Optically Active Gold Nanoparticles

Abstract

Nanoparticles play a key role in our daily life from high-end quantum technologies to medical diagnostics and therapies. Chiral nanoparticles have emerged as a promising class of nanomaterials for their application in asymmetric catalysis, biosensing, spintronics and chiroptical devices. We have previously shown that cysteine-capped silver nanoparticles achieve strong optical activity. Herein we apply the same synthetic approach with the goal to form chiral gold nanoparticles. The chiroptical properties of the obtained nanostructures were investigated by circular dichroism spectroscopy in the ultraviolet and visible wavelength range. We show that using a ligand exchange procedure, exchanging the citrate ligands with amino acids (L-glutathione) keeps the structure of the spherical nanoparticles mostly unchanged failing to generate optical activity in the optical transitions of the metal. While direct reduction of gold salt to from nanoparticles does not lead to transfer of chirality in gold NPs, supramolecular polymeric chiral nanoparticles are formed at higher relative concentrations of cysteine that can show relatively high optical activities. Promoting the reduction of gold with a more potent reducing agent in presence of cysteine seems to be necessary to form chiral plasmonic gold nanoparticles opening new possibilities for the design of bio-compatible chiral plasmonic materials.