Design of a 1532 nm-driven red upconverter with high color purity for optical thermometry and anti-counterfeiting applications

The development of high color purity red upconversion (UC) materials operating within the second near-infrared (NIR) biological window (NIR-II) holds significant research importance for enhancing the penetration depth of such materials in biological tissues. Herein, near-pure red UC luminescence excited by a 1532 nm wavelength is achieved in CaSc₂O₄:Er³⁺ through Ho³⁺ doping, showing an approximately 19-fold improvement in the red-to-green emission ratio. Such a huge improvement in emission color purity results from the effective modulation of energy transfer (ET) mechanisms by Ho³⁺ ions, which is fully evidenced by the steady state and transient spectroscopic data. Utilizing the Stark splitting of Er³⁺:⁴F_9/2 → ⁴I_15/2 and Er³⁺:⁴I_11/2 → ⁴I_15/2 transitions, highly sensitive optical temperature sensing is realized with detection depths in biological tissues of about 6 mm and 8 mm, respectively. Furthermore, CaSc₂O₄:Er³⁺/Ho³⁺ exhibits different luminescence colors under the excitation of 980 nm and 1532 nm wavelengths, enabling its optical anti-counterfeiting application with high concealment and security. These findings present a novel strategy to design NIR II-responsive red UC materials with high color purity for biomedicine and anti-counterfeiting applications.