Colloidal Gold Nanoparticles (AuNPs) Solution Preparation

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    We used HAuCl4.3H2O to produce colloidal gold nanoparticles solution. The very first thing to remember is to avoid any HAuCl4.3H2O and metal contact because HAuCl4.3H2O is very corrosive and can immediately react with the metal. A glass spatula should be used instead of a metal spatula. All the glassware must be thoroughly cleaned with aqua regia (3:1 :: HCl:HNO3), rinsed with DI water, and dried under a stream of nitrogen gas to avoid unwanted nucleation during the synthesis, as well as aggregation of gold colloid solutions. Here's the procedure that was followed:
  1. Prepare ~1% trisodium citrate aqueous solution: Add 1 g sodium citrate dihydrate to 100 ml DI water.
  2. Prepare ~1% HAuCl4 aqueous solution: Add 0.01 g HAuCl4 to 1 ml deionized (DI) water.
  3. Prepare ~0.11% KBH4 solution (in trisodium citrate solution): Add 0.11 g KBH4 to 100 ml 1% trisodium citrate aqueous solution.
  4. Add 1 ml of ~1% HAuCl4 aqueous solution (obtained in step 2) to 100 ml DI water; stir vigorously for 1 minute.
  5. Add 1 ml of ~1% trisodium citrate aqueous solution to the solution obtained in step 4.
  6. (After 1 minute) Add 1 ml of ~0.11% KBH4 solution (obtained in step 3) to the solution obtained in step 5.
The solution turns dark purple immediately during the addition of 0.11% KBH4 solution. The solution container is labeled and stored in a refrigerator until use.

Gold nanoparticle size determination

    Wolfgang Haiss and coworkers, The University of Liverpool, published an article in 2007 on the determination of size and concentration of gold nanoparticles using UV-Vis spectrophotometer. It was found that the surface plasmon resonance (spr) peak for particles in the range 2.5 nm - 100 nm appears between 520 nm and 580 nm, and that this peak dampens for small particles due to the reduced mean free path of the electrons. For particles larger than 25 nm (35 nm - 100 nm), an exponential relation was found between the surface plasmon resonance peak ({lambda_spr}) and the particle diameter. The equation obtained was:
d = {ln({{lambda_spr}-{lambda_0}}/{L_1})}/{L_2} [equation 1]

Where, {lambda_0} = 512, L1 = 6.53, and L2 = 0.0216. This equation determines the value of the particle diameter (d) within 3% of the experimental results.

For particles smaller than 35 nm, a different method was used that determined the particle size within 11% of the experimental results. In this method, the ratio of the absorbance at two different wavelengths was used to determine the particle size. The equation obtained was:
d = exp({B_1}[{A_spr}/{A_450}]-{B_2}) [equation 2]

Where, A450 is the absorbance at 450 nm and the experimentally determined fit parameters are B1 = 3.00 and B2 = 2.20. It should be noted that both the wavelengths used in this method need to be below 600 nm.

Furthermore, the particle size can also be determined with better accuracy if the initial concentration of gold (mol/liter) used to prepare the colloidal gold nanoparticles solution is known. The equation obtained was:
d = {{{{A_spr}(5.89 x {10^{-6}}}}/{{c_Au}{exp{{C_1}}}})^{1/{C_2}}} [equation 3]

Where, C1 = -4.75 and C2 = 0.314, and cAu is the initial concentration of gold in moles/liter.

The graph below shows the absorbance vs wavelength of the colloidal gold nanoparticles solution prepared in our laboratory. {lambda_spr} was found to be 523 nm and {A_spr} was found to be 1.139.

Equation 1 determined the size of the gold nanoparticles to be 24 nm, whereas equation 3 (concentration method) determined it to be 16 nm. Although the particle size was not confirmed by the TEM analysis, it would be reasonable to assume the particle size to be around 16 nm (as determined by the concentration method) because equation 1 is more suited to particles larger than 35 nm. Furthermore, Jiandong Hu et al (whose recipe we followed) found their gold nanoparticles to be around 10 nm (average) by the TEM analysis.

Electroless gold deposition / plating procedure.