The citrate synthesis procedure 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 borohydride synthesis procedure and extraction test is based on M. N. Martin, J. I. Basham, P. Chando, and S. Eah, "Charged Gold Nanoparticles in Non-Polar Solvents," Langmuir (2010) 26(10), 7410-7417. The laser pointer activity was added by G. Lisensky. The electrolyte analysis of sports drinks was developed by Andrew Greenburg.
Au+3 ions are reduced to neutral gold atoms. Citrate ions act as both a reducing agent and a capping agent. Borohydride acts as the reducing agent but the reference reports the capping agent is unknown. The formation of gold nanoparticles can be observed
by a change in color since small nanoparticles of gold are red. The presence of this colloidal suspension can be detected by plasmon emission
when the particles are excited by a laser beam. Switching to a smaller anion for the capping agent allows the
particles to approach more closely and a color change is observed.
Never look directly into a laser
or shine a laser at another person.
Citrate Synthesis Method
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, bring the solution to a rolling boil, and move on to the next step.
To the rapidly-stirred boiling solution, quickly add 2 mL of a 1% solution of trisodium citrate
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.)Borohydride Synthesis Method
Rinse all glassware with pure water before starting. Add 10 mL of 1.0 mM HAuCl4 to a 50 mL beaker or Erlenmeyer flask on a stirring plate. Add a magnetic stir bar and begin stirring but do not heat. Be ready to proceed with the first test. Quickly add 10 mL of 3.0 mM NaBH4 and stir for 30 seconds. Do the first test immediately with no waiting.
Repeat with a wait between additions. Add 10 mL of 1.0 mM HAuCl4 to a 50 mL beaker or Erlenmeyer flask on a stirring plate. Add a magnetic stir bar and begin stirring but do not heat. Be ready to proceed with the first test. Quickly add 5 mL of 3.0 mM NaBH4 and stir for 30 seconds, then add a second 5 mL of 3.0 mM NaBH4, and stir for 30 seconds. Do the first test immediately with no waiting. Does the wait between additions make a difference?
Add product solution to a screw cap vial or test tube to make it about 1/3 full. Add half as much acetone, cap and shake for 1 s, then add 0.2 mM dodecanethiol in hexane to double the original product volume. Cap and shake to mix. Gold particles smaller than 5 nm will transfer to the top layer. What happens for your products?
The presence of metal nanoparticles can be detected by their interaction with a beam of light since the oscillating electric field causes quantized light (plasmon) emission from the particles. Can you see a laser beam as it passes through the solution?
(Does a solution dyed with red food coloring do the same thing?)
The light from a laser pointer may be polarized with the electric field oscillation in only in one plane. Test your laser pointer with a polarizing filter to see if the laser emits polarized light. Is your laser polarized? If so the plasmon emission from gold nanoparticles would occur only in one plane. Shine the laser through the solution and rotate the laser.
What fraction of a full rotation separates the maximum and minimum observed brightness?
Record the visible spectrum of the solution. If necessary, add additional water to the cuvette to get the absorbance on scale. What is the peak wavelength?
What is the peak width at half height?
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: this part could be done in a cuvet with the visible spectrum recorded after each addition.
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" (pdf) from IVD Technology.
What is the wavelength of the visible absorption peak maximum? What is the peak width at half the maximum height (fwhm) of the visible absorption? For the latter you may need to measure the width of the longer wavelength half and then double it to get the full width.
Which method gives smaller nanoparticles? How do you know? (NaBH4 is a stronger reducing agent than Na3Citrate.)
How would the results change if you did not stir the solutions during the reduction step? Does adding the reducing agent not all at once make a difference?
Summarize the evidence that you made solid nanoparticles.
What do you think is the spacer in the borohydride synthesis? Explain your reasoning. Hint: what possible negative ions might be present?
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.
3.0 mM NaBH4: Dissolve 0.0284 g NaBH4 in 250 mL water. This solution should be made fresh before the experiment. BH4 slowly decomposes in neutral water to produce H2 gas. Solutions should not be kept overnight or tightly sealed.
0.2 mM dodecanethiol in hexane: 1.2 uL dodecanethiol in 25 mL hexane with glass pipet for dispensing.
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. Colorless or as little color as possible solutions work better. You can test electrolyte content in sports drinks by counting drops needed to change the color of 7 drops of gold nanoparticle solution.
50 mL Erlenmeyer flask or beaker (a beaker works better for the laser tests)
Graduated cylinders: 25 mL (HAuCl4), 10 mL (Na3Citrate) and 10 mL (NaBH4)
1" or 1 cm stir bar. After the experiment clean stir bars with aqua regia (3:1 HCl:HNO3 with eye protection, chemical gloves, and a fume hood.)
Stirring hotplate (Corning digital setting about 220.)
Laser pointer, polarizing filter
Dropper bottles and test tubes or cuvets for NaCl tests