Polarity inversion of gold nanorod materials

1. A method for preparing an anhydrous nanoparticle dispersion, comprising the steps of:

a. providing an aqueous medium comprising a surfactant and a quantity of surfactant-coated nanoparticles suspended in the aqueous medium, wherein the surfactant is cetyltrimethylammonium bromide (CTAB);

b. i. estimating an average particle length and an average particle diameter of the surfactant-coated nanoparticles suspended in the aqueous medium, and

ii. estimating a quantity of the surfactant-coated nanoparticles in the aqueous medium;

c. estimating a surface area (SA) of surfactant-coated nanoparticles in the aqueous medium based on the average particle length and the average particle diameter estimated in step b. i. and the quantity of the surfactant-coated nanoparticles estimated in step b. ii.;

d. adjusting a ratio of a quantity of a nanoparticle surface area (SA NP ) to a quantity of the surfactant to a desired value to form a precursor aqueous solution having a CTAB/SA NP value between 10 to 20;

e. diluting the precursor aqueous solution with an organic solvent to provide a suspension of surfactant-coated nanoparticles in the organic solvent, and

f. heating the suspension at about 80-120° C. for about 20 min to 24 hours to remove remaining water.

2. The method of claim 1 , wherein the step of adjusting the quantity of the surfactant-coated nanoparticles to the quantity of surfactant comprises:

separating the surfactant-coated nanoparticles from the surfactant to provide a supernatant component and a solid component;

decanting a portion of the supernatant component to reduce the quantity of the surfactant to provide a residual mixture; and

diluting the residual mixture with water to provide the precursor aqueous solution.

3. The method of claim 1 , wherein the organic solvent comprises a polar aprotic solvent of boiling point higher than 200° C.

4. The method of claim 3 , wherein the polar aprotic solvent is selected from the group consisting of n-methylpyrrolidinone, propylene carbonate, and combinations thereof.

5. The method of claim 1 , wherein the heating temperature is about 100° C.

6. The method of claim 1 , wherein the heating time is about 20 minutes.

7. The method of claim 1 , wherein the nanoparticles comprise gold nanoparticles.

8. The method of claim 1 , wherein the step of estimating surface area of the surfactant-coated nanoparticles includes estimating a concentration of the nanoparticles and a concentration of the surfactant in the aqueous medium using ultraviolet-visible spectroscopy.

9. The method of claim 1 , further comprising:

g. diluting an anhydrous nanoparticle dispersion with organic solvents to provide a suspension of surfactant-coated nanoparticles in the organic solvents.

10. The method of claim 9 , wherein organic solvent is selected from any polar solvent and combinations thereof.

11. The method of claim 9 , wherein organic solvent is selected from the group consisting of toluene, chloroform, cyclohexane, hexane, and combinations thereof.

12. A method for preparing an anhydrous nanoparticle dispersion, comprising the steps of:

a. providing an aqueous medium comprising a surfactant and a quantity of surfactant-coated nanoparticles suspended in the aqueous medium, wherein the surfactant is cetyltrimethylammonium bromide (CTAB);

b. i. estimating an average particle length and an average particle diameter of the surfactant-coated nanoparticles suspended in the aqueous medium, and

ii. estimating a quantity of the surfactant-coated nanoparticles in the aqueous medium;

c. estimating a surface area (SA) of the surfactant-coated nanoparticles in the aqueous medium based on the average particle length and the average particle diameter estimated in step b. i. and the quantity of the surfactant-coated nanoparticles estimated in step b. ii.;

d. adjusting a ratio of a quantity of a nanoparticle surface area (SA NP ) to a quantity of the surfactant to a desired value to form a precursor aqueous solution, wherein the quantity of the surfactant to a quantity of a nanoparticle surface area (SA NP ) is adjusted to form a precursor aqueous solution having a CTAB/SA NP value between 10 to 20;

e. diluting the precursor aqueous solution with an organic solvent to provide a suspension of surfactant-coated nanoparticles in the organic solvent,

f. heating the suspension at about 80-120° C. for about 20 min to 24 hr to remove remaining water and create an anhydrous nanoparticle dispersion; and

g. diluting the anhydrous nanoparticle dispersion with organic solvents to provide a suspension of surfactant-coated nanoparticles in the organic solvents.

13. The method of claim 12 , wherein the step of adjusting the quantity of the surfactant-coated nanoparticles to the quantity of surfactant comprises:

separating the surfactant-coated nanoparticles from the surfactant to provide a supernatant component and a solid component;

decanting a portion of the supernatant component to reduce the quantity of the surfactant to provide a residual mixture; and

diluting the residual mixture with water to provide the precursor aqueous solution.

14. The method of claim 12 , wherein the heating temperature is about 100° C., and the heating time is about 20 min.

15. The method of claim 12 , wherein the organic solvent is selected from the group consisting of toluene, chloroform, cyclohexane, hexane, and combinations thereof.