Synthesis of Colloidal Gold

The synthesis procedure shown here is from A. D. McFarland, C. L. Haynes, C. A. Mirkin, R. P. Van Duyne and H. A. Godwin, "Color My Nanoworld," J. Chem. Educ. (2004) 81, 544A.

The formation of gold nanoparticles can be observed by a change in color since small nanoparticles of gold are red. A layer of absorbed citrate anions on the surface of the nanoparticles keep the nanoparticles separated. The presence of this colloidal suspension can be detected by the reflection of a laser beam from the particles. Switching to a smaller anion allows the particles to approach more closely and another color change is observed.

Procedure

Wear eye protection

Chemical gloves recommended

Never look directly into a laser or shine a laser at another person.

Add 20 mL of 1.0 mM HAuCl₄ to a 50 mL Erlenmeyer flask on a stirring hot plate. Add a magnetic stir bar and bring the solution to a boil.

To the boiling solution, add 2 mL of a 1% solution of trisodium citrate dihydrate, Na₃C₆H₅O₇^.2H₂O. The gold sol gradually forms as the citrate reduces the gold(III). Gaps in the movie indicate equal gaps in time. The total elapsed time is approximately 10 times the movie length.

The presence of a colloidal suspension can be detected by the reflection of a laser beam from the particles.

Because a laser pointer emits polarized light, the pointer can be oriented such that the beam appears to disappear. When the beam from the laser is visible in one view, it is invisible in the view perpendicular to the first.
Larger movie in a new window

5-10 drops of 1 M NaCl solution are added to the tube on the right. Note the change of color of the solution as the nanoparticles get closer together.

Materials

Stock Solutions for 25 batches

• 1.0 mM hydrogen tetrachloroaurate: Dissolve 0.1 g HAuCl₄ in 500 mL distilled water. This stock solution of gold(III) ions can be prepared in advance if stored in a brown bottle.
• 1% trisodium citrate: Dissolve 0.5 g Na₃C₆H₅O₇^.2H₂O (sodium citrate) in 50 mL distilled water.
• 1 M NaCl: Dissolve 0.5 g of NaCl in 10 mL distilled water.

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