Synthesis of gold nanoparticles

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This section of the module first provides some background information about the most common method used for the synthesis of gold nanoparticles which involves the reduction of tetrachloroauric acid (HAuCl₄) by small amounts of citric acid. This method was introduced by Turkevich et al. in 1951 and further refined by Frens in 1973. In this method, hot tetrachloroauric acid reacts with a small amount of sodium citrate solution. The citrate ions act as both a reducing agent, and a capping agent. Reduction occurs according to the reactions:

\[\ce{Na3C6H5O7 \:(sodium\: citrate) + 3H+ → C6H8O7 \:(citric\: acid) + 3Na+}\nonumber\]

\[\mathrm{2HAuCl_4 + 3C_6H_8O_7 \:(citric\: acid) → 2Au + 3C_5H_6O_5 \:\textrm{(3-ketoglutaric acid)} + 8HCl + 3CO_2}\nonumber\]

In the traditional Frens’ method, small citrate to tetrachloroauric acid ratios are used without particular attention to pH and the typical trend observed is that particle size decreases as the ratio of citrate to tetrachloroauric acid increases. This is consistent with the mechanism where part of the sodium citrate is used to reduce Au³⁺ to Au and the remaining sodium citrate ions are available for stabilizing the particles (capping agent). Nanoparticles continue to aggregate until the total surface area of all particles becomes small enough to be covered by the existing citrate ions. Therefore, higher concentrations of sodium citrate result in less particle aggregation and the final size of the particles decrease. However, Ji et al. (reference 7) observed that if the ratio is increased beyond 3.5:1, the particle size actually increases and eventually levels off for very large ratios. This can be attributed to the effect the citrate plays on the final pH of the solution. Upon addition of larger amounts of Na₃Ct to HAuCl₄, the pH increases and shifts the gold complex equilibrium toward more hydrolyzed forms. In addition, oxidation products of citrate are formed. All these processes create a continuously changing Turkevich reaction system with a great complexity and a variety of possible pathways for the reduction of Au³⁺. pH also affects the ionization of citrate depending where the pH falls with respects to three pKa values of 3.2, 4.8 and 6.4. As more citric acid is in the form of citrate (with a -3 charge), the ionic strength of the solution increases and the colloidal stability of the seed particles decreases. As consequence, the seed particle size increases leading to less particles and larger final sizes.

Students are directed to investigate both the effect of changing the Na₃Ct to HAuCl₄ ratio as well as the pH of the solution at three different values slightly above each respective pKa. These values were chosen because of the larger particle size observed which yield larger extinction coefficients and are more suitable for sensor application. However, to thoroughly investigate the role that pH plays in the synthesis of gold nanoparticles, the instructor will have to widen the pH range from 1 to 8 (see reference 7).

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

The oxidation number is +3.

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

\[\ce{HAuCl4 + 3e- → Au + H+ + 4Cl-}\nonumber\]

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

Since citric acid is a triprotic acid with pKa values of 3.2, 4.8 and 6.4, the acid will be completely dissociated to citrate at pH values above 6.4.