Green synthesis of copper and silver nanoparticles mediated by extracts of medicinal plant species of Ethiopia for potential antimicrobial applications.

Abstract

The research on the fabrication of plant mediated copper and silver NPs for multifunctional applications has gained prominence in the recent years. However, the toxicity of Ag and Cu NPs is found to depend on numerous morphological and physicochemical properties of the NPs. These consist of the nano features of the nanoparticles as well as the coating of Ag and Cu NPs by phytoconstituents which may decrease the risk of cytotoxicity. Hence it is a great challenge for researchers to synthesize green Cu and Ag NPs which are environment friendly with suitable size and shape. Thus we present the eco-friendly green synthesis of copper and silver nanoparticles using medicinal plant leaf extract at low temperature to investigate synergistic influence of phytochemicals present around the copper and silver nanoparticles on bacterial strains.Ethiopian medicinal plants, Vernonia amygdalina del., Hagenia abyssinica (Brace) JF. Gmel., Artemisia absinthium L., Carum copticum L., Echinops sp. and Syzygium guineense (Willd.) DC mediated green Ag NPs and Cu/CuO NPs were successfully synthesized for the first time. The surface amalgamation of biomolecules of plant leaf extract around Ag and Cu NPs has also been approved by the most advanced techniques which were employed to characterize the NPs. The presence of absorbance maxima in UV-Vis (Ultra Violet-visible) spectra confirms the formation of Ag and Cu NPs. The DRS (Diffused Reflectance spectra) studies revealed the band gap varying in the range from 1.95 eV to 2.23 eV and 1.95 eV to 2.17 eV for Ag and Cu/CuO NPs respectively. The role of biomolecules as capping agents for NPs was authenticated by FT-IR (Fourier Transform-Infra Red) spectra. The presence of sharp peaks in the XRD (X-Ray Diffraction) pattern of NPs confirmed the highly crystalline nature of Ag, Cu and CuO NPs. The Cu NPs synthesised by using Echinops sp. plant root extract only yielded partially amorphous NPs which is believed to be due to the ineffective nucleation and capping of Cu NPs by the phytoconstituents. The TGA-DTA (Thermogravimetric analysis) and Differential thermal analysis) studies revealed that Ag and Cu/CuO NPs were thermally stable above 570 oC and 480 oC, respectively. The superficial morphology and purity of the NPs were corroborated by SEM-EDS (Scanning Electron Microscopy and Energy Dispersive Spectroscopy) analysis. The average particle sizes of Ag NPs and Cu NPs were deduced to vary between 18.72 nm to 36.88 and 27.1 to 34.76 nm, respectively. In addition, TEM micrographs revealed the presence of varieties of nano-sized shapes for Ag, Cu and CuO NPs. TEM-HRTEM-SAED (Transmission Electron Microscopy- High Resolution TEM- Selective Area Electron Diffraction) analysis of Ag and Cu NPs corroborated the existence of crystalline nature in biogenic silver and copper NPs. The interplanar d-spacing values varied from 0.126 nm to 0.262 nm for various planes of Ag and Cu NPs. The synergistic influence of surface amalgamated bioactive compounds and biogenic Ag and Cu NPs proved to exhibit highly effective antibacterial mechanism against pathogens, S. aureus, E. coli, P. aeruginosa, and E. aerogenes with 18 mm as a highest zone of inhibition. The antibacterial activity of all the Ag NPs was found to be superior when compared with that of Cu NPs. Thus it can be concluded that the cumulative effect of Ag and Cu NPs coupled with phytochemicals of plant extract proved to be detrimental for bacteria. The medicinal plants proved to have served as the better source of bioactive compounds for the synthesis of Ag and Cu NPs.