Design of a bi-functional NaScF4: Yb3+/Er3+ nanoparticles for deep-tissue bioimaging and optical thermometry through Mn2+ doping

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 H-2(11/2)/S-4(3/2) of Er3+ to red emission state F-4(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+: I-4(13/2) -> I-4(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/T-2, 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.

Automated summary

An approximately monochromatic red upconversion emission was achieved in NaScF4: Yb3+/Er3+ nanoparticles by doping Mn2+ ions, which acted as bridges during the energy transfer process, leading to a significant acceleration of the red UC enhancement. The sample with 10% Mn2+ ions exhibited the strongest red luminescence and enabled ultrasensitive optical thermometry in the physiological temperature region.