2.4. Green Synthesis of Gold Nanoparticles (AuNPs)

Next, 1 mL of precursor solution (HAuCl₄) was mixed with the Ssp extract for the green synthesis of gold nanoparticles (AuNPs). The concentration of the precursor solution and the extract volume were those of the experimental parameters evaluated. For concentration analysis, HAuCl₄ solution was varied in 3 mM, 5 mM, and 7 mM, while the extract and precursor volumes used were 0.25 mL for the three first experiments. Then, in a second set of experiments, the volume extract was varied from 0.25 to 1 mL against 1 mL of HAuCl₄ solution at 7 mM. All the synthesis reactions were carried out at room temperature (~25 °C).

A purification process was then carried out and liquid samples containing the AuNPs were subjected to centrifugation at 10,000 rpm for 15 min. The nanoparticles were then precipitated and the supernatant containing extract residues and unreacted Au ions were removed. Distilled water was then added and the nanoparticles were redispersed using an ultrasonic bath for 10 min. The centrifugation and redispersion process were repeated 3 times to obtain clean nanoparticles for characterization and application.