The effect of polycations on the formation of magnetite nanoparticles Nanoparticles of magnetite (Fe3O4) are envisioned to ﬁnd used in diverse applications ranging from magnetic data storage inks ferroﬂuids as well as in magnetic resonance imaging drug delivery and hyperthermia cancer treatment. Their magnetic properties strongly depend on their size and morphology two properties that can be synthetically controlled. Achieving appropriate control under soft chemical conditions has so far remained a challenging endeavor. One proven way of exerting this desired control has been using a biomimetic approach that emulates the proteome of magnetotactic bacteria by adding poly L arginine in the co precipitation of ferrous and ferric chloride. The objective of the work presented here is to understand the impact of this polycation on the formation mechanism of magnetite and through rational design to enhance the control we can exert on magnetite nanoparticle size and morphology. We developed a SAXS setup to temporally and structurally resolve the formation of magnetite in the presence of poly L arginine ...