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Synthesis of gold nanoparticles

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  • There are several different ways by which gold nanoparticles can be synthesized but the most common reaction involves the reduction of tetrachloroauric acid (HAuCl4) to gold (Au) using trisodium citrate (Na3C6H5O7) (6). This is a red-ox reaction where gold in the tetrachloroauric acid is reduced to elemental gold while the citrate (C6H5O7 3–) is oxidized to dicarboxy acetone (C5H4O5) (7). Figure 3 shows the structures of these species. This reaction was first described by John Turkevich in 1951 (8) and the mechanisms of particle formation under different experimental conditions are well discussed in the literature (9).

    Q6. What is the oxidation number of gold in HAuCl4?

    Q7. Write the half reaction for the reduction of HAuCl4 to Au.







    Figure 3. Chemical structure of citric acid (a); trisodium citrate (b); dicarboxy acetone (c)

    Excess citrate ions not involved in the red-ox reaction are also adsorbed on the surface of the particles, thus playing a role in stabilizing the nanoparticles.  They are called “capping” agents.  Adsorption of citrate ions gives the gold particles an overall negative charge. Mutual repulsion of the small, negatively charged particles keeps them suspended in the solution and prevents them from coagulating to form larger particles that might eventually settle out of solution.  This suspension of the gold nanoparticles is known as a colloid.

    Since citric acid is a triprotic acid with distinct pKa values of 3.2, 4.8 and 6.4, pH will affect the chemical equilibrium involving the dissociation of the three hydrogens (10).

    Q8. Above what pH value will citric acid (C6H8O7) be completely dissociated to citrate (C6H5O73–)?

    From an application standpoint, it is very important that particle size distribution be uniform.  For example, if one was to develop a colorimetric sensor for a given analyte based on the color shift of gold nanoparticles, the optical characteristics of the nanoparticles would have to be highly reproducible to ensure repeatability of the analytical method.  As you observe from data in Table 1, size strongly affects the particle wavelength of maximum absorption as well as the molar extinction coefficient which, in turns, defines the sensitivity of the analytical method.

    Thus it is important to understand how different synthetic conditions influence the size and uniformity of size distribution.  Controlling the size and size distribution is necessary to control the optical properties of the nanoparticles.

    In the next section of this module you will explore the synthesis of gold nanoparticles using the reduction of tetrachloroauric acid (HAuCl4) by citrate and identify experimental parameters that may affect the size, size distribution and optical properties of gold nanoparticles.