Seeded-growth synthesis of 20–60 nm monodisperse citrate-capped gold nanoparticles in a millifluidic reactor

Abstract Gold nanoparticles have diverse applications, requiring advancements in their synthesis that facilitate scale up, size control and reproducibility. Using a seeded-growth method in a 20 mL two-phase flow reactor (ID 2.4 mm) at 35 °C, highly monodisperse gold nanoparticles of any chosen size from 20 to 60 nm were produced. Heptane was utilised as the segmenting fluid to transport the aqueous reagent-containing droplets through a coiled PTFE reactor preventing their interaction with the reactor walls and thus reactor fouling. Gold seeds ~ 12 nm were produced via a passivated Turkevich synthesis by reduction of high pH Au(III) solution using citric acid as reducing agent. For the seeded-growth in flow, the reagents utilised were the seed solution (diluted accordingly), a stabilising Tris base solution, tetrachloroauric(III) acid trihydrate and hydrogen peroxide as reducing agent. Seeded-growth synthesis was also performed using as seeds commercial 10 nm gold nanoparticles, with excellent Coefficient of Variation (CoV) and Optical Density (OD) of the grown particles (CoV < 8% and OD ≥ 1) demonstrating that they are monodisperse and have high concentration. The synthesis was able to produce 18 mL/h of grown nanoparticles solution at 2.2–2.8 mg Au/h without any divergence in the quality of the produced particles for over eight hours.