From the Unimolecuclar Template of Water-Soluble Multiarm Star Brush Copolymers to One-Dimensional Colloidal Nanocrystal Clusters: Facile Synthesis via a Combination of Magnetic Assembly with Photoinduced Cross-Linking

Abstract

An effective process for the fabrication of one-dimensional (1D) colloidal superparamagnetic magnetite (Fe3O4) nanocrystal clusters was developed by combining magnetic assembly and photoinduced cross-linking under an external magnetic field. First, a series of water-soluble multiarm star brush copolymers with different molecular weights using α-cyclodextrin as a multifunctional initiator, star brush copolymer PEO-g-PAA, which consisted of poly(ethylene oxide) (PEO) as the main chain and poly(acrylic acid) (PAA) as functional side chains, were prepared by combining anionic ring-opening copolymerization with atom transfer radical polymerization (ATRP). Sequentially, star brush copolymer PEO-g-PAA as a spherical unimolecuclar template was utilized to structure-direct the in situ formation of spherical superparamagnetic Fe3O4 colloidal nanocrystal clusters by loading precursors of Fe3O4 (FeCl3 and FeCl2) into the PAA compartment. Surface-tethered PAA grafting chains of spherical Fe3O4 colloidal nanocrystal clusters, produced by ATRP, retain terminal bromine atoms that were subsequently converted into photo-cross-linkable azide groups (-N3) through nucleophilic substitution. The spherical Fe3O4 colloidal nanocrystal clusters coated with photo-cross-linkable azide groups were assembled into temporary 1D secondary nanostructures by being exposed to external magnetic fields. The resulting temporary 1D nanoclusters were efficiently cross-linked by exposure to UV light to stabilize the 1D secondary nanostructures of Fe3O4 nanocrystals. The dimension of the resulting stable and cross-linked 1D clusters can be tuned by adjusting the strength of the external magnetic field.