Bismuth-stabilized perovskites with water-boosted fluorescence for accurate quantification of high-concentration chloride in wastewater.

The rapid anion exchange between chloride (Cl-) and bromide (Br-), accompanied by remarkable photoluminescence (PL) shifts, enables CsPbBr3 perovskites to exhibit excellent selectivity for Cl- detection. However, CsPbBr3 nanocrystals stabilized by organic ligands usually exhibit limited applicability for Cl- detection in water, because the hydrophobic ligands prevent direct contact with aqueous Cl- and restrict interfacial ion-exchange kinetics. Ligand-free CsPbBr3-based perovskites synthesized by solid-state methods generally suffer from poor stability in aqueous environments and weak fluorescence, arising from their intrinsic ionic nature, abundant defect states, and large grain size, which severely limit their application in fluorescence sensing. Herein, metallic bismuth (Bi) with strong affinity toward Cl- was introduced to form composites with ligand-free CsPbBr3/Cs4PbBr6 perovskites, constructing multi-branched CsPbBr3/Cs4PbBr6/Bi composites (CPB-Bi) for Cl- detection in ethanol-water environments. The incorporation of Bi provides a robust structural framework and interfacial protection, significantly improving the stability of perovskite components in polar solvent environments. Additionally, the amount of water in ethanol induces partial reconstruction of Cs4PbBr6 into CsPbBr3 nanocrystals and passivates surface defects, resulting in enhanced radiative recombination and greatly improved PL intensity. Owing to the rapid reaction between the nanosized perovskite materials and chloride ions, the detection time was significantly reduced to 10 min. Based on the Cl-/Br- anion exchange mechanism, a linear relationship between PL peak positions and Cl- concentrations was established. The optimized CPB-Bi10 (10 mmol Bi) sensor achieved a chloride detection accuracy of 95.65% in wastewater samples at Cl- concentrations up to 10,000 ppm. The sensor exhibits outstanding sensitivity, high selectivity, and robust anti-interference capability, offering a facile strategy for designing ligand-free perovskite-based fluorescent sensors for environmental ion monitoring.