A composition for dry reagent colorimetric sensing of nanoparticles in aqueous media, including sodium borohydride (NaBH4), methylene blue (MB), and 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) in the form of a powdered mixture. The composition may be formed by combining MB, HEBES, and water to yield an aqueous mixture, removing water from the aqueous mixture to yield a solid mixture, and combining NaBH4 powder with the solid mixture to yield the composition. The composition may be used to detect metallic nanoparticles an aqueous solution by combining the composition with an aqueous solution to yield a test solution, and assessing a concentration of the metallic nanoparticles in the test solution based on absorbance of light by the test solution. The composition may be provided in an assay kit for sensing nanoparticles in aqueous media.