Reduction of Au(III) by a β-cyclodextrin polymer in acid medium. A stated unattainable reaction

Gold nanoparticles (AuNPs) can be prepared from the reduction of Au(III) with cyclodextrins acting as both, reducing and capping agent. It has been stated that a basic medium (pH = 9-12) is a mandatory condition to achieve such reduction. We demonstrated, for the first time, the reduction of Au(III) by a crosslinked beta-cyclodextrin-epichlorohydrin polymer ((beta CDP) in acid medium (pH similar to 3). The coordination of Au(III) to the beta CD in beta CDP polymer required a pCD: [AuCl4](-) ratio of 4:1. The same ratio was necessary to achieve a 50% of the reduction of Au(III) to Au within the first 24 h of reaction. During this initial time, the reaction showed a concentration-dependent reduction rate while for longer times the reduction rate was diffusion -dependent. An overall mechanism to explain this dependency has been proposed. The C-13 NMR spectrum identified the oxidation of the C-OH groups to carboxylic ones by recording a signal at 175.6 ppm. Gold nanoparticles cores (AuNPs) with a diameter of 34.2 + 7.7 nm, determined by Transmission Electron Microscopy (TEM), was prepared by refluxing HAuCl4 in an aqueous solution of pCDP. The AuNPs core was capped by dimers of the beta CDP polymer as determined by Dynamic Light Scattering measurements. (C) 2017 Elsevier Ltd. All rights reserved.