Real-time monitoring of the evolution of magnetism during precipitation of superparamagnetic nanoparticles for bioscience applications

The magnetic response as measured in real-time during aqueous co-precipitation of superparamagnetic magnetite nanoparticles is reported for the first time. An integrated AC magnetic susceptometer within the mixing zone of the reactants allows for continuous monitoring of the precipitation reaction. The methodology is illustrated by demonstrating how a rapid mixing (RM) of the reactants on the order of milliseconds yields smaller magnetic particles and a much narrower particle size distribution as compared to a slow mixing (SM) on the order of a second. The RM particles display an ultra-low coercivity Hc 8.5 A m(-1), the lowest reported value to our knowledge. The reaction is completed within one minute at room temperature, whereas a slow mix (SM) results in a slower reaction (> 1 h) and particles with ca. 4 times higher coercivity. Reaction dependences on iron ion and base concentrations are reported as well as a comparison between reactions with ammonia and sodium hydroxide. The results are explained in terms of different pH stabilities of ferrous and ferric ions within the pH range covered during the course of the reactions. It is suggested that RM is necessary to avoid uneven particle growth leading to an exaggerated particle size distribution with less than ideal magnetic properties. This in-situ magnetic measurement during the actual synthesis may become an important tool for real-time quality control. RM gives uniform batch to batch properties which is especially important for bioscience applications.