In situ formation of chiral core–shell nanostructures with raspberry-like gold cores and dense organic shells using catechin and their catalytic application

Abstract

Chiral core–shell nanostructures containing raspberry-like gold cores and well-defined dense organic shells are synthesized by an in situ method using a natural antioxidant catechin as the reducing agent. This method is flexible and enables control over the shell thickness by adjusting the molar ratio of catechin to HAuCl₄. Transmission electron microscopic analysis shows the formation of core–shell nanostructures with somewhat raspberry-shaped gold cores. The proposed mechanism explains that catechin reduces Au³⁺ to metallic Au to gold nanostructures and gets oxidized to different oligomeric products that are adsorbed in situ and assembled through H-bonding and form a thick organic shell around the generated gold nanostructures. Each of these oxidized forms of catechin is well-characterized by FTIR, ESI-MS, and MALDI-TOF-MS spectroscopies. This reaction follows a radical pathway as confirmed by electron paramagnetic resonance spectroscopy. Due to the presence of a compact and dense shell, the rate of gold core dissolution sharply decreases compared to that of the dissolution of monolayer protected Au nanoparticles when etched with KCN solution. The optical activity of the core–shell nanostructure is the result of the interaction between chiral shells and gold cores as observed by circular dichroism (CD) spectroscopy. The chiral core–shell Au nanostructures exhibit a CD band at the plasmon resonance frequency (∼596 nm). Finally, these chiral core–shell nanostructures are used as an effective catalyst in the borohydride reduction of p-nitrophenol.