Magnetic-optical bifunctional properties of sub-20 nm β-NaYF4:Yb3+, Er3+ @NaGdF4 core-shell nanocrystals

In this work, the monodisperse spherical pure β-NaYF4:Yb3+,Er3+ core NCs with average size of 7.2 nm were synthesized by using NaOA as sodium source instead of the conventional NaOH, based on which the sub-20 nm β-NaYF4:Yb3+,Er3+@NaGdF4 core-shell NCs with magnetic-optical bifunctional properties were successfully obtained with seed-mediated growth method. The magnetization of the core-shell NCs enhanced proportionally with increased shell thickness, and the maximum upconversion intensity was three orders of magnitude increased in the 18.3 nm NaYF4: Yb3+,Er3+@NaGdF4 core-shell NCs compare with the corresponding core NCs. Besides, the upconversion mechanism for the core and core-shell NCs were discussed. This work may lead to an ideal platform for the preparation of sub-20 nm spherical NCs with magnetic and optical properties for application in biomedical imaging and drug delivery.