Aim: In this study, the biosynthesis of silver nanoparticles was studied using biomass of Chaetoceros sp. diatom, cultivated diatom, biomass from frustuls diatom (silica) and silica/Fe3O4 composite for the measurement of water-soluble ammonia. Methodology: Live cells of Chaetoceros sp. diatom was used as a reducing and coating agent for the biosynthesis of silver nanoparticles. The resulting nanoparticles for determining functional groups, structural properties and optical properties were analyzed with infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and ultraviolet-visible spectroscopy (UV-Vis) respectively. Assay of soluble ammonia using biosynthesized nanoparticles was evaluated by ultraviolet-visible spectrophotometer. Results: Spectroscopic and microscopic analyzes confirmed the biosynthesis of silver nanoparticles in the presence of diatom biomass. The synthesized nanoparticles had spherical shapes and the average particle size was about 49.07 nm. Based on the results of XRD spectroscopy, silver nanoparticles were synthesized together with other silver compounds including silver chloride in the presence of silica and silica/Fe3O4 composite. The optimal conditions for the biosynthesis of silver nanoparticles in the presence of diatom biomass are at pH=7-7.5, duration 18h, 20?M silver nitrate and 50ml of diatom biomass at room temperature. The results of assaying soluble ammonia using silver nanoparticles synthesized in the presence of diatom biomass, showed that with increasing ammonia concentration, light absorption peaks due to the formation of [Ag(NH3)2] complexes that repel between silver nanoparticles and consequently leads to the formation of smaller silver nanoparticles tends to absorb more light and shift to shorter wavelengths (blue shift). In contrast, with increasing ammonia concentration, light absorption peaks obtained from nanoparticles synthesized in the presence of
