Strategies to enhance the sensitivity of NaGdF_4:Yb-Tm based nanothermometers

摘要

The Yb~(3+)-Tm~(3+)-doped NaGdF4 upconversion nanoparticles were studied as contactless nanothermometers for the firstbiological tissue transparency window under 980 nm excitation. The single hexagonal phase NaGdF4:Yb~(3+)-Tm~(3+)nanoparticles were synthesized by solvothermal technique. The influence of dopants concentration and pumping powerdensity on thermal sensitivity and temperature resolution of obtained nanoparticles was analyzed. It was shown, that anincrease of Yb~(3+) doping concentration leads to a strong increase in near-infrared Tm~(3+) luminescence intensity, whichcorresponds to transitions ~3F_2-~3H_6, ~3F_3-~3H_6, ~3H_4-~3H_6 and could be used for thermometry. The measured efficiency ofupconversion luminescence for 80% of Yb~(3+) and 2% of Tm~(3+) doped nanoparticles was 5.0% compared to 0.2%efficiency for 30% of Yb~(3+) and 0.5% of Tm~(3+) doped nanoparticles. Laser induced heating of synthesized nanoparticleswith ratiometric temperature measurement was studied. The increase of pumping power density negatively affected thesensitivity, but increased the accuracy of measurement due to the increased near-infrared luminescence. In addition, thecomparison of different wavelengths for ratiometric thermal calibration was performed. It was shown, that the use of680-720 nm luminescence peak to 730-750 nm valley intensity ratio for thermometry promotes significant enhancementof thermal sensitivity and temperature resolution. Thermal sensitivity of 4%∙C~(-1) and temperature resolution of 0.6˚C in30-36˚C region were obtained for NaGdF4 nanoparticles doped with 80% of Yb~(3+) and 2% of Tm~(3+).