Microfluidic generation of metal nanoparticles by borohydride reduction

In this work a microfluidic system for the production of very small gold nanoparticles (GNP) by reduction with borohydride will be presented, as well as their direct surface modification with thiol ligands. Furthermore, the microsystem was tested for the generation of other metal nanoparticles, as for example, silver, copper and complex particles. The size distributions were determined using scanning electron microscopy and an analytical centrifugation method. Additionally, in situ UV–vis spectroscopy was applied for online monitoring and kinetic measurements. Reactor fouling in the course of the syntheses was monitored with a microscope camera, and methods like silanization and pH-adjustment were explored to minimize the fouling. The direct reduction of chloroauric acid with borohydride yielded reproducibly gold nanoparticles in the size range between 4 and 7 nm without recognizable fouling of the reactor. Between 200 and 4000 μl/min, the flow rate does not affect the size distribution of the obtained gold nanoparticle dispersions. An increasing dilution of the reactants leads to a decrease in the mean diameter of the particles. The influence of the concentration ratio (BH4−)/Au and the effect of the pH were also investigated, but no significant trend was observed. Surface modification of these particles was achieved in the microsystem and monitored via UV–vis measurement and size distribution determination.