Green synthesis of Au, Pd and Au@Pd core–shell nanoparticles via a tryptophan induced supramolecular interface

Abstract

A facile, eco-friendly, room-temperature method for rapid one-pot synthesis of Au@Pd bimetallic nanoparticles exhibiting a core–shell morphology (∼60 nm) has been developed based on the successive reduction of Au(III) and Pd(II) precursors with tryptophan (Trp) in an aqueous environment. The unique supramolecular chemistry arising due to the hydrogen bonded indole group layer over the Au core seemed critical in the formation Pd shell. The core–shell morphology and surface analysis of the resulting Au@Pd nanoparticles were confirmed by aberration corrected scanning transmission electron microscopy followed by X-ray photoelectron spectroscopy. The formation of the core (Au) and shell (Pd) was also confirmed by Energy Dispersive X-Ray elemental scanning analysis. The resulting Au, Pd and Au@Pd nanoparticles were also analysed by UV-Vis spectroscopy, X-ray diffraction and Dynamic Light Scattering. Our results suggest a simple coordination mechanism where the pre-stabilized poly-Trp Au core serves as a template to facilitate the subsequent reduction of Pd(II) via active carboxyl groups. This study effectively demonstrates for the first time that core–shell nanoparticle synthesis (reduction and stabilization) can be effectively achieved by simple amino acids like Trp in an aqueous reaction mixture.