Green synthesis and stabilization of earthworm-like gold nanostructure and quasi-spherical shape using Caesalpinia sappan Linn. extract

Green synthesis of nanoparticles (NPs) has received much attention due to biocompatibility and being facile and eco-friendly. Caesalpinia sappan Linn. (CS) is a plant found in Southeast Asia and is commonly known as sappan wood. In this research, we are the first to successfully use CS extract as reducing and stabilizing agents for the green synthesis of gold NPs (AuNPs). Colloidal solutions of AuNPs after reduction show natural red-wine and dark purple color depending on the size of AuNPs, without the interference of the orange/reddish-orange color from the CS extract. The concentration of CS was investigated for the formation and stabilization of AuNPs in colloidal solution (i.e. morphology, size and zeta potential). It was found that the optimized concentration of the reducing agent for the green synthesis of AuNPs was in the range of 0.004-0.04 wt% of CS. As the concentration of CS was increased, the sizes of AuNPs decreased due to the adsorption and stabilizing effect of CS. Transmission electron microscopy (TEM) analysis showed differences in size and shape of the NPs with earthworm and quasi-spherical characteristics. The zeta potential of AuNPs synthesized in the presence of CS was approximately in the range of -10 mV to -20 mV, indicating the stability of AuNPs synthesized by CS.