Supraparticles\nfrom Cubic Iron Oxide Nanoparticles:\nSynthesis, Polymer Encapsulation, Functionalization, and Magnetic\nProperties

Supraparticles (SPs) consisting of superparamagnetic\niron oxide\nnanoparticles (SPIONs) are of great interest for biomedical applications\nand magnetic separation. To enable their functionalization with biomolecules\nand to improve their stability in aqueous dispersion, polymer shells\nare grown on the SPs’ surface. Robust polymer encapsulation\nand functionalization is achieved via atom transfer radical polymerization\n(ATRP), improving the reaction control compared to free radical polymerizations.\nThis study presents the emulsion-based assembly of differently sized\ncubic SPIONs (12–30 nm) into SPs with diameters ranging from\n∼200 to ∼400 nm using dodecyltrimethylammonium bromide\n(DTAB) as the surfactant. The successful formation of well-defined\nspherical SPs depends upon the method used for mixing the SPION dispersion\nwith the surfactant solution and requires the precise adjustment of\nthe surfactant concentration. After purification, the SPs are encapsulated\nby growing surface-grafted polystyrene shells via activators generated\nby electron transfer (AGET) ATRP. The polymer shell can be decorated\nwith functional groups (azide and carboxylate) using monomer blends\nfor the polymerization reaction. When the amount of the monomer is\nvaried, the shell thickness as well as the interparticle distances\nbetween the encapsulated SPIONs can be tuned with nanometer-scale\nprecision. Small-angle X-ray scattering (SAXS) reveals that cubic\nSPIONs form less ordered assemblies within the SPs than spherical\nSPIONs. As shown by vibrating sample magnetometer measurements, the\nencapsulated SPs feature the same superparamagnetic behavior as their\nSPION building blocks. The saturation magnetization ranges between\n10 and 30 emu/g and depends upon the nanocubes’ size and phase\ncomposition.