What is the role of phytochemical compounds as capping agents for the inhibition of aggregation in the green synthesis of metal oxide nanoparticles? A DFT molecular level response

Abstract

To control the size of metal oxide nanoparticles, capping agents are often employed in the green synthesis of metal oxide nanoparticles. To guide the role of phytochemical-based capping agents, the interactions between the phytochemicals and nanoparticles have been investigated by DFT method. For this purpose, aucubin, an iridoid glycoside compound, was selected as a model. In this work, an in-depth analysis of the interactions between aucubin and ZnO nanoparticles with different sizes were conducted in gas and water phases. The adsorption Gibbs free energies of all bio-nanocomposites with different sizes were observed to be negative, which indicates the spontaneous nature of the adsorption process. The Eg and Gibbs free energy values of aucubin was found to depend on the size of ZnO nanoparticles. The bio-nanocomposite aucubin-(ZnO)6 has the most negative Gibbs free energy due to the formation of four hydrogen bonds between the aucubin and the nanoparticles. The evaluation of charge decomposition analysis showed that the direction of charge transfer is from aucubin to ZnO. Indeed, we found that both the vdW and electrostatic interactions are present in ZnO-aucubin bio-nanocomposites. The molecular level information provided in this work can be considered as a model for using phytochemicals as capping agents in green synthesis of nanoparticles.