Colloidal solution of silver nanoparticles for label-free colorimetric sensing of ammonia in aqueous solutions

Silver nanoparticles were synthesized in the presence of saccharides and ammonia (NH₃) in the concentration range from 10^-2 to 10³ ppm to develop an optical sensor for NH₃ in aqueous solutions. Ammonia affects the features of the nanoparticles obtained in a concentration-dependent manner as determined by UV-vis absorption analysis and TEM observations. Structural and morphological analysis provides the basis for the production of a colorimetric label-free sensor for ammonia. Overall, surface plasmon resonance increases when ammonia concentration rises, although the functional trend is not the same over the entire investigated ammonia concentration range. Three different ranges have been identified: very low ammonia concentrations from 0.01 to 0.2 ppm, high ammonia concentrations from 20 to 350 ppm and, most importantly, the intermediate or physiological range of ammonia from 0.5 to 10 ppm.