Study on Fluorescence Recognition of CrO₄²⁻ and Cr₂O₇²⁻ Based on Zn-MOF

Abstract

In contemporary society, as industrialization and urbanization advance rapidly, the problem of chromium-containing wastewater discharge has grown increasingly conspicuous. Hexavalent chromium salts (CrO₄²⁻ and Cr₂O₇²⁻), as common heavy metal ions, pose a severe threat to both the environment and human health. Therefore, the progress of efficient and sensitive detection technologies is crucial for monitoring such environmental pollutants. In this study, 2,6-naphthalenedicarboxylic acid and 4,7-bis (4-pyridyl) benzothiadiazole were employed as ligands to synthesize a zinc-based metal-organic framework (MOF-1) through a solvothermal approach. The successful preparation of the MOF-1 was verified through powder X-ray diffraction (PXRD) analysis. MOF-1 exhibits specific recognition capabilities for CrO₄²⁻ and Cr₂O₇²⁻ through the mechanism of fluorescence quenching. Interference experiments, conducted using Cr₂O₇²⁻ as a representative anion, confirmed that MOF-1 can accurately identify Cr₂O₇²⁻ even in the presence of other coexisting anions, demonstrating excellent selectivity. Titration experiments revealed that the fluorescence intensity of MOF-1 diminishes as the concentrations of CrO₄²⁻ and Cr₂O₇²⁻ increase, showing a typical concentration-dependent quenching behavior. The quenching constants (Ksv) were determined to be 1.96×10⁴ M⁻¹ and 2.68×10⁴ M⁻¹ for CrO₄²⁻ and Cr₂O₇²⁻, respectively, indicating high sensitivity. This sensing mechanism is attributed to the significant spectral overlap between the ultraviolet absorption spectra of CrO₄²⁻/Cr₂O₇²⁻ and the excitation spectrum of MOF-1. The competitive absorption of excitation energy by these ions reduces the energy available for MOF-1, thereby decreasing its fluorescence intensity. The combination of high sensitivity and selectivity highlights the significant advantages of MOF-1 as a fluorescent sensing agent for hexavalent ion detection, providing reliable technical support for environmental monitoring and analytical testing applications.