Three Syntheses 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 was modified by E. Koenig and G. Lisensky from 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 photochemical synthesis procedure was modified by S. Rudisill and G. Lisensky from K. L. McGilvray, M. R. Decan, D, Wang and J. S. Scaiano, "Facile Photosynthesis of Unprotected Aqueous Gold Nanoparticles," J. Amer. Chem. Soc. (2006) 128, 15980-15981. The laser pointer activity was created by G. Lisensky. The electrolyte analysis of sports drinks was developed by Andrew Greenburg.

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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 nucleation begins. The remaining dissolved gold atoms bind to the nucleation sites and growth occurs with the solution concentration of gold atoms at saturation. See "Producing gold colloids" (pdf) from IVD Technology.

Au+3 ions can be reduced to neutral gold atoms by several methods:

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. The peak width of the plasmon emission is a measure of the size distribution of the particles. 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 (Read all steps before starting.)

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 size test. Quickly add 10 mL of 3.0 mM NaBH4 and stir for 30 seconds. Do the size test immediately with no waiting. (You have less than 30 seconds to complete the next test.)

Size test: 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? Discard the hexane, alkanethiol, and acetone in an organic waste bottle.

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 size 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. Repeat the size test if your previous trial showed small particles. Does the wait between additions make a difference?

Photochemical Synthesis Method

Rinse all glassware with pure water before starting. Add 2.0 mL of 1.0 mM HAuCl4 to a small beaker. Add 4.0 mL of 1.5 mM 2-hydroxy-4'-(2-hydroxyethoxy)-2-methyl-propiophenone.

Put the beaker on an illuminator designed for examing gels.

To protect yourself block the apparatus with a light shield. Illuminate with intense 300 nm light for 60-90 seconds. Record the illumination time.

Never look at the UV lamps. This movie shows what is happening in the photoreactor behind the light shield.

Tests

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? (Would 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. (Which of the lasers in the video of a rotating polarizer are polarized light sources?) Test laser pointers with a polarizing filter until you find one that emits polarized light. What fraction of a full rotation separates the maximum and minimum observed brightness?

If the light source is polarized, the plasmon emission from gold nanoparticles would occur only in one plane. Shine a polarised laser (DO NOT USE A POLARIZING FILTER!) 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 the maximum height? You may need to measure the width of the longer wavelength half and then double the value to get the full width.

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? When finished put the gold solutions with chloride in the "bad gold" container.
Option: this part could be done in a cuvet with the visible spectrum recorded after each addition.

Conclusions
  1. What is the wavelength of the visible absorption peak maximum for each of the metal nanoparticle syntheses? What is the peak width at half the maximum height (fwhm) of the visible absorption? Tabulate your results for all products.
  2. Which method gives the most monodispersed metal nanoparticles? How do you know?
  3. Why do more concentrated or stronger reducing agents tend to give more uniform nanoparticles? NaBH4 is a stronger reducing agent than Na3Citrate. The reducing strength for the photochemical method depends on the light intensity and the reducing strength in a microwave depends on the power. How would you rank the reducing power based on your observed spectra?
  4. How should the results change if you did not stir the solutions during the reduction step? Did adding the NaBH4 reducing agent in two portions to the gold ion make a difference?
  5. Explain your observations when you added salt solutions to the gold nanoparticle solutions. (The results are less clear for silver nanoparticles since any excess silver ion will result in a silver chloride precipitate.)
  6. Summarize the evidence that you made solid metal nanoparticles. Are the particles spherical?

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 February 12, 2017 .