Mohd Zeeshan, Asma S Al-Wasidi, Mohammad Yasir Khan, M Shahid, Ehab A Abdelrahman
A water-stable dinuclear Cd(II) coordination cluster, [Cd₂(pdm)₂(benzoate)₄] (pdm = pyridine-2,6-dimethanol), was developed as a multifunctional luminescent probe for the selective aqueous detection of Zn2+, Fe3+, and Cr₂O₇2-. The probe exhibits a distinct fluorescence turn-on response toward Zn2+ and pronounced turn-off responses toward Fe3+ and Cr₂O₇2-, with a characteristic emission maximum at approximately 410 nm. Quantitative fluorescence analysis afforded detection limits of 2.245 × 10-6 M for Zn2+, 0.562 × 10-6 M for Fe3+, and 1.900 × 10-6 M for Cr₂O₇2-, with corresponding LOQ values of 7.484 × 10-6, 1.874 × 10-6, and 6.335 × 10-6 M, respectively. The quenching responses toward Fe3+ and Cr₂O₇2- yielded Stern-Volmer constants of 5.123 × 104 and 1.775 × 104 M-1, respectively. Mechanistic investigations using XPS, time-resolved photoluminescence (TRPL), and cyclic voltammetry revealed analyte-dependent electronic interactions and excited-state quenching pathways. The fluorescence lifetime decreased from 8.56 ns to 1.84 ns for Fe3+ and 1.88 ns for Cr₂O₇2-, corresponding to energy-transfer efficiencies of 78.5% and 78.0%, respectively. The sensing responses were further evaluated by interference experiments, pH-dependent fluorescence measurements, complete emission spectral analysis, and photostability studies, confirming the reliability of the probe under aqueous conditions. Furthermore, recycling experiments combined with PXRD analysis demonstrated retention of the crystalline structure after repeated sensing-regeneration cycles, while ICP-OES measurements showed low Cd2+ leaching of 0.00682, 0.00135, and 0.00496 mg g-1 under acidic, neutral, and alkaline conditions, respectively. These results establish [Cd₂(pdm)₂(benzoate)₄] as a robust and reusable luminescent sensing platform with complementary structural, photophysical, and electrochemical evidence supporting its analyte-specific recognition.