By loading microemulsion containing both sensitizer and emitter into the treated eco-friendly hydrogel, a new solid-state material of triplet−triplet annihilation based upconversion (TTA-UC) material was first reported. This UC hydrogel has shown an excellent stable emission of the green-to-blue luminescence, owing to the honeycomb-like nanoporous structure of the hydrogel. Moreover, this emission of UC hydrogel was strongly dependent on the temperature. The UC fluorescence intensity is close to the linear growth from 30 to 60 °C. This thermoresponsive upconversion hydrogel, which could be easily molded into various shapes, has provided a new perspective in manufacturing a novel temperature indicator and as well other applications in stabilizing air-sensitive species in aerated systems.

Air-stable, highly efficient and bio-safe triplet–triplet annihilation upconversion (TTA-UC) is achieved in a terpene-based green solvent d-limonene.

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