Aqueous phase synthesis of upconversion nanocrystals through layer-by-layer epitaxial growth for in vivo X-ray computed tomography

Abstract
Lanthanide-doped core-shell upconversion nanocrystals (UCNCs) have tremendous potential for applications in many fields, especially in bio-imaging and medical therapy. As core-shell UCNCs are mostly synthesized in organic solvents, tedious organic-aqueous phase transfer processes are usually needed for their use in bio-applications. Herein, we demonstrate the first example of one-step synthesis of highly luminescent core-shell UCNCs in the "aqueous" phase under mild conditions using innocuous reagents. A microwave-assisted approach allowed for layer-by-layer epitaxial growth of a hydrophilic NaGdF4 shell on NaYF4:Yb, Er cores. During this process, surface defects of the nanocrystals could be gradually passivated by the homogeneous shell deposition, resulting in obvious enhancement in the overall upconversion emission efficiency. In addition, the up-down conversion dual-mode luminescent NaYF4:Yb, Er@NaGdF4:Ce, Ln (Eu, Tb, Sm, Dy) nanocrystals were also synthesized to further validate the successful formation of the core-shell structure. More significantly, based on their superior solubility and stability in water solution, high upconversion efficiency and Gd-doped predominant X-ray absorption, the as-prepared NaYF4:Yb, Er@NaGdF4 core-shell UCNCs exhibited high contrast in in vitro cell imaging and in vivo X-ray computed tomography (CT) imaging, demonstrating great potential as multiplexed luminescent biolabels and CT contrast agents.

Citation
Li, F., Li, C., Liu, J., Liu, X., Zhao, L., Bai, T., … Feng, S. (2013). Aqueous phase synthesis of upconversion nanocrystals through layer-by-layer epitaxial growth for in vivo X-ray computed tomography. Nanoscale, 5(15), 6950. doi:10.1039/c3nr01530k

Acknowledgements
This work was supported by the Foundation of the National Natural Science Foundation of China (no. 20971054, 90922034 and 21131002), Specialized Research Fund for the Doctoral Program of Higher Education (no. 20110061110015), National High Technology Research and Develop Program (863 program) of China (no. 2013AA031702) and the Special Program of China Postdoctoral Science Foundation (no. 2012T50288). X. G. Kong would like to thank the NSF of china (no. 11174277 and 61275202). ESI is available online from Wiley InterScience or from the author.

Publisher
Royal Society of Chemistry (RSC)

Journal
Nanoscale

DOI
10.1039/c3nr01530k

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