Engineering the Structure and Optical Properties of Gold Nanostars with Microfluidics

Gold nanostars (AuNS) are anisotropic gold nanoparticles with structure-tunable optical properties. Most synthetic protocols rely on multistep procedures and on the use of cytotoxic and/or strongly bound chemicals, which can hamper AuNS applications. While the synthesis of AuNS with HEPES overcomes these limitations by using a biocompatible buffer that acts as both a shape-directing and a reducing agent, the resulting nanoparticles are highly heterogeneous and polydisperse. In this paper, we use a microfluidic chip to manipulate the morphology and optical properties of AuNS, while significantly improving their monodispersity. Notably, by adjusting microfluidic parameters, including viscosity of the organic phase, flow rate ratio, and buffer-to-gold ratio, this protocol can manipulate the growth mechanism of the nanoparticles, switching between seedless and seed-mediated-like growth, and it does so without the need to add a presynthesized seed. Such control is not possible with one-pot bench synthesis. Our results provide new opportunities to tailor the growth and formation of gold nanoparticles while significantly improving their monodispersity.