Dry reagent colorimetric sensing of nanoparticles in aqueous media

1. A composition comprising:

sodium borohydride (NaBH 4 );

methylene blue (MB); and

4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),

wherein the composition is in the form of a powdered mixture.

2. The composition of claim 1 , wherein a molar ratio of NaBH 4 :HEPES:MB is about 500:500:1.

3. The composition of claim 1 , further comprising a base.

4. The composition of claim 3 , wherein the base is sodium hydroxide.

5. The composition of claim 4 , wherein the composition, when dissolved in water, has a pH of about 7.

6. An assay kit comprising:

a vial containing a composition comprising:

sodium borohydride (NaBH 4 );

methylene blue (MB); and

4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),

wherein the composition is in the form of a powdered mixture.

7. The assay kit of claim 6 , wherein a molar ratio of NaBH 4 :HEPES:MB in the composition is about 500:500:1.

8. The assay kit of claim 6 , wherein the composition further comprises sodium hydroxide.

9. The assay kit of claim 8 , wherein the composition, when dissolved in water, has a pH of about 7.

10. 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 (NaBH 4 ) powder with the solid mixture to yield the composition, wherein the composition is in the form of a powder.

11. The method of claim 10 , wherein removing the water from the aqueous mixture comprises freeze-drying the aqueous mixture.

12. 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 (NaBH 4 );

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.

13. The method of claim 12 , comprising assessing the concentration of the metallic nanoparticles in the test solution less than five minutes after combining the composition with the aqueous solution.

14. The method of claim 12 , wherein a concentration of MB in the test solution is about 20 μM.

15. The method of claim 12 , wherein a concentration of HEPES in the test solution is about 10 mM.

16. The method of claim 12 , wherein a concentration of the NaBH 4 in the test solution is about 10 mM.

17. The method of claim 12 , wherein assessing the concentration of the metallic nanoparticles comprises assessing a concentration in range of about 5 μg/L to about 500 μg/L for 5-nm gold nanoparticles.

18. The method of claim 17 , wherein the concentration of 5 μg/L for 5-nm gold nanoparticles is equivalent to a reactive nanoparticle surface area of about 0.3 m 2 /m 3 .

19. The method of claim 12 , wherein assessing the concentration of the metallic nanoparticles comprises comparing the absorbance of light having a wavelength of 663 nm by the test solution with the absorbance of light having a wavelength of 663 by solutions having known concentrations of the metallic nanoparticles.

20. The method of claim 12 , wherein the metallic nanoparticles catalyze reduction of MB to leuco methylene blue (LMB).