Inorganic Chemistry recently published a study on upconversion (UC) luminescent materials by researchers from Chongqing University of Posts and Telecommunications and the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. The team developed an erbium-doped calcium scandate crystal (CaSc2O4:Er3⁺) that enables high-sensitivity temperature sensing in deep biological tissues and advanced anticounterfeiting under near-infrared (NIR) light. Upconversion materials, which convert low-energy NIR light into higher-energy visible emissions, are vital for biomedical imaging and security technologies. However, conventional materials face challenges: limited tissue penetration and reliance on external sensitizers like Yb3+ or Nd3+.
The material was synthesized using a high-temperature solid-state method, ensuring structural stability and phase purity. X-ray diffraction confirmed Er3+ ions successfully replaced smaller Sc3+ ions in the crystal lattice. This substitution not only stabilized the matrix but also enhanced its luminescent efficiency. The crystal demonstrated resilience under varying temperatures and environmental conditions, critical for real-world applications.
Under 1532 nm excitation, CaSc2O4:Er3⁺ exhibited dual emission bands: visible red/green light and NIR signals. Researchers exploited thermally coupled energy levels of Er3+ to create a multi-path optical thermometer. The system achieved a maximum temperature sensitivity of 0.44% per Kelvin, with a detection depth exceeding 6 mm in biological tissues—ideal for non-invasive medical diagnostics.
For anticounterfeiting, the material's emission color shifted from yellow to red when switching excitation wavelengths between 980 nm and 1532 nm. This tunability, invisible under normal light, allows secure encoding on documents or products. Tests using screen-printed patterns confirmed its effectiveness, as hidden messages only appeared under specific NIR illumination.
The study highlights CaSc2O4:Er3⁺'s dual advantages: eliminating rare-earth sensitizers reduces costs, while deep-tissue compatibility opens avenues for minimally invasive medical tools. Compared to earlier UC materials, its excitation within the second NIR window minimizes light scattering, improving imaging resolution. For anticounterfeiting, the wavelength-dependent color shift offers a simple yet robust security solution.
The team plans to optimize the material for integration into portable medical devices and high-security labels. Further research may explore scaling production or adapting the crystal for other photonic applications, such as optical communications.
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