Design of a bi-functional NaScF₄: Yb³⁺/Er³⁺ nanoparticles for deep-tissue bioimaging and optical thermometry through...

An approximately monochromatic red upconversion (UC) emission is successfully realized in NaScF₄: Yb³⁺/Er³⁺ nanoparticles (NPs) through Mn²⁺ ions doping without phase transition. The Mn²⁺ ions play a role of bridge during the energy transfer process from green emission state ²H_11/2/⁴S_3/2 of Er³⁺ to red emission state ⁴F_9/2 of Er³⁺, which significantly accelerates the red UC enhancement. The strongest red luminescence is observed in the sample containing 10% Mn²⁺ ions (Mn-10) with an enhancement factor of 7.5 times. Meanwhile, an ultrasensitive optical thermometry in the physiological temperature region can be realized by utilizing the fluorescence intensity ratio (FIR) between two thermally coupled Stark transitions of Er³⁺: ⁴I_13/2 → ⁴I_15/2, locating in the near-infrared (NIR) long wavelength region of the second biological window. Its relative sensitivity S_R can be expressed by 340/T², which is much higher than most optical thermometers based on thermally coupled Stark sublevels reported by the previous papers. Beyond that, an ex vivo experiment is designed to evaluate the penetration depth of the red and NIR emission of Mn-10 in the biological tissues, revealing that they can reach depth of at least 3 mm and 5 mm respectively. More importantly, the increasing tissue thickness has almost no effect on the FIR values. All the results show that the present sample is a promising bi-functional nano probe which can be used for bioimaging and temperature sensing in the deep tissues through the strong red UC emission and ultrasensitive NIR optical thermometer, respectively.