Block copolymer-regulated synthesis of gold nanocrystals with sharp tips and edges

Abstract

Gold decahedra and triangular plates of small size (30 and 50 nm, respectively) were grown by a green one-step synthesis process of HAuCl₄ in the presence of citric acid and amphiphilic copolymer Tetronic T904^® (from BASF), which both act as reducing and stabilizing agents. Tetronic T904^® is a star-shaped copolymer composed of polyethylene oxide and propylene oxide units. Both compounds are key in the formation of decahedral particles due to their specific adsorption on certain crystallographic planes of the nanoparticles. Withdrawal or substitution of one of them involves the formation of irregular particles. On the other hand, by changing the reactions conditions the nanoparticle size and shape could be modified. In particular, by varying the molar ratio between the block copolymer a more efficient protection provided by the block copolymer is achieved, which allows the reaction to turn into kinetic control and, thus, a change from decahedral to triangular shape is observed. Due to the presence of acute tips and sharp edges, which sustain large electromagnetic fields upon excitation with light of appropriate energy, these nanoparticles demonstrated their utility as SERS substrates. Concentrations of up to ∼10⁻¹¹ M of the Raman probe 4-nitrobenzenethiol displayed a readable SERS spectrum, with electromagnetic enhancement factors of up to ∼4.2 × 10⁷.