Synergistic pH-driven chiral engineering for enhanced circularly polarized upconversion luminescence in Yb3+/Er3+-doped Bi4O5Br2 toward optical thermometry and advanced anti-counterfeiting

Upconversion luminescent materials can convert near-infrared excitation into higher-energy emissions, showing great potential in applications such as temperature sensing and optical anti-counterfeiting. However, it remains challenging to simultaneously achieve high thermal stability, sensitive temperature response, and circularly polarized luminescence within a single system. In this work, chiral Bi 4 O 5 Br 2 : Yb 3+ /Er 3+ nanosheets were synthesized via a hydrothermal method combined with chiral modification and pH regulation, and their structural and luminescent properties were systematically investigated. The results indicate that pH regulation during precursor synthesis facilitates the optimization of the Bi 4 O 5 Br 2 morphology and improves its crystallinity, thereby promoting a more ordered crystal growth process. Subsequently, chiral induction introduces local lattice distortion while suppressing defect formation. The synergistic effect of both helps to significantly enhance upconversion luminescence performance. Under variable-temperature conditions, the 849 nm near-infrared emission ( 4 I 9/2 - 4 I 15/2 ) exhibits negligible thermal quenching over the temperature range of 293−533 K, in sharp contrast to the thermally quenched behavior of visible emissions. Based on this feature, a fluorescence intensity ratio (FIR) thermometry strategy was established, achieving high sensitivity with an absolute sensitivity ( S a ) of 20.73%/K and a relative sensitivity ( S r ) of 3.45%/K. Furthermore, the material exhibits pronounced emission intensity differences under circularly polarized excitation and detection. This enables the construction of a polarization-color synergistically tunable luminescent anti-counterfeiting system. This work provides a new strategy for designing multifunctional upconversion materials that integrate optical thermometry and polarization-based anti-counterfeiting capabilities.