LAUSR

Thermal decomposition of iron oleate is a simple and widespread method for synthesizing monodispersed iron oxide nanoparticles (IONPs) with well‐defined morphology. However, the complexity of the underlying mechanism makes this method rather sensitive to variations in experimental conditions, and the lack of simple techniques to monitor the reaction progress in situ usually results in poor reproducibility and time‐consuming optimizations. Here, a simple, robust, and versatile in situ marker to monitor particle formation based on a sudden change in the temperature during reflux is reported. A linear relationship between the onset of particle formation and the concentration of surfactants is unveiled, corroborating a ‘chemically activated’ burst nucleation mechanism. Using this linear relationship as a guide, highly uniform spherical, cubic, and star‐shaped particles between 12 and 30 nm can be obtained. This temperature marker and the derived linear relationship not only deepen the understanding of the reaction process, but also provide a powerful tool for the straightforward optimization of IONPs.