Green synthesis, characterisation and application of metal nanoparticles using chemical constituents of selected South African plants

Abstract

Diabetes mellites and bacterial infections are among the leading health challenges facing the world. These challenges are not restricted by age or gender as it affects every individual, causing a great deal of burden not only on health but also on the world's economy. Concerted efforts, therefore, have been continuously made to find other alternatives (including the trials of medicinal plants). Leucosidea sericea and Hypoxis hemerocallidea are probably the most popular medicinal plants used by many ethnic groups in South Africa. These plants have played a significant role in the management of many diseases, including diabetes and bacterial infections. Scientific investigations have confirmed the traditional uses of the plants. In the modern-day nanotechnology advancement, nanoscale materials have continued to gain more attention as ''smart" alternatives due to numerous advantages. In the biomedical sector, nanomaterials made from gold and silver have demonstrated interesting abilities in the management of many diseases. However, their methods of synthesis dictate their application. Gold and silver nanoparticles synthesized using the physical and chemical methods suffer several setbacks. These include longer reaction time, use of sophisticated instruments, and harsh chemicals. The toxic chemicals may also cause environmental pollution. Green synthesis is, therefore, a better alternative. In this study, Leucosidea sericea and Hypoxis hemerocallidea were exhaustively extracted and partitioned into hexane, dichloromethane, ethyl acetate, methanol and water extracts. Preliminary screening showed that only the ethyl acetate, methanol and aqueous extracts formed gold and silver nanoparticles. Therefore, the ethyl acetate and methanol extracts were chromatographed using both silica gel and Sephadex column chromatography to further purify the extracts. Subsequent use of High-Performance Liquid Chromatography and Liquid Chromatography-Mass spectrometry led to the identification/isolation of the compounds. These techniques were corroborated by nuclear magnetic resonance spectroscopy. The individual total extracts and fractions/compounds were then used in the green synthesis of gold and silver nanoparticles which were fully characterized using various spectroscopic, optical and microscopic techniques. In vitro stability studies were also conducted using various media and buffers. The properties and behaviour of the nanoparticles (between those of the total extract and fractions/compounds) from the above characterization and studies were compared. Thereafter, their potential biological applications were evaluated. First, the nanoparticles were diluted to various concentrations to confirm that they still retain their characteristic features even at lower concentrations. The in vitro antioxidant, antidiabetic and antibacterial activities were then examined at low concentrations. The antioxidant activities were done using three assays. 2, 2’-Azino-bis (3-ethylbenzo thiazoline-6-sulfonic acid), Ferric Reducing Antioxidant Power and the total phenolic content. The total extract and fractions/compounds were evaluated alongside the nanoparticles for comparison. The results were expressed based on the standard curve of the standard antioxidant compounds employed in the case of each assay. The antidiabetic activity was conducted using alpha-glucosidase and alpha-amylase enzymatic assays. Like the antioxidant studies, a comparison was made between the total extract, fractions/compounds, and the standard antidiabetic drug, acarbose. In the case of antibacterial studies, the aqueous solutions of the total extracts and fractions/compounds did not show activity even at high concentrations (˃2000 μg/mL). The nanoparticles displayed activity against six bacterial species, consisting of both Gram-positive and Gram-negative types. The results were compared to those of the standard antibiotics. Having identified the chemical structure of the compounds (phytochemicals), the study confirmed the involvement of the functional groups in the green synthesis of gold and silver nanoparticles. The fact that individual phytochemical and fractions successfully formed stable nanoparticles also showed their reducing and capping abilities. In addition to the above, the comparison of the nanoparticles with those of the total extracts revealed that procyanidins and hypoxoside were the major compounds contributing to the easy formation of nanoparticles in the respective extracts. This study has avoided the use of toxic chemicals, external stabilizers, and have resulted in the production of bioactive gold and silver nanoparticles. However, toxicity concerns remained a challenge even in the green synthesis. This should, therefore, be further explored for better applications in the biomedical field.