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 β-cyclodextrin-epichlorohydrin polymer (βCDP) in acid medium (pH ∼3). The coordination of Au(III) to the βCD in βCDP polymer required a βCD:[AuCl₄]⁻ ratio of 4:1. The same ratio was necessary to achieve a 50% of the reduction of Au(III) to Au⁰ 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 ¹³C NMR spectrum identified the oxidation of the C[sbnd]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 βCDP. The AuNPs core was capped by dimers of the βCDP polymer as determined by Dynamic Light Scattering measurements.