Tuning Gold Nanorod Synthesis through Prereduction with Salicylic Acid

Successful synthesis of gold nanorods requires subtle combination of additives and reducing species. The latter are of major importance, as the reducing power determines the rate of metallic gold formation, which often defines the final shape and anisotropy. Ascorbic acid is a common reducing agent in the synthesis of gold nanorods, but its relatively strong reducing power limits the tunability of the final shape and optical response. We propose here a bimodal reducing agent system comprising a combination of salicylic acid and ascorbic acid. While salicylic acid prereduces Au(III) to Au(I) in the growth solution, ascorbic acid participates in the autocatalytic reduction of Au(I) to Au(0), selectively occurring on the metallic surface. This combination provides a fine control over gold reduction at any stage of nanorod formation, which in turn leads to improved monodispersity, better reduction yield, and morphology control.