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Abstract: Multifunctional colloidal nanoparticles which exhibit fluorescence, superparamagnetism, and thermosensitivity are produced by two step seed emulsifier-free emulsion polymerization in the presence of oleic acid (OA) and sodium undecylenate (NaUA) modified Fe3O4 nanoparticles. In the first step, St and NIPAM polymerize the NaUA on the surface of Fe3O4 nanoparticles to form Fe3O4/poly(St-NIPAM) nanoparticles which act as seeds for the polymerization of Eu(AA)3Phen with the remaining St and NIPAM in the second step to form an outer fluorescent layer. The core–shell composite nanoparticles show reversible dimensional changes in response to external temperature stimuli. Fluorescence spectra acquired from the composites exhibit characteristic emission peaks of Eu3+ at 594 and 619 nm and vivid red luminescence can be observed by 2-photon confocal scanning laser microscopy (CLSM). In vitro cytotoxicity tests based on the MTT assay demonstrate good cytocompatibility and the composites also possess paramagnetic properties with a maximum saturation magnetization of 6.45 emu/g and high transverse relaxivity rates (r2) of 411.78 mM−1 s−1. In vivo magnetic resonance imaging (MRI) studies show significant liver and spleen contrast with relative signal intensity reduction of about 86% 10 min after intravenous injection of the composites. These intriguing properties suggest that these nanocarriers have large clinical potential as multimodal optical/MRI probes. [Copyright &y& Elsevier]