Synthesis, Characterisation, and Antimicrobial Activity of Silver Nanoparticles Using Leaf Extracts of Ocimum gratissimum from Ondo State, South West Nigeria

Biogenic silver nanoparticles synthesised using plant extracts rich in phenolic and flavonoid reducing agents offer a sustainable alternative to physicochemical nanoparticle synthesis methods and have demonstrated broad-spectrum antimicrobial activity relevant to combating multidrug-resistant pathogens prevalent in Nigerian clinical environments. This study synthesises silver nanoparticles using aqueous leaf extracts of Ocimum gratissimum collected from three cultivation sites in Ondo State, South West Nigeria. Synthesis parameters including extract concentration, silver nitrate precursor concentration, pH, reaction temperature, and reaction time are optimised using a Taguchi orthogonal array design. Nanoparticle characterisation employs UV-visible spectrophotometry for surface plasmon resonance confirmation, X-ray diffraction for crystallinity and size determination, Fourier transform infrared spectroscopy for surface ligand identification, transmission electron microscopy for morphological analysis, dynamic light scattering for hydrodynamic size and polydispersity index, and zeta potential measurement for colloidal stability. Antimicrobial activity is evaluated against Staphylococcus aureus, methicillin-resistant Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, and Candida albicans by the broth microdilution method for minimum inhibitory concentration determination and time-kill kinetics. The antibiofilm activity of the nanoparticles against Pseudomonas aeruginosa is assessed by crystal violet staining. Cytotoxicity is screened using the brine shrimp lethality assay. Findings advance knowledge on the relationship between synthesis parameters, nanoparticle physicochemical properties, and biological activity for South West Nigerian plant-derived nanoparticle systems. Keywords: silver nanoparticles, green synthesis, Ocimum gratissimum, antimicrobial activity, Ondo State