Scientists Made Color-changing Paint Out of Gold Nanoparticles

Materials:

Stock Solutions for 25 batches

• 1.0 mM hydrogen tetrachloroaurate: The solid is hygroscopic so purchase HAuCl4.3H2O (Aldrich 244597 or 520918) in 1.0 g quantities and use the entire bottle. Dissolve 1.0 g HAuCl4.3H2O in 250 mL distilled water to make a 10.0 mM stock solution of gold(III) ions that can be kept for years if stored in a brown bottle. Dilute 25 mL of stock to 250 mL to make the 1.0 mM concentration for this experiment.
• 1% trisodium citrate: Dissolve 0.5 g Na3C6H5O7.2H2O (sodium citrate) in 50 mL distilled water. NOTE: Sodium citrate does not keep well so it is best to make fresh solutions prior to each synthesis.
• NaCl solution: Dissolve at least 0.5 g of NaCl in 10 mL distilled water or use a saturated solution.
• Optional sports drinks: Gatorade Ice, Powerade, Flavorless Pedialyte, Pickle Juice. Solutions with little to no color work best.

Equipment:

• 50 mL Erlenmeyer flask or beaker
• 1″ or 1 cm stir bar
• Stirring hotplate
• Laser pointer, polarizing filter
• Droppers and test tubes or cuvettes

Protocol:

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 (5-10 drops) of the gold nanoparticle solution into 5 test tubes or microcentrifuge tubes. Use one tube as a color reference and add an equal volume of NaCl solution, water, sports drink, or pickle juice to the other tubes. Does the color of the solution change as the addition of chloride (or other electrolyte) makes the nanoparticles clump closer together?

• Option 1: this part could be done in a cuvette 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.
• Option 3: you can ask students to retrieve a clear liquid of their choice that they would like to test in addition to some of the other solutions.

Conclusions

Before the addition of the reducing agent, the gold is in solution in the Au+3 form. When the reducing agent is added, gold atoms are formed in the solution, and their concentration rises rapidly until the solution exceeds saturation. Particles then form in a process called nucleation. The remaining dissolved gold atoms bind to the nucleation sites and growth occurs.

See "Producing gold colloids" at https://chem.beloit.edu/edetc/nanolab/gold/Manufacturing%20high-quality%20gold%20sol.pdf

1. Why do more concentrated or stronger reducing agents tend to give smaller nanoparticles?
2. Why does rapid mixing of the reducing agent give more monodispersed particles?
3. What is the wavelength and fwhm (peak width at half the maximum height) of the visible absorption?
4. Summarize the evidence that you made solid nanoparticles.
5. If you tested several salt solutions, report your results and rank the solutions in order from the least to the most electrolytes.

YouTube Link

http://www.youtube.com/watch?v=urmi99jQSZY

Scientists Made Color-changing Paint Out of Gold Nanoparticles

Source: https://education.mrsec.wisc.edu/citrate-synthesis-of-gold-nanoparticles/

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