Time resolved spectroscopy of infrared emitting Ag₂S nanocrystals for subcutaneous thermometry

We report a systematic investigation on the temperature dependence of fluorescence decay dynamics of infrared emitting colloidal Ag₂S nanocrystals (NCs) with different surface coatings. The drastic lifetime reduction in the biological temperature range (20-50 °C) makes Ag₂S NCs outstanding candidates for high sensitivity subcutaneous lifetime-based thermal sensing in the second biological window (1000-1400 nm). Indeed, the lifetime thermal sensitivity of Ag₂S NCs has been found to be as large as 3-4% °C^-1 at an operating wavelength of 1250 nm. Their application for lifetime-based luminescence nanothermometry has been demonstrated through simple ex vivo experiments specially designed to elucidate the magnitude of subcutaneous thermal gradients. Experimental data were found to be in excellent agreement with numerical simulations.