Simultaneous enhancement of upconversion luminescence and thermometric property of upconversion nanoparticles by tuning crystal field

Lanthanide luminescence nano-thermometers, with great potential applications in photo-thermal therapy for temperature detecting and controlling, have aroused wide interest recently. Enhancing the sensitivity and accuracy of temperature sensing property of those nano-thermometers are of great importance. In the present paper, NaGdF4 upconversion nanoparticles with high doping Bi ions concentration were investigated. With as high as 60% Bi ions doping concentration, no impurity phase was observed in the products and no morphological change occurred. Enhanced upconversion luminescence and thermal sensing properties were obtained, due to the tailoring of crystal field and increasing asymmetry of host matrix. Judd-Ofelt calculation was performed to testify the increasing lattice asymmetry, which was consistent with experimental results. This work provides guidance of tuning thermal-sensing properties of optical nano-thermometer, which are of great significance in bio-sensing field.