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 beaker or Erlenmeyer flask on
a stirring hot plate. Add a magnetic stir bar and bring the solution just 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). Remove from heat when the solution has turned red.
(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
Put a small amount of the gold nanoparticle solution in two test tubes. Use one tube as a color reference and add 5-10 drops of NaCl solution to the other tube.
Does the color of the solution change as the addition of chloride makes the nanoparticles closer together?
Materials
Stock Solutions for 25 batches
1.0 mM hydrogen tetrachloroaurate: Dissolve 0.1 g HAuCl4 (Aldrich G4022) 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.
