Microfluidic systems are a powerful tool to study and optimize a wide range of biological and chemical reactions,[1] and their use for the synthesis of nanoparticles is attracting more and more attention. Compared to conventional bulk synthesis strategies, microfluidic systems allow more precise control of the reaction conditions, which can lead to reductions in particle size and polydispersity.[2] A range of different nanoparticles have been synthesized in microfluidic systems: CdSe, CdS, TiO2, boehmite, Au, Co, Ag, Pd, Cu, BaSO4, and CdSe–ZnS core–shell nanoparticles.[2, 3] Although there are some recent preliminary results on the synthesis of iron oxide nanoparticles in a millifluidic system,[4] to date a microfluidic synthesis of iron oxide nanoparticles has not been demonstrated. Spinel iron oxide nanocrystals have attracted attention for their use as high-density data storage media,[5] or in biomedical applications, for example as contrast enhancement agents for magnetic resonance imaging (MRI) and for drug delivery.[6, 7] Controlling the synthesis conditions of these particles is critical, as these determine their physical properties.[8]

While single-phase microfluidic systems are subjected to diffusion-limited mixing and reagent dispersion, dropletbased microfluidic systems overcome these limitations by fast mixing in spatially isolated microreactors (droplets) containing well-defined quantities of materials [9–11] and therefore provide a high level of control of the synthesis