Folate receptor targeted, rare-earth oxide nanocrystals for bi-modal fluorescence and magnetic imaging of cancer cells

Targeted cancer imaging using rare-earth oxide nanocrystals, free from heavy metals (Cd, Se, Te, Hg and Pb), showing bright red-fluorescence and magnetic resonance imaging (MRI) is presented. Y2O3 nanocrystals (YO NC) doped in situ with fluorescent (Eu3+) and paramagnetic (Gd3+) impurities and conjugated with a potential cancer targeting ligand, folic acid (FA), were prepared using an all-aqueous wet-chemical process. Structural, optical and magnetic properties of these multifunctional nanocrystals were investigated by X-ray diffraction, electron microscopy, photoluminescence and magnetization studies. Highly monodisperse nanocrystals of size ∼20 nm with cubic bixbyite crystal structure showed bright red-fluorescence when doped with Eu3+. Co-doping with Gd3+ and mild air drying resulted significantly enhanced fluorescence quantum efficiency of ∼60% together with paramagnetic functionality, enabling T1-weighted MR contrast with ∼5 times higher spin-lattice relaxivity compared to the clinically used Gd3+ contrast agent. Cytotoxicity and reactive oxygen stress studies show no toxicity by YO NC in both normal and cancer cells up to higher doses of 500 μm and longer incubation time, 48 h. Cancer targeting capability of FA conjugated NCs was demonstrated on folate receptor positive (FR+) human nasopharyngeal carcinoma cells (KB) with FR depressed KB (FRd) and FR negative (FR-) lung cancer cells A549 as controls. Fluorescence microscopy and flow-cytometry data show highly specific binding and cellular uptake of large concentration of FA conjugated NCs on FR+ve cells compared to the controls. Thus, the present study reveals, unique bi-modal contrast imaging capability, non-toxicity and cancer targeting capability of multiple impurities doped rare-earth oxide nanocrystals that can find promising application in molecular imaging.