Monodisperse NaYbF₄:Tm³⁺/NaGdF₄ core/shell nanocrystals with near-infrared to near-infrared upconversion photoluminescence and magnetic resonance properties

We report core/shell NaYbF₄:Tm³⁺/NaGdF₄ nanocrystals to be used as probes for bimodal near infrared to near infrared (NIR-to-NIR) upconversion photoluminescence (UCPL) and magnetic resonance (MR) imaging. The NaYbF₄:Tm³⁺ nanocrystals were previously reported to produce the intense NIR-to-NIR UCPL peaked at ∼800 nm under excitation at ∼975 nm. We have found that the growth of a NaGdF₄ shell on surface of the NaYbF₄:Tm³⁺ nanocrystals results in the increase in the intensity of UCPL of Tm³⁺ ions by about 3 times. Unlike biexponential PL decay of NaYbF₄:Tm³⁺ nanocrystals, the PL decay of NaYbF₄:Tm³⁺/NaGdF ₄ core/shell nanocrystals is single exponential and of longer lifetime due to the suppression of surface quenching effects for Tm³⁺ PL. The growth of a NaGdF₄ shell on surface of the NaYbF ₄:Tm³⁺ nanocrystals also provides high MR relaxivity from paramagnetic Gd³⁺ ions contained in the shell. The T1-weighted MR signal of the (NaYbF₄:2% Tm³⁺)/NaGdF₄ nanoparticles was measured to be about 2.6 mM^-1s^-1. Due to the combined presence of efficient optical and MR imaging capabilities, nanoprobes based on NaYbF₄:Tm³⁺/NaGdF₄ fluoride nanophosphors can be considered as a promising platform for simultaneous bimodal PL and MR bioimaging.