts, medicine, electronics and optics. The biological activity of plant extracts and nanoparticles have each been studied and are well known. In addition to being green, nanoparticles produced by the plant-mediated synthesis route are being utilised in biomedical applications with the added advantage of increased activity arising from the synergistic effects of both the biologically active nanoparticles and plant extracts or phytocompounds. In this project, extracts and phytocompounds from the indigenous, South African, medicinal plant species Chrysanthemoides monilifera and Harpephyllum caffrum were obtained and used to synthesise and stabilise silver, zinc oxide and selenium nanoparticles. The plant extracts and phytocompounds were characterised using spectroscopic techniques. Quercetin was isolated from the extract of C. monilifera and catechin from the extract of H. caffrum. The growth of nanoparticles was investigated at various concentrations of phytocompounds. The synthesised nanoparticles were characterised using spectroscopic and microscopic techniques. Growth seemed to occur by agglomeration and subsequent re-orientation. Changes in concentration had an effect on the yield, shapes and sizes of the synthesised nanoparticles. To investigate the synergistic or antagonistic effect of the capping agents on the biological activity of synthesised nanoparticles, freestanding (uncapped) nanoparticles were synthesised, using sodium borohydride as a reducing agent, for comparison. Results showed biosynthesised nanoparticles to be capped with plant biomolecules. The biosynthesised nanoparticles had a wider size distribution than freestanding nanoparticles. The particles were tested for their antioxidant activity using three different assays (1,1-diphenyl-2-picrylhydrazyl radical (DPPH•) radical scavenging, ferric reducing antioxidant power (FRAP) and H2O2 radical scavenging. The results showed plant biomolecules to enhance the antioxidant activity of the biosynthesised nanoparticles. The anti-microbial activities of the nanoparticles were evaluated with the Kirby-Bauer disc diffusion susceptibility test. Capping of selenium and zinc oxide nanoparticles with plant biomolecules did not have any effect on antibacterial activity. H. caffrum was found to enhance the activity of the silver nanoparticles whilst quercetin promoted its anti-quorum sensing ability.