A ferritin mediated photochemical method to synthesize biocompatible catalytically active gold nanoparticles: size control synthesis for small (∼2 nm), medium (∼7 nm) or large (∼17 nm) nanoparticles

Abstract

Ferritin (Ftn) undergoes photo-induced charge separation reactions that oxidize organic substrates. The liberated electrons are transferred through the protein shell to reduce Au ions to gold nanoparticles (AuNPs). We systematically varied the concentrations of citrate (electron donor), Au³⁺ or Au⁺ (electron acceptor), and ferritin (photo catalyst) to determine if careful control of these reactant concentrations would: (1) provide size control; (2) alter the morphology of the resulting AuNPs; and (3) alter the catalytic activity of the resulting AuNPs. The size and phosphate content of the ferritin iron core was also evaluated for its influence in this photocatalysis reaction. We report that as the Ftn concentration was increased to an optimal range, the number of AuNPs increased and showed smaller size, more spherical shape, and narrower distribution. Increasing the citrate concentration (electron donor) increased the rate of AuNP formation producing more spherical, uniform sized AuNPs. Increasing the Au³⁺ concentrations increased the number and sizes of the AuNPs. Since Au³⁺ reduction requires 3-electrons we proposed that using Au⁺ would increase the rate of the reaction. The photochemical reaction with Au⁺ was faster and produced 2.4 ± 1.0 nm diameter AuNPs providing another method of size control. AuNPs were tested as reduction catalysts to convert 4-nitrophenol into 4-aminophenol. The smaller spherical AuNPs were better reduction catalysts than the larger AuNPs. In summary, using a single photochemical synthesis method we can reproducibly control the size, uniformity and catalytic activity of the resulting AuNPs simply by varying the concentrations or oxidation states of the reactants.