Synthesis of gold nanoparticles in an interdigital micromixer using ascorbic acid and sodium borohydride as reducers

Continuous-flow process conditions for the reduction to gold nanoparticles were varied following a variation scheme aiming at impacting each of the elementary steps—mixing, reaction, polymer diffusion and polymer binding. While the last three variations are done by proper material choice (reducer; polymeric stabilizer), flow processing using a multilamination micromixer intensified the mass transfer in all these steps and particularly improved the first mixing step of the reactants. As reducers L-ascorbic acid (H2Asc) and sodium borohydride (NaBH4) were used and as gold particle precursor chloroauric acid (HAuCl4). All processing runs were analysed by plasmon absorption and transmission electron microscopy to reveal size, shape and polydispersity. For the HAuCl4–H2Asc system, the smallest particles were 0.8–4 nm (7.5 mL/min) and 0.6–3 nm (10.0 mL/min) for the HAuCl4–NaBH4 system. This is at the upper edge of performance reported for batch systems in literature, i.e. among the smallest gold particles generated. While the particles made for the HAuCl4–H2Asc system were polydisperse and polyshaped for most process conditions (however being spherical and relatively uniform at the two largest flow rates investigated), processing with the HAuCl4–NaBH4 system resulted in particles sizes with small mean deviation and almost ideal spherical shape. With polyvinyl alcohol (PVA) as stabilizer, stable colloids were obtained for one week (violet coloured) up to at least 2 months (pink coloured) in case of reaction with ascorbic acid, and for at least 1 month in case of reaction with NaBH4.

► NaBH4 and H2Asc reducers of gold precursor were used to run reaction with different rates in micromixer as a potential device to conduct AuNPs synthesis.
► Different flow rates were used to optimize the process.
► High flow rate makes possible to synthesize spherical AuNPs with the size of about 1 nm.