Flow-controlled synthesis of gold nanoparticles in a biphasic system with inline liquid–liquid separation

We present the flow synthesis of 4-(dimethylamino)pyridine (DMAP)-stabilised gold nanoparticles in a biphasic flow reactor system. In the first stage, the reduction of gold chloride using sodium borohydride took place via gas permeable tubing to minimise pressure build-up in the lines due to hydrogen gas liberation. The use of liquid–liquid membrane separators enabled automatic washing of the intermediate tetraoctylammonium bromide (TOAB)-stabilised gold nanoparticles and for subsequent phase transfer of the organic nanoparticles to the aqueous phase using DMAP. Through a systematic investigation of DMAP concentration and ratio, the amount of DMAP necessary for full transfer of the gold nanoparticles was elucidated. This semi-automated, scalable flow process enabled the production of gold nanoparticles in high yield, with good batch-to-batch reproducibility and improved monodispersity compared to conventional batch methods.