Expanding luminescence thermometry detection range to the SWIR for biomedical applications

High-resolution thermal sensing and bioimaging at the cellular level and in animal models is interesting for both early diagnosis and controlled treatment via photothermal conversion of several diseases. Despite excellent in vitro results have been obtained with visible emitting luminescent nanothermometers, their application for in vivo studies is very limited due to the reduced penetration depth of visible light in biological tissues. This can be overcome if materials with emitting in the so-called biological windows (650-1350 nm) are used. Despite all this work, the number of studies exploring the possibilities of longer emission wavelengths in luminescence thermometry are scarce. This includes those lying in the so called short-wavelength infrared (SWIR) that extends from 1.35 to 2.3 μm. SWIR light transmits more effectively (up to three times) through specific biological tissues (oxygenated blood and melanin-containing tumors), achieving higher penetrations depths. Due to the reduced tissue absorbance and scattering within this region. Here, we analyze the possibilities for temperature sensing purposes of emissions in the SWIR region generated by Er3+, Tm3+ and Ho3+ ions in KLu(WO4)2 nanoparticles. The thermometric responses of these particles are compared with those shown by other Ln3+-doped nanoparticles of the same family of materials operating in the other biological windows, and demonstrate the potentiality of SWIR emitting nanoparticles for temperature measurements in biological tissues. The results indicate that SWIR emitting nanoparticles are good candidates for luminescent thermometry in biomedical applications.