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. 2021 Feb 1;11(2):354. doi: 10.3390/nano11020354

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© 2021 by the authors.

Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Up-conversion fluorescence images for Gd₂O₃:Yb³⁺-Ln³⁺ (Ln = Ho, Er or Tm) colloids under excitation at 980 nm, obtained using a digital camera without any filter. Reproduced from [228], with permission from DOVE Press Publisher, 2016.