Polymer-Assisted Self-Assembly of Superparamagnetic Iron Oxide Nanoparticles into Well-Defined Clusters: Controlling the Collective Magnetic Properties Christian Schmidtke,* ,†,‡ Robin Eggers, †,‡ Robert Zierold, § Artur Feld, †,‡ Hauke Kloust, †,‡ Christopher Wolter, †,‡ Johannes Ostermann, †,‡ Jan-Philip Merkl, †,‡ Theo Schotten, ∥ Kornelius Nielsch, § and Horst Weller* ,†,‡,∥,⊥ † Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany ‡ The Hamburg Center for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany § Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, 20355 Hamburg, Germany ∥ Center for Applied Nanotechnology, Grindelallee 117, 20146 Hamburg, Germany ⊥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia * S Supporting Information ABSTRACT: The combination of superstructure-forming amphi- philic block copolymers and superparamagnetic iron oxide nano- particles produces new nano/microcomposites with unique size- dependent properties. Herein, we demonstrate the controlled clustering of superparamagnetic iron oxide nanoparticles (SPIOs) ranging from discretely encapsulated SPIOs to giant clusters, containing hundreds or even more particles, using an amphiphilic polyisoprene-block-poly(ethylene glycol) diblock copolymer. Within these clusters, the SPIOs interact with each other and show new collective properties, neither obtainable with singly encapsulated nor with the bulk material. We observed cluster-size-dependent magnetic properties, influencing the blocking temperature, the magneto- viscosity of the liquid suspension, and the r 2 relaxivity for magnetic iron oxide nanoparticles. The clustering methodology can be expanded also to other nanoparticle materials [CdSe/CdS/ZnS core/shell/shell quantum dots (QDs), CdSe/CdS quantum dots/quantum rods (QDQRs), gold nanoparticles, and mixtures thereof]. ■ INTRODUCTION The self-assembly of block copolymers leads to ordered superstructures (micelles, vesicles, tubules, and gels), 1,2 which can be used as vehicles for drugs or contrast agents in targeted delivery. 3 For instance, water-soluble micelles form a hydro- phobic inner core, in which hydrophobic dyes or drugs can be encapsulated and well-protected. 4 The outer sphere, consisting of hydrophilic blocks, provides solubility of the micellar container and ideally minimizes interactions with the biological environment. However, in order to exploit the outstanding size-dependent properties of inorganic nanoparticles (NPs) in material and life science, 5 these particles need to be rendered water-soluble, because high-quality NPs are usually synthesized in organic high-boiling solvents−incompatible with biological systems. By means of block copolymers, it is possible to encapsulate the hydrophobic particles under conservation of their character- istics and render them water-soluble. 6−8 Furthermore, multiple 9 or different NPs 10 can be assembled into ordered clusters within the polymeric container. 11 This leads to new functional materials with novel collective properties (magnetic−magnetic, magnetic−plasmonic, plasmonic−plasmonic, plasmonic−fluo- rescent, or magnetic−fluorescent interactions) not existent in singly encapsulated NPs or in the bulk material. 12−14 Another advantage of the clustering is the signal intensity enhancement for biological imaging applications. Gaining full control over the self-assembly of NPs into well-defined monodisperse clusters is still a formidable challenge, because of multifactorial parameters influencing the self-assembly process. Clusterization depends on the nature of the polymer, the NPs-to-polymer ratio, 11,15 the solvent composition, 16 temperature, and injection speed (shearing forces 17 ). Recently, we published a report on the micellar encapsulation of NPs by using an amphiphilic poly(isoprene)-b-poly(ethylene glycol) diblock copolymer (PI-b-PEG). 18,19 This system can be further stabilized by cross-linking 20 or by introducing a Received: June 5, 2014 Revised: August 21, 2014 Published: August 25, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 11190 dx.doi.org/10.1021/la5021934 | Langmuir 2014, 30, 11190−11196