Characterisation of Optical Properties of Colloidal Gold Nanoparticles Synthesised Using Nigerian Plant Extracts

Green synthesis of gold nanoparticles using plant extracts represents a sustainable and chemically benign approach to nanomaterials production with potential applications in biomedical diagnostics, catalysis, and photonic devices. Nigerian medicinal plants offer a rich source of phytochemical reducing and capping agents for nanoparticle synthesis. This study synthesises and characterises gold nanoparticles using aqueous extracts from three Nigerian medicinal plants: bitter leaf (Vernonia amygdalina) from Benue State, moringa leaf (Moringa oleifera) from Sokoto State, and neem leaf (Azadirachta indica) from Kano State. Synthesis is performed by mixing 1 mM HAuCl4 solution with plant extract at 60 degrees Celsius, and progress is monitored by UV-Vis spectrophotometry. Nanoparticle size distribution and zeta potential are characterised by dynamic light scattering. TEM imaging confirms particle morphology and size at the nanometre scale. Results indicate surface plasmon resonance absorption peaks at 520 to 548 nm for all three extract-synthesised samples, confirming gold nanoparticle formation. Moringa-derived particles exhibit the narrowest size distribution with a mean diameter of 14.2 nm and zeta potential of minus 32 mV, indicating good colloidal stability. Nanoparticle optical extinction cross-section is computed from Mie theory. Keywords: gold nanoparticles, green synthesis, plasmon resonance, optical properties, Nigeria