Safety:
•Wear eye protection
•Chemical Gloves Recommended
•Never look directly into a laser or shine a laser at another person
Step 1. Rinse all glassware with pure water before starting. 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 to a rolling boil.
Step 2. To the rapidly-stirred boiling solution, quickly 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 deep red or 10 minutes has elapsed.
•(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.
•The light from a laser pointer may be polarized. When polarized light causes plasmon emission the beam may disappear at some angles. When the beam from the laser is visible, is it invisible in a view perpendicular to the first?
Step 3. Record the visible spectrum of the solution. If necessary, add additional water to the cuvette to get the absorbance on scale.
Step 4. 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?
•Option 1: this part could be done in a cuvet with the visible spectrum recorded after each addition.
•Option 2: test electrolyte content in sports drinks by counting drops needed to change the color of 7 drops of gold nanoparticle solution.