Comparison of the bioactivity of gold, silver and copper oxide nanoparticles derived from Aloe africana leaf extract and Magnetospirillum magnetotacticum

Metallic nanoparticles have emerged as novel therapeutic agents due to their distinctive physicochemical properties and broad-spectrum activity, with applications in antimicrobial therapy, drug delivery and bioremediation. Conventional methods for metallic nanoparticle synthesis often utilize toxic chemicals and energy intensive processes that are expensive. Green synthesis offers a sustainable and cost-effective alternative by using biomolecules from plants and microorganisms. In this study, gold (AuNPs), silver (AgNPs), and copper oxide (CuO NPs) nanoparticles were biosynthesized using leaf extracts of Aloe africana Mill., a South African medicinal plant rich in phytochemicals, and the magnetotactic bacterium Magnetospirillum magnetotacticum that naturally produces intracellular nanoparticles. GC-MS analysis revealed 13 known phytochemicals in the A. africana extract including esters, terpenoids, monoglycerides, and fatty acids which served as reducing and capping agents for nanoparticle synthesis. A. africana-derived AgNPs were spherical (11-30 nm) in shape, capped with dihydrosqualene, a known antibacterial compound; and was found to display activity against Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Enterococcus faecalis and Staphylococcus aureus) bacteria. These AgNPs however exhibited cytotoxicity to HEK293 and HeLa cell lines. A. africana AuNPs (17-62 nm) displayed diverse morphologies and CuO NPs (55-115 nm) were irregular shaped, and both nanoparticles exhibited limited antibacterial activity and low cytotoxicity. M. magnetotacticum-derived AuNPs (12-21 nm) and AgNPs (51-126 nm) were spherical, with the CuO NPs (42-66 nm) having irregular shapes. Except for A. africana-derived AgNPs, all other metallic nanoparticles displayed poor antibacterial activity. These findings are novel and highlight a dual-function green synthesis platform where A. africana phytochemicals contribute to both nanoparticle synthesis and bioactivity, positioning A. africana AgNPs as promising antibacterial agents.