Excellent Upconversion Luminescence Obtained by Er³⁺ Self-Sensitization for Temperature Sensing in Deep Tissues and Anticounterfeiting Application

The development of high-efficiency upconversion (UC) luminescent materials with multifunctional applications, particularly those excited by wavelengths within the second near-infrared (NIR) biological window, holds a paramount scientific significance. Herein, Er³⁺ self-sensitization UC performance excited by 1532 nm wavelength is first studied in the CaSc₂O₄ matrix. Strong green and red UC emission as well as ⁴I_11/2 → ⁴I_15/2 NIR transition are observed, though multipath optical thermometry with high sensitivity is realized. Moreover, the penetration depth of the ⁴I_11/2 → ⁴I_15/2 transition in the biological tissues can reach at least 6 mm, benefiting from its ideal excitation and emission wavelengths located in the biological window. Beyond that, CaSc₂O₄: Er³⁺ phosphor displays variable color luminescence under dual-wavelength excitation, including 980 and 1532 nm, which can be used for enhancing the security of anticounterfeiting materials. Our findings suggest that CaSc₂O₄: Er³⁺ phosphor with excellent UC properties is a promising candidate for the application in optical thermometry and anticounterfeiting field.