Demystifying the morphology and size control on the biosynthesis of gold nanoparticles using Eucalyptus globulus bark extract

Nowadays, the "green synthesis" of different types of nanostructures, e.g. metallic nanoparticles (NPs), using plant extracts, emerged as a powerful, sustainable and economical design tool due to the panoply of plants available. However, the main challenge in this emerging area is still the strict control of the morphology (shapes) and dimensions of the NPs that is fundamental for the implementation at large scale of these cleaner synthetic methodologies. Here, an Eucalyptus globulus Labill. bark aqueous extract was used as a model extract and we systematically investigated the effect of different reaction parameters, namely temperature, pH, extract concentration and reaction time, on the morphology and size of gold NPs, by Ultraviolet visible (UV-vis) spectroscopy, transmission electron microscopy (TEM), and dynamic light scattering (DLS). The obtained results demonstrated that the extract concentration has a major role in the size of the gold NPs (interestingly for very low concentrations a "size plateau" can be reached) and the pH of the medium slightly affects their morphology. Temperature does not present a significant effect in both size and morphology of spherical nanostructures however a clear impact in the size of anisotropic structures was evidenced. Reaction time has also a poor effect in both size and morphology of the NPs, except when varying extract concentration. This study presents for the first time an unequivocally and clear establishment of the reduction conditions of these eco-friendly approaches that should be selected to control the size and morphology of gold NPs prepared in view of their final application.