Larvicidal Efficacy of Green-Synthesized Silver Nanoparticles from Alstonia boonei and Terminalia catappa against Anopheles gambiae

Abstract

This study investigates the eco-friendly synthesis of silver nanoparticles (AgNPs) using aqueous extracts of Alstonia boonei (AA) and Terminalia catappa (TCA) and evaluates their larvicidal efficacy against the fourth-instar larvae of Anopheles gambiae. The bio-reduction of silver ions (Ag⁺) to elemental silver (Ag⁰) was confirmed by a color change and the Surface Plasmon Resonance (SPR) peak at 435 nm, indicating successful AgNP formation. FTIR analysis identified functional groups, such as O-H, C-H, and N groups, which acted as both reducing and capping agents. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDXs) confirmed the presence of silver (77.20 %) in the biosynthesized nanoparticles. The larvicidal activity was evaluated following WHO (2005) protocol, different concentrations of the nanoparticles: 50, 100, 150, 200 and 250 mgL^-1 was used, 25 fourth-instar larvae of was exposed to each concentration, with three replicate for each test. Both AA and TCA-synthesized AgNPs exhibited concentration-dependent larvicidal activity. AA proved to be the most potent, with a lethal concentration (LC₅₀) of 65.2 % Cl after just one hour of exposure. Upon the initial treatment, the TCA was not very effective as compared to AA. After 12 hours, it became much more potent, with an LC₅₀ of 34.5 %. The tiny size of the nanoparticles helped them get through the larval membrane easily. Once inside, they invade the cells, which caused the larvae to die. This shows that the silver nanoparticles, made from these plant extracts, could be a great, eco-friendly way to control mosquitoes.