We report the synthesis, characterization and application of highly water-soluble fluoresceinpolyethyleneimine (PEI) coated gadolinium oxide (Gd 2 O 3 ) nanoparticles to magnetic resonance imaging (MRI) and cell labeling (CL). The average particle diameter and average hydrodynamic diameter were estimated to be 3.92 and 7.5 nm, respectively. Fluorescein-PEI was prepared from EDC/NHS coupling method. The surface coating was characterized by the FT-IR absorption spectrum and the surface coating

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... unt was estimated to be 22.42 wt% from a TGA analysis, corresponding to 0.65 nm 22 grafting density. The fluorescein-PEI coated gadolinium oxide nanoparticles showed r 1 and r 2 of 6.76 and 20.27 s 21 mM 21 , respectively, and a strong fluorescence at y527 nm. A pronounced positive contrast enhancement was clearly observed in 3 tesla T 1 MR images of a rat with a liver tumor after injection of an aqueous sample solution into a rat tail vein. After treatment of DU145 cells with a sample solution, a strong fluorescence in confocal images was also observed. These two results together confirm the excellent MRI-CL dual functionality of fluorescein-PEI coated gadolinium oxide nanoparticles. { Electronic supplementary information (ESI) available: XRD patterns of both as-prepared and TGA analyzed powder samples of fluorescein-PEI coated gadolinium oxide nanoparticles, a HRTEM image of the TGA analyzed powder sample, and NMR spectra. See