Boltzmann- and Non-Boltzmann-Based Thermometers in the First, Second and Third Biological Windows for the SrF2:Yb3+, Ho3+ Nanocrystals Under 980, 940 and 915 nm Excitations

Abstract Spectrally determination of temperature based on the lanthanide-doped nanocrystals (NCs) is a vital strategy to noninvasively measure the temperature in practical applications. Here, we synthesized a series of SrF 2 :Yb 3+ /Ho 3+ NCs and simultaneously observed the efficient visible upconversion luminescence (UCL) and near-infrared (NIR) downconversion luminescence (DCL) under 980, 940 and 915 nm excitations. Subsequently, these NCs were further utilized for thermometers based on the Boltzmann (thermally coupled levels, TCLs) and non-Boltzmann (non-thermally coupled levels, NTCLs) of Ho 3+ ions in the first (~ 650 nm), second (~ 1012 nm) and third (~ 2020 nm) biological windows (BW-I, BW-II and BW-III) under tri-wavelength excitations. The thermometric parameters including the relative sensitivity ( $$S_{{\text{r}}}$$ S r ) and temperature uncertainty ( $$\delta T$$ δ T ) are quantitatively determined on the I 648 / I 541 (BW-I), I 1186 / I 1012 (BW-II), and I 1950 / I 2020 (BW-III) transitions of Ho 3+ ions in the temperature range of 303–573 K. Comparative experimental results demonstrated that the thermometer has superior performances.