Upconverting nanoparticles are attracting extensive interest as a multimodal imaging tool. In this work, we report on the synthesis and characterization of gadolinium-enriched upconverting nanoparticles for bimodal magnetic resonance and optical luminescence imaging. NaYF4:Gd3+,Yb3+,Tm3+ core upconverting nanoparticles were obtained by a thermal coprecipitation of lanthanide oleate precursors in the presence of oleic acid as a stabilizer. With the aim of improving the upconversion emission and increasing the amount of Gd3+ ions on the nanoparticle surface, a 2.5 nm NaGdF4 shell was grown by the epitaxial layer-by-layer strategy, resulting in the 26 nm core-shell nanoparticles. Both core and core-shell nanoparticles were coated with poly(ethylene glycol) (PEG)-neridronate (PEG-Ner) to have stable and well-dispersed upconverting nanoparticles in a biological medium. FTIR spectroscopy and thermogravimetric analysis indicated the presence of ∼20 wt % of PEG-Ner on the nanoparticle surface. The addition of inert NaGdF4 shell resulted in a total 26-fold enhancement of the emission under 980 nm excitation and also affected the T 1 and T 2 relaxation times. Both r 1 and r 2 relaxivities of PEG-Ner-modified nanoparticles were much higher compared to those of non-PEGylated particles, thus manifesting their potential as a diagnostic tool for magnetic resonance imaging. Together with the enhanced luminescence efficiency, upconverting nanoparticles might represent an efficient probe for bimodal in vitro and in vivo imaging of cells in regenerative medicine, drug delivery, and/or photodynamic therapy.

Department of Histology and Embryology 3rd Faculty of Medicine Charles University Ruská 87 Prague 10 Prague 100 00 Czech Republic

Department of Polymer Particles Institute of Macromolecular Chemistry Czech Academy of Sciences Heyrovského nám 2 Prague 6 Prague 162 06 Czech Republic

Faculty of Health Studies Technical University of Liberec Studentská 1402 2 Liberec 461 17 Czech Republic

Institute of Condensed Matter Chemistry and Technologies for Energy National Research Council and Department of Chemical Sciences University of Padova via F Marzolo 1 Padova 35131 Italy

Radiodiagnostic and Interventional Radiology Department Institute for Clinical and Experimental Medicine Vídeňská 1958 9 Prague 4 Prague 140 21 Czech Republic

University of Chemistry and Technology Prague Technická 5 Prague 6 Prague 166 28 Czech Republic

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