Monodisperse NaYbF₄:Tm³⁺/NaGdF₄core/shell nanocrystals with near-infrared to near-infrared upconversion photoluminescence and magnetic resonance properties

We report core/shell NaYbF4:Tm3+/NaGdF4nanocrystals 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 NaGdF4shell 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+/NaGdF4core/shell nanocrystals is single exponential and of longer lifetime due to the suppression of surface quenching effects for Tm3+PL. The growth of a NaGdF4shell on surface of the NaYbF4: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+)/NaGdF4nanoparticles 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+/NaGdF4fluoride nanophosphors can be considered as a promising platform for simultaneous bimodal PL and MR bioimaging.