Colloidal nanothermometers based on neodymium doped alkaline-earth fluorides in the first and second biological windows

Strontium fluoride nanoparticles activated with Nd3+ ions were studied as nanosized optical thermometers in the first (800-950 nm) and second (1000-1300 nm) biological windows, by monitoring the Nd3+ emission as a function of the temperature. The variations of the Nd3+ emissions were correlated with the temperature changes and the thermometric properties of SrF2 nanoparticles were evaluated using a ratiometric method. The best thermal sensitivity was found to be 0.60% K-1, a value which is the highest found for water dispersible colloidal nanothermometers based on the near-infrared luminescence of Nd3+ ions only in single phase materials. The excellent thermometric performance is also demonstrated by a thermal sensitivity of 1.2 °C, that can be achieved in temperature evaluation of real systems. Temperature measurements using a phantom tissue confirmed that the samples serve as efficient ratiometric nanothermometers that can work in two biological windows