Continuous synthesis of gold nanorods in a two-phase millifluidic flow reactor

We report the successful translation of a batch-based synthesis procedure for gold nanorods (AuNRs) to two-phase flow in a millifluidic channel, with a view to enhancing reaction control and scalability. Aqueous solutions of gold seed-particles and growth-mixture are mixed and dispersed as droplets inside a stream of organic carrier liquid, before passing through a length of gently warmed polytetrafluoroethylene (PTFE) tubing where the seeds grow into nanorods. Following inline separation of the aqueous reaction mixture from the carrier liquid, the product is collected in a solution of sodium sulfide to quench further growth of the AuNRs outside of the reactor. The size of the AuNRs is readily tuned by varying the concentration of seed particles added to each droplet and the relative flow-rates of the aqueous reagent streams, with high nanorod shape-yields of > 80 percent and low length-dispersities of < 10 percent being obtained under optimised conditions. In contrast to typical single-phase reactors, no issues relating to reactor fouling are encountered using the two-phase procedure deployed here, which may offer a scalable and reliable approach to shape- and size-selective synthesis of AuNRs. • Successful translation of seed-mediated batch-based synthesis of gold nanorods into continuous droplet flow-based process. • The effect of aging of growth and seed solutions, and the concentration of seed particles per droplet volume are investigated. • At similar reaction conditions, gold nanorods could be produced in flow with higher shape yeild and lower polydispersity than in batch process.