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

## TL;DR

This paper introduces a new nanothermometer that changes color with temperature, useful for monitoring cryogenic conditions in biological storage.

## Contribution

The novel contribution is the development of Sm³⁺, Li⁺-doped calcium hydroxyapatite nanocrystals with tunable luminescence for cryogenic temperature sensing.

## Key findings

- The material shows a temperature-dependent emission color shift from blue-green to orange-red.
- It achieves high relative sensitivity of up to 0.025 K⁻¹ between 200–225 K.
- The nanocrystals are suitable for real-time cryogenic temperature monitoring.

## Abstract

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.