Temperature Dependence of Water Absorption in the Biological Windows and Its Impact on the Performance of Ag₂S Luminescent Nanothermometers

The application of nanoparticles in the biological context generally requires their dispersion in aqueous media. In this sense, luminescent nanoparticles are an excellent choice for minimally invasive imaging and local temperature sensing (nanothermometry). For these applications, nanoparticles must operate in the physiological temperature range (25-50 degrees C) but also in the near-infrared spectral range (750-1800 nm), which comprises the three biological windows of maximal tissue transparency to photons. In this range, water displays several absorption bands that can strongly affect the optical properties of the nanoparticles. Therefore, a full understanding of the temperature dependence of water absorption in biological windows is of paramount importance for applications based on these optical properties. Herein, the absorption spectrum of water in the biological windows over the 25-65 degrees C temperature range is systematically analyzed, and its temperature dependence considering the coexistence of two states of water is interpreted. Additionally, to illustrate the importance of state-of-the-art applications, the effects of the absorption of water on the emission spectrum of Ag2S nanoparticles, the most sensitive luminescent nanothermometers for in vivo applications to date, are presented. The spectral shape of the nanoparticles' emission is drastically affected by the water absorption, impacting their thermometric performance.