Synthesis of Gold Nanoparticles

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.

Procedure

Wear eye protection

Chemical gloves recommended
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 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.)

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?

Tests

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.

Conclusions

Click image for larger view
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.
  1. 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.
  2. Which method gives smaller nanoparticles? How do you know? (NaBH4 is a stronger reducing agent than Na3Citrate.)
  3. 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?
  4. Summarize the evidence that you made solid nanoparticles.
  5. 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

Equipment


Developed in collaboration with the
University of Wisconsin Materials Research Science and Engineering Center
Interdisciplinary Education Group   |   MRSEC on Nanostructured Interfaces
This page created by George Lisensky, Beloit College.  Last modified September 7, 2016 .