Accurate thermometry based on the red and green fluorescence intensity ratio in NaYF_4: Yb, Er nanocrystals for bioapplication

A biological temperature measurement method based on the fluorescence intensity ratio (FIR) was developed to reduce uncertainty. The upconversion luminescence of NaYF4:Yb, Er nanocrystals was studied as a function of temperature around the physiologically relevant range of 300–330 K. We found that the green–green FIR Fe and red–green FIR (I660/I540) varied linearly as temperature increased. The thermometric uncertainties using the two FIRs were discussed and were determined to be almost constant at 0.6 and 0.09 K for green–green and red–green, respectively. The lower thermometric uncertainty comes from the intense signal-to-noise ratio of the measured FIRs owing to their comparable fluorescence intensities.