Dry reagent colorimetric sensing of nanoparticles in aqueous media

An assay kit comprising:

• a vial containing a composition comprising:
• sodium borohydride (NaBH4);
• methylene blue (MB);
• and
• 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),
• wherein the composition is in the form of a powdered mixture.

A method of forming a composition for detecting metallic nanoparticles in aqueous media, the method comprising:

• combining methylene blue (MB), 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES), and water to yield an aqueous mixture;
• removing water from the aqueous mixture to yield a solid mixture;
• and
• combining sodium borohydride (NaBH4) powder with the solid mixture to yield the composition, wherein the composition is in the form of a powder.

A method of detecting metallic nanoparticles in an aqueous solution, the method comprising:

• combining a composition with the aqueous solution to yield a test solution, wherein the composition comprises:
• sodium borohydride (NaBH4);
• methylene blue (MB);
• and
• 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),
• wherein the composition is in the form of a powdered mixture;
• and
• assessing a concentration of the metallic nanoparticles in the test solution based on absorbance of light by the test solution.