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

Abstract

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 the biological tissues. Herein, near-pure red UC luminescence excited by 1532 nm wavelength is achieved in CaSc2O4: Er3+ through Ho3+ doping, showing an approximate 19-fold improvement of 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 proved via the steady state and transient spectroscopic data. Utilizing the Stark splitting of Er3+: 4F9/2 → 4I15/2 and Er3+: 4I11/2 → 4I15/2 transition, highly sensitive optical temperature sensing capability is realized with detection depth in the 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 wavelength, 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.