Synthesis of Gold Nanoparticles via Citrate Reduction

Gold nanoparticles were prepared by the reduction of gold(iii) chloride trihydrate (99.9%, Sigma-Aldrich, USA) solutions with sodium citrate (99% Synth, Brazil) by the standard procedure, following the citrate method.^{44 (link)} Solutions were prepared with deionized water obtained from a commercial Millipore Elix 3 system. All the chemicals used in this work were of analytical grade and were used as received with no further purification. A volume of 100 mL of an aqueous solution of HAuCl₄ (1.0 mmol L⁻¹) was heated and stirred gently with a magnetic Teflon-coated bar. When the temperature of the HAuCl₄ solution reached 90 °C, 1.0 mL of a 0.3 mol L⁻¹ solution of sodium citrate preheated to 90 °C was added, and the pH of the mixture, measured at room temperature, was maintained at 3 to 4. After 8 minutes of reaction, the reaction mixture was removed from the heat and cooled to room temperature (Fig. 5).