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 SrF2:Yb3+/Ho3+ 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 Ho3+ ions in the first (~ 650 nm), second (~ 1012 nm) and third (~ 2020) biological windows (BW-I, BW-II and BW-III) under tri-wavelength excitations. The thermometric parameters including the relative sensitivity (\({S_{\text{r}}}\)) and temperature uncertainty (\(\delta T\)) are quantitatively determined on the I648/I541 (BW-I), I1186/I1012 (BW-II), and I1950/I2020 (BW-III) transitions of Ho3+ ions in the temperature range of 303–573 K. Comparative experimental results demonstrated that the thermometer has superior performances.