Boltzmann- and Non-Boltzmann-Based Thermometers in the First, Second and Third Biological Windows for the SrF(2):Yb(3+), Ho(3+) Nanocrystals Under 980, 940 and 915 nm Excitations

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 ([Formula: see text] ) and temperature uncertainty ([Formula: see text] ) 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. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s11671-022-03718-z.