Accurate thermometry based on the red and green fluorescence intensity ratio in NaYF sub(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 NaYF sub(4):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 (I sub(660)/I sub(540)) 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.