Surface engineering superparamagnetic nanoparticles for aqueous applications: design and characterization of tailored organic bilayers

Engineered superparamagnetic nanoparticles (NPs) have broad potential in biotechnologies, high contrast magnetic resonance imaging, and advanced environmental sensing and remediation technologies, among others. For successful environmental, aqueous-based applications, particle stability (as highly monodisperse and single domain nanocrystals) and specific surface functionality are critical to control. In this report, aqueous stabilization of 8 nm superparamagnetic iron oxide NPs is described and optimized using a series of surface engineered bilayers, exploring 13 ionic surfactants, which are systematically varied with regard to hydrophobic tail (size and properties) and polar head groups. As monodispersed aqueous suspensions, material libraries were evaluated through particle-particle aggregation kinetics (with varied ionic types and strengths) and long-term aqueous stabilities (up to one year). Optimal phase transfer approaches are presented, along with corresponding particle stability characterization data sets for each of the 13 ligands studied, which, when taken together, allow for flexible surface design strategies for a variety of superparamagnetic particle-based aqueous applications.