Highly selective turn-on fluorescent chemosensor for the detection of Cr (III) ion in drinking water

An effective and simple carbazole-based Schiff base (CrbPy2) chemosensor for selective recognition of Cr(III) was designed and synthesized. The molecular structure of CrbPy2 was confirmed by NMR and FT-IR spectroscopies, its crystal structure was investigated by XRD analysis and the results were in good agreement with the DFT calculations. The CrbPy2 probe showed high selectivity toward Cr3+ ions in THF/H2O (95/5%) solution. Adding Cr3+ ions to CrbPy2 dilute solution induced a naked eye color change from colorless to yellow accompanied with a fluorescence turn-on. The proposed sensing mechanism was related to chelation-enhanced fluorescence (CHEF) process. The stoichiometry ratio of CrbPy2-Cr3+ complex is 1:1 with an association constant exceeding 3 x 105 M-1. The study of the sensing properties in different pH showed that CrbPy2 can tolerate the detection of Cr3+ ions in a pH range from 6 to 8. The chelating sites of the chemosensor and the coordination geometry of the complex were determined using 1H NMR titrations and MEP calculations. The detection was completed within 2-3 min and the detection limit reaches 1.08 mu M. The linear recognition range of Cr(III) is from 1.08 mu M to 4 mu M making CrbPy2 a suitable probe for real-time identification of unsafe Cr3+ concentration in drinking water.