Silver nanoparticles synthesized by a reagent less method involving only UV radiation have been used in colorimetric assay for the detection of ammonia in solution. The silver nanoparticles were synthesized by the exposure of a silver nitrate solution to a low-power UV source in the presence of poly(methacrylic acid) (PMA), which acted both as reducing and capping agent. The synthesis of the silver nanoparticles was studied by monitoring the changes in position and amplitude of the localized plasmon resonance (LSPR) band using UV-vis spectroscopy. The morphology of the particles was studied using transmission electron microscopy which confirmed the formation of spherical particles with an average particle size around 8 nm. Interestingly, the silver nanoparticles solution was found to display a strong color shift from purple to yellow upon mixing with increasing concentration of ammonia ranging from 5 to 100 ppm. Hence, the nanoparticles prepared with this method could be used as colorimetric assay for sensing applications of ammonia in water.