Optimization studies of conditions for biological synthesis of AuNPs in various shapes using plant extract (Ocimum sanctum)

We demonstrate the effects extract amount, pH and temperature on the formation of gold nanoparticles (AuNP) using Ocimum santum plant extract. The average AuNP size, shape, and particle number can be controlled by simply varying the pH, temperature and amount of plant extract in the reaction medium. Acidic pH was found to be ideal for the formation of AuNPs. UV-visible spectra for the reaction of the plant extract with 1 mM HAuCl₄ solution showed that the highest intensity of surface plasmon resonance (SPR) occurred at pH 3. Transmission electron microscopy (TEM) showed that temperature can be used to control the aspect ratio and the relative amount of gold nanoplatelets (triangular and hexagonal). Increasing the temperature decreased the population of gold nanoplatelets and increased the number of spherical NPs, and nanoplatelet population was found to be highest at 20°C and 30°C.