Tailoring the emission color in Sm3+/Li+-doped calcium hydroxyapatite nanocrystals: a path toward cryogenic luminescent thermosensors

This study reports the synthesis and physicochemical characterization of Sm³⁺, Li⁺ ions-co-doped nanocrystalline calcium hydroxyapatite (nHAp) as a cryo-compatible luminescent nanothermometer. This material exhibits a temperature-dependent emission color shift from blue-green at 77 K to orange-red at 300 K, with a high relative sensitivity (a maximum of 0.025 K−¹ at 200–225 K). The presence of a broadband self-activated emission band originates from the host lattice and is related to charge-compensation phenomena. Due to the linear color tuning below room temperature, the obtained materials enable precise, real-time temperature monitoring under cryogenic conditions. This type of nanosized thermometer could be particularly useful in cryopreservation, enabling accurate control of freezing and thereby improving the survival of stored biological materials.