Magnetic iron oxide nanoparticles have attracted increasing attention owing to their appealing applications as contrast agents for magnetic resonance imaging (MRI). Thermal decomposition synthesis of high quality iron oxide nanoparticles is widely investigated so far based on batch preparation. However due to the complexity of thermal decomposition synthesis, poor batch-to-batch reproducibility sets a huge hurdle in the front of practical application of iron oxide nanoparticles. Flow synthesis has been adopted for preparing kinds of nanomaterials owing to its steady production under automated continuous condition. Although RTD theory is relevant for explaining the particle size distribution in the flow synthesis of inorganic nanoparticles, the monomer concentration distribution in the fluid as well as the effect of Ostwald ripening process were not taken into consideration in previous studies. PEGylated Fe₃O₄ nanoparticles were prepared through flow synthesis upon the pyrolysis of ferric acetylacetonate (Fe(acac)₃) in anisole at 250°C under pressure of 33 bars, in the presence of α,ω-dicarboxyl-terminated polyethylene glycol (HOOC–PEG–COOH) and oleylamine. In combination with theoretical analysis, it has clearly been revealed that lowering the linear velocity of laminar flow narrows the particle size distribution due to effectively suppressed residence time distribution, but simultaneously prolonged residence time encourages Ostwald ripening leading to reverse variation tendency for particle size distribution. Moreover, the monomer concentration distribution within the tube reactor, strongly associated with the flow parameters, i.e., linear velocity of the reaction flow and tube reactor diameter, largely affects the particle size distribution. In accordance to these findings, monodispersed PEGylated nanoparticles with size distribution (relative standard deviation: 10.6%) sufficiently narrower than that achieved through batch preparation are obtained. Very interestingly, the resulting 4.6 nm particles present fairly high longitudinal relaxivity up to 11.1 mM^-1 ?s^-1. In addition, the excellent colloidal stability of the resultant Fe₃O₄ nanoparticles makes them reliable for MRI application. The relative results have been accepted by Chemistry of Materials. The current investigations have systematically uncovered the impacts of the flow parameters, i.e. residence time, linear velocity of fluid, and tube reactor dimension on the particle size distribution, and thus provide a new insight for guiding flow synthesis of advanced functional nanoparticles.
                                                                                                                                                                                                         Mingxia Jiao et al.