(Gd,Yb,Tb)PO4 up-conversion nanocrystals for bimodal luminescence–MR imaging

Abstract

Up-conversion (Gd,Yb,Tb)PO₄ materials and their potential for bimodal imaging have received little attention in the literature. Herein, we report the first study on the up-conversion emission of (Gd,Yb,Tb)PO₄ nanocrystals synthesized via a hydrothermal method at 150 °C. These materials exhibit ultraviolet, blue and green up-conversion emissions upon excitation with a 980 nm continuous wave laser diode. The intensity of the blue-emission band at 479 nm, ascribed to the cooperative up-conversion emission of a pair of excited Yb³⁺ ions, depends on the Yb³⁺/Tb³⁺ concentration ratio, calcination temperature and particle size. Strong green up-conversion emission of Tb³⁺ is observed at 543 nm for the ⁵D₄ → ⁷F₅ transition. Relaxometry measurements reveal that the nanocrystals are efficient T₂-weighted (negative) contrast agents which, combined with visible-light emission generated by infrared excitation, affords them considerable potential for being used in bimodal, photoluminescence–magnetic resonance, imaging.