Silver and gold nanoparticles in plants: sites for the reduction to metal

Abstract

Induced formation of metal nanoparticles in living plants is poorly understood. The sites for the reduction of Ag⁺ and Au³⁺ to Ag⁰ and Au⁰ metal nanoparticlesin vivo in plants were investigated in order to better understand the mechanism of the reduction processes. Brassica juncea was grown hydroponically, followed by growth in solutions of AgNO₃, [Ag(NH₃)₂]NO₃ or HAuCl₄. Harvested plants were sectioned and studied by transmission electron microscopy. Total metal content was analysed by atomic absorption spectroscopy. The chemical state of the metals was determined by X-ray absorption spectroscopy. Nanoparticles of Ag⁰ and Au⁰ were found in leaves, stem, roots and cell walls of the plants at a concentration of 0.40% Ag and 0.44% Au in the leaves. Particles which were approximately spherical were formed with sizes of 2–100 nm. The sites of the most abundant reduction of metal salts to nanoparticles were the chloroplasts, regions of high reducing sugar (glucose and fructose) content. We propose that these sugars are responsible for the reduction of these metals and other metal salts with reduction potentials over +0.16 V and that the amount of reducing sugar present or produced determines the quantity of metal nanoparticles that may be formed.